Subcutaneous Transmitter (A3028)

© 2013-2021, Kevan Hashemi, Open Source Instruments Inc.

Contents

Warnings
Description
Versions
Electrodes
Leads
Antenna
Design
Modifications
Analog Inputs
Synchronization
Input Noise
Power Measurement
Body Capacitance
Battery Life
Battery Recharging
Encapsulation
Development

Warnings

Warning: Check to make sure devices are inactive upon arrival at your facility. Do not store within 10 cm of iron tools, power transformers, computers, or external hard drives. During implantation, take note that stainless steel tools and work surfaces are often magnetic, and can activate and deactivate your transmitters.

Description

The Subcutaneous Transmitter (A3028) is an implantable telemetry sensor for rats and mice that provides high-fidelity amplification and recording of one or two biopotential inputs. The device comes in rechargeable and non-rechargeable versions, as well as fully-implantable and head-mounting versions. Non-rechargeable versions have three times the operating life per unit volume as rechargeable device. Head-mounting devices allow for battery replacement, but are larger and preclude co-housing the animals.


Figure: Subcutaneous Transmitter A3028A-CCC-B45-B. Body 13 mm × 13 mm × 8 mm, volume 1.3 ml, mass 2.2 g. The leads are 45 mm long and 0.7±0.1 mm in diameter. First recording channel is Red − Blue, second is Yellow − Blue. Lead terminations are M0.5 screws soldered to the stainless steel coil of the lead.

The A3028A displaces 1.3 ml and provides two EEG channels with bandwidth 0.3-160 Hz for 26 days. The A3028E displaces 3.0 ml, provides one 0.3-160 Hz EEG channel and runs for 133 days. Both the A3028A and A3028E are fully-implantable beneath the skin of an animal. The A3028MX is a head fixture with a replaceable battery that provides two EEG channels with 640-Hz and 2048 SPS for four days. The A3028M may be mounted on the skull of an animal, but may not be implanted beneath the skin.


Figure: Subcutaneous Transmitter A3028B-AA-B45-B and A3038P1-AA-C37-C Seen From Top (Left) and Side (Right). The A3028B is on the left in each picture. It has volume 1.3 ml and is equipped with 0.7-mm diameter leads and a 30-mm loop antenna. The A3028P is on the right in each picture. It has volume 0.65 ml and is equipped with 0.5-mm leads and a 13-mm helical antenna.

All versions of the A3028 operate within our Subcutaneous Transmitter System. The A3028 may be equipped with rechargeable (secondary) or non-rechargeable (primary) batteries. When the battery is rechargeable, we recharge it through the red and blue leads using a Battery Charger (A3033). Primary batteries offer three times the charge density of secondary batteries. As a consequence, non-rechargeable transmitters offer two to three times the operating life of rechargeable transmitters. Rechargeable transmitters may be restored to full capacity after each implantation, but their electrode leads suffer the same damage and length reduction as those of a non-rechargeable transmitter. We warranty all our implantable devices against corrosion and manufacturing defect for one year after delivery. We do not warranty the devices against damage during explantation, which is traumatic to the leads and antenna. You can follow our recharging and soldering instructions to recharge batteries and replace damaged leads. Or you can send your rechargeable transmitters back to us and we will recharge their batteries, test their frequency response and gain, and repair damaged leads as needed. Our price list gives the cost of these services and of the transmitters themselves. Transmitters with battery holders are intended to be mounted in head fixtures, and we deliver them with a plastic enclosure that may be cemented to an animal's skull. Their batteries may be replaced at any time.


Figure: Subcutaneous Transmitter A3028Q-PPP-B150-A. Volume 2.8 ml, diameter 24 mm, depth 7 mm, mass 6.2 g. The three leads are 140-mm long and 0.7-mm in diameter with 3-mm bare wire terminations (B-Leads with P-Coils). The antenna is a 50-mm loop of stranded steel (A-Antenna).

We turn the A3028 on and off with a magnet. We recommend a cow magnet. The A3028A comes with three helical stainless-steel leads coated with silicone. The three leads provide us with two biopotential inputs, red and yellow, that share a common ground, blue. The sensor measures the voltage difference Red − Blue and Yellow − Blue. The lead terminations can be screws, pins, or bare wire, as we discuss in Electrodes and The Source of EEG.

PropertyValue
Volume of Transmitter Body1.2 ml
Mass of Transmitter Body2.2 g
Lead Dimensionsdiameter 0.7±0.1 mm, length up to 140 mm
Maximum Dimensions14 mm × 14 mm × 8 mm
Minimum Operating Life14 days
Shelf Life5.5 years
Number of Inputs2
Type of InputDifferential
Input Impedance10 MΩ || 2 pF
Sample Rate (Each Input)512 SPS
Sample Resolution16-bit
Input Dynamic Range27 mV
Input Bandwidth0.3-160 Hz
Input Noise≤8 μV rms
Input Mains Hum<1 μV
Total Harmonic Distortion<0.1%
Table: Specifications of the A3028A. The mains hum is for an implanted sensor. Shelf life and operating life are the time it takes the off-state and on-state currents to run down a fresh battery.

The A3028D is the same circuit with the same functionality as the A3028A, but with a 255 mA-hr battery instead of the A3028A's 48 mA-hr battery. The A3028D's volume is 3.0 ml and its operating life is 1900 hours. All versions of the A3028 turn on and off with the application of a magnetic field. We like to use cow magnets. Component U2 is a micro-power hall effect switch. When it detects a magnetic field, it asserts its output. When we remove the field, the transmitter changes state. If it is inactive, it activates. If it is active, it deactivates. The state change does not occur when we bring the magnet close to the transmitter, it occurs when we move the magnet away.

We determine the version of a transmitter during assembly, programming, and encapsulation. We might change capacitors on the board to set the filter frequencies. We might change resistors to set the gain. We program the logic chip to set the sample rate for each channel. We can equip the circuit with various sizes of battery. The larger the battery, the longer the operating life.


Figure: Subcutaneous Transmitter A3028MX-AAA-A45-B. Bandwidth 0.3-640 Hz, 2048 SPS each channel, replaceable battery, 100 hrs operating life per battery. The bottom side of the non-implantable head fixture devices is bare, sanded epoxy. The device may be bonded directly to the skull or placed in a head fixture designed and tested by one of our customers.

The A3028 transmits one or two signals on one or two SCT channel numbers. Each sample of each signal is transmitted as a separate radio-frequency message. Each message contains the channel number and the sample value, as we describe in Message Encoding. Channel numbers lie in the range 1-222. A two-channel transmitter transmits on channels n and n+1, where n is an odd number. In two-channel transmitters we have a red lead and a yellow lead. The red lead signal is transmitted on the first channel number, the yellow lead signal on the second channel number.


Figure: A Bare Pup Transmitter Circuit. We have not yet attached the leads and antenna, but the 10-mm diameter battery is loaded.

We can program the A3028 to take anything from 128-4096 SPS. We can enable either or both of its inputs. The A3028PV1 circuit is a variant of the A3028 circuit that provides only input. The circuit board itself is only 10.2 mm wide and 9.6 mm high, compared to 12 mm square for the two-input circuit.

Versions

[29-SEP-21] The Subcutaneous Transmitter (A3028) can be equipped with a dozen different batteries, any lead length up to 140 mm, several lead diameters, one or two recording channels, a dozen varieties of lead terminations, several types of antenna, and a range of bandwidths, gains, and sample rates. You specify which transmitter you want with a full SCT part number.


Figure: Comparison of Subcutaneous Transmitter Dimensions. From left: A3028T1-AA-C32-C (0.50 ml, 1.0 g), A3028P1-AA-C37-C (0.65 ml, 1.4 g), A3028S2-AA-C37-C (0.98 ml, 1.7 g), A3028B-DD-A45-B (obsolete encapsulation and thick leads, 1.4 ml, 2.4 g), A3028B-AA-B45-B (newer encapsulation and thin leads, 1.2 ml, 2.2 g), and A3028E-AA-A150-A (thick leads, 3.0 ml, 5.9 g).

The part number begins with A3028 and is followed by the primary version code, which can be a single letter, as in A3028A, or a letter followed by more letters and numbers, as in A3028S2Z. We can create new versions to your specifications, in which case we will assign the new version a new primary version code.


Figure: A3028 Part Numbering Scheme. Click on the large boxes to jump to tables listing letter codes and options.

The table below lists the A3028 primary version codes. Battery capacities are usually expressed in units of mA-hr (milliamp-hours). We convert to μ;A-dy (microamp-days) so it is easier to divide the capacity by the active current and obtain the operating life in days. We give frequency response in Hertz, sample rate in samples per second, and sixteen-bit sample conversion factors in microvolts per count. Input dynamic range is 90% of conversion factor multiplied by 65536. Input impedance is 10 MΩ for all versions

Version X Y Battery
Capacity
(μA-dy)
Volume
(ml)
Dimensions
(mm)
L × W × H
Mass
(g)
Minumum
Operating
Life (dy)
Shelf
Life
(yr)
A3028T1R 0.3-40 Hz, 128 SPS, 0.38 μV/cnt Omitted 208 (ML621S/DN) 0.50 16 × 11 × 3.7 1.0 5.6 0.6
A3028P1 0.3-40 Hz, 128 SPS, 0.41 μV/cnt Omitted 1250 (CR1025) 0.65 19 × 11 × 3.7 1.4 33 3.4
A3028P2 0.3-80 Hz, 256 SPS, 0.41 μV/cnt Omitted 1250 (CR1025) 0.65 19 × 11 × 3.7 1.4 24 3.4
A3028P3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Omitted 1250 (CR1025) 0.65 19 × 11 × 3.7 1.4 15 3.4
A3028S1 0.3-40 Hz, 128 SPS, 0.41 μV/cnt Omitted 2000 (CR1225) 0.85 21 × 13 × 3.7 1.6 54 5.5
A3028S2 0.3-80 Hz, 256 SPS, 0.41 μV/cnt Omitted 2000 (CR1225) 0.85 21 × 13 × 3.7 1.6 38 5.5
A3028S3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Omitted 2000 (CR1225) 0.85 21 × 13 × 3.7 1.6 24 5.5
A3028S2Z 0.0-80 Hz, 256 SPS, 4.1 μV/cnt Omitted 2000 (CR1225) 0.78 21 × 13 × 3.7 1.7 36 5.5
A3028W 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 1250 (CR1025) 1.0 12 × 12 × 7 1.8 24 3.4
A3028WZ 0.0-40 Hz, 128 SPS, 4.1 μV/cnt 0.0-40 Hz, 128 SPS, 4.1 μV/cnt 1250 (CR1025) 1.0 12 × 12 × 7 1.8 23 3.4
A3028A 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 14 5.5
A3028B 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled 2000 (CR1225) 1.2 14 × 14 × 7 2.2 24 5.5
A3028C Disabled 0.3-80 Hz, 256 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 38 5.5
A3028K Disabled 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 54 5.5
A3028F 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 7.5 5.5
A3028H 0.3-80 Hz, 256 SPS, 0.41 μV/cnt 0.3-80 Hz, 256 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 24 5.5
A3028J 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 3-200 Hz, 512 SPS, 1.4 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 14 5.5
A3028M 0.3-640 Hz, 2048 SPS, 0.41 μV/cnt 0.3-640 Hz, 2048 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 3.9 5.5
A3028U 0.0-160 Hz, 512 SPS, 4.1 μV/cnt 0.0-160 Hz, 512 SPS, 4.1 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 13 5.5
A3028V 0.3-160 Hz, 496 SPS, 0.41 μV/cnt 3-200 Hz, 16 SPS, 0.41 μV/cnt 2000 (CR1225) 1.2 14 × 14 × 7 2.2 24 5.5
A3028BX 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled 2000 (CR1225) 2.6 16 × 16 × 10 3.2 24 5.5
A3028MX 0.3-640 Hz, 2048 SPS, 0.41 μV/cnt 0.3-640 Hz, 2048 SPS, 0.41 μV/cnt 2000 (CR1225) 2.6 16 × 16 × 10 3.2 3.9 5.5
A3028D 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 11000 (CR2330) 3.0 24 × 24 × 8 6.3 76 30
A3028G 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 11000 (CR2330) 2.8 24 × 24 × 8 6.2 41 30
A3028E 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled 11000 (CR2330) 2.8 24 × 24 × 8 6.2 132 30
A3028Q3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 22000 (CR2450) 4.5 24 × 24 × 11 8.8 155 62
A3028Q4 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 22000 (CR2450) 4.5 24 × 24 × 11 8.8 84 62
A3028L 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt 42000 (CR2477) 6.0 27 × 27 × 14 13.0 156 114
Table: Primary Version Codes of A3028 Subcutaneous Transmitters. Minimum operating life at 37°C in days. Typical operating life is 10% higher. Shelf life for calculating fraction of battery capacity lost while on the shelf at 25°C. Rechargeable devices have "R" at the end of their code. Those with frequency response extending down to 0.0 Hz have "Z" at the end. Head-mounting versions have an "X" at the end.

The shelf life for a non-rechargeable transmitter is how long it takes to exhaust the battery permanently when we leave it inactive on the shelf. The shelf life of a rechargeable transmitter is how often we must recharge it to keep its battery in good condition. See below for details of current consumption and how to calculate battery life of new versions of the A3028. Transmitters with battery holders are intended for use in head fixtures. We can replace the battery, but we cannot implant the device because the battery holder is not water-proof. The transmitter comes with a head fixture enclosure that is cemented to the animal skull.

By default, we set the top of the frequency range at one third the sample rate. When its cut-off frequency is one third the sample rate, the A3028's low-pass filter provides 20 dB of attenuation at one half the sample rate. Frequencies above one half the sample rate will be distorted by sampling, and so compromise the fidelity of the recording. Because the EEG signal contains less and less power as frequency increases, this attenuation is sufficient to ensure that distorted frequencies are insignificant.

After the primary version number, the A3028 part number continues with a specification of the lead terminations. An A3028 has up to four leads: red, yellow, blue, and green. We specify the lead terminations with letters, as defined in our Termination Table. We give the lead terminations in order red, yellow, blue, green. The A3028C, for example, has yellow and blue leads. The A3028C-DA is a gold pin on the yellow lead and a bare wire on the blue lead. The A3028H-DDK has gold pins on red and yellow and a small screw on blue.

We specify the leads with a letter that gives the type of lead, and the length of the lead in millimeters. We present the lead letter codes in our Lead Table. Thus A3028C-DA-B45 is equipped with two 45-mm long leads each 0.7 mm in diameter. The final section of the part number specifies the antenna with a single letter, as specified in our Antenna Table. Thus A3028C-DA-B45-B uses a 30-mm loop antenna made out of stranded stainless steel.

Electrodes

Each lead of the transmitter has its own termination. We specify the termination of each lead with its own letter code: two letters for single-channel transmitters, three letters for a dual-channel transmitters with a common reference potential, and four letters for a dual-channel transmitter with separate reference potentials. The lead terminations is: Red, Yellow, Blue, Green. For a transmitter with Red, Yellow, and Blue leads, the letters "DDA" specify D-Pin termination on the red and yellow leads and an A-Coil termination on the blue lead. For a transmitter with Yellow and Blue leads "PA" specifies a P-Coil termination on the yellow lead and an A-Coil termination on the blue lead.

NameTypeDescription
A Coil Stainless steel helix, 1 mm long.
B Screw Thread 0-80, diameter 1.6 mm, length 3.2 mm.
C Screw Screw, thread M0.5, diameter 0.5 mm, length 0.6 mm.
D Pin Diameter 0.30 mm, length 2.1 mm, Mill-Max 4353-0-00-15-00-00-33-0
E Socket For pin diameters 0.20-0.33 mm, Mill-Max 4428-0-43-15-04-14-10-0
F Pin Diameter 0.64 mm, length 4.1 mm, Mill-Max 5035-0-00-15-00-00-33-0.
Mates with Plastics One socket E363/0.
G Pin Diameter 0.51 mm, length 4.4 mm, Mill-Max 5063-0-00-15-00-00-33-0.
Mates with Plastics One socket MS303/6.
K Screw Thread 00-90, diameter 1.2 mm, length 1.6 mm.
L Screw Thread 000-120, diameter 0.86 mm, length 1.6 mm.
M Wire Bare silver, diameter 125 μm, length 20 mm.
N Pin Diameter 0.38 mm, length 3.2 mm, Mill-Max 4689-0-00-15-00-00-33-0.
P Coil Stainless steel helix, length 3.0 mm, for crimp contacts, B-Leads only.
Table: Termination Letter Codes for the A3028. Precede letter-code with lower-case "s" to specify parallel orientation.

By default, we solder screws, pins, and sockets perpendicular to leads and wires. But if we precede the electrode letter code with a lower-case "s", the pin, screw, or socket is parallel to the lead or wire, as shown below.


Figure: Perpendicular (Left) and Straight (Right) Attachment of D-Electrode Pins. We specify perpendicular with code "D" and straight with code "sD".

Some of the above lead terminations can act as electrodes themselves. The screws we can push into a skull hole. The bare helix we can straighten and bend to fit in a skull hole.


Figure: Bare Wire Termination of B-Leads. Left: 1-mm A-Coil. Right: 3-mm P-Coil. P-Coil is designed for crimp connections to depth electrodes and is not available for C-Leads.

If we want a more sophisticated electrode, however, we must use the termination to connect to that electrode, and perform the connection during surgery. Whatever we attach to the ends of the SCT leads must be small enough to be drawn up under the skin of the neck to the head. If we want to use a depth electrode with our transmitter, we connect the depth electrode to the transmitter lead during surgery, for which we use a pin and socket contact or a crimp constact. Order depth electrodes at the same time as your transmitters, and we will try to make sure your transmitter leads have matching terminations. The pin and socket connection is slightly easier to perform than the crimp contact, but the crimp contact generates less noise.

NameDescription
H Depth electrode, wire 125-μm dia Pt-Ir, insulation 200-μm dia teflon.
Locate with guide cannula, provides E-Socket to mate with D-Pin termination.
J Depth electrode, 125-μm dia 316SS wire, insulation 200-μm dia teflon.
Locate with guide cannula, provides E-Socket to mate with D-Pin termination.
W Depth electrode, straight wire 125-μm dia 316SS, insulation 200-μm dia teflon.
Locate with hypodermic tube, provides E-Socket to mate with D-Pin termination.
X Depth electrode, wire 125-μm dia 316SS, insulation 200-μm dia teflon.
Locate with hypodermic tube, bare wire crimps to P-coil termination using 2-mm crimp ferrule.
Table: Electrode Letter Codes.

See SCT Implantation for an introduction to the implantation procedure. Join the OSI-Neuroscience mailing list to ask our customers about details of implantation.

Leads

[26-FEB-19] Our electrode leads are a flexible helix of stainless steel wire insulated in silicone. Standard leads are 20-140 mm long. We can make longer leads by joining shorter leads, but we will charge extra for doing so. We describe how we arrived at this design in Flexible Wires.

Lead
Code
Outer Diameter (mm) Spring Diameter (μm) Wire Diameter (μm) Resistance (Ω/cm) Names Common Application
A 1.0±0.2 450 100 6.3 Thick Lead Obsolete
B 0.7±0.1 450 100 6.3 Thin Lead 45-mm leads for mice,
100-mm leads for rat pups,
150-mm leads for adult rats
C 0.5±0.1 250 50 25 Very Thin Lead 35-mm leads for mouse pups,
45-mm leads for mice
50-mm leads for rat pups
D 0.8±0.1 500 150 1.6 Stimulator Lead 150-mm lamp leads for adult rats
Table: Lead Types. All leads are insulated with three coats of silicone, the first coat contains a dye to give the leads a bright color. The final coat consist only of unrestricted medical-grade silicone.

We order our springs in 150-mm sections, which makes it easy to manufacture leads up to 140 mm long. In principle, we could make longer leads by joining two 140-mm leads together and insulating the join with silicone (see video). We use the C-Leads (0.5-mm diameter) with our smallest transmitters, those with volume less than 1.0 ml such as the T, P, and S series devices. We use the B-Leads (0.7-mm diameter) with transmitters larger than 1.0 ml. The wire in the A-Lead (1.0-mm diameter) is the same as the wire in the 0.7-mm lead, but the A-Lead, being larger is more likely to irritate the animal. When we reduced the diameter of our leads from 1.0 mm to 0.7 mm, we noticed a sharp drop in the number of reports we received of skin irritation around the implanted wires. The 0.5-mm lead is half the volume of the 0.7-mm lead, which is important in the smallest animals. But the wire in the 0.5-mm lead is one half the diameter of the wire in the 0.7-mm lead, so the spring itself is one quarter as strong. The 0.5-mm leads must be handled with care. They will survive the fatigue of animal movement, but they are easy to damage with a scalpel during implantation or extraction.

Antenna

[23-JUN-21] We make three type of antenna: 50-mm loop, 30-mm loop, and 13-mm helix. We use the 50-mm loop in rat transmitters, the 30-mm loop in mouse transmitters, and the 13-mm helix in pup transmitters. The loop antennas are made of stranded steel wire. The loop antennas are tough and efficient. The helical antenna is made of a steel spring. It is small and flexible, but less efficient. We specify the Antenna with a letter code in the transmitter part number.

Antenna
Code
Length (mm) Description
A 50 Stranded steel loop antenna for rats
360-μm diameter 7×7 316SS wire
insulated in clear MED-6607 silicone.
B 30 Stranded steel loop antenna for rats,
360-μm diameter 7×7 316SS wire
insulated in clear MED-6607 silicone.
C 13 Straight antenna of helical wire, for pups,
450-μm diameter 316SS helix
insulated in clear MED-6607 silicone.
D 30 Stranded steel loop antenna for pups
250-μm diameter 7×7 316SS wire
insulated in clear MED-6607 silicone.
Table: Antenna Versions.

[02-MAR-18] The antenna connection is marked OUT on the schematic. The antenna transmits the sample values with 7-μs bursts of 902-928 MHz radio waves. The power transmitted during these bursts is roughly 300 μW (−5 dBm). The receiving antenna, which is usually a Loop Antenna (A3015C) connected to an Octal Data Receiver (A3027), must pick up at least 25 pW (−76 dBm) to overcome noise in the receiving antenna amplifiers, and at least four times (12 dB) more than the 902-928 MHz interference power picked up by the receive antenna. In our office, interference power ranges from 100 pW to 100 nW. In order to guarantee reception outside a faraday enclosure, we must receive 1 μW (−30 dBm). Inside an FE2F faraday enclosure, which offers at least ×1000 (30 dB) isolation from interference, we must receive 1 nW (−60 dBm).


Figure: Subcutaneous Transmitter A3028S2-DA-C45-D. Bandwidth 0.3-80 Hz, 256 SPS single-channel, 950 hrs operating life, 45-mm long 0.5-mm diameter helical leads terminated with pin and bare wire, 30-mm thin stranded loop antenna.

We compared (see here and here) the 50-mm and 30-mm antennas moving in air and water with a self-propelled sphere. We found that the 30-mm antenna performed just as well as the 50-mm in water, and almost as well in air. Given that an animal body is more closely approximated by water than air, we conclude that, when implanted, the 30-mm loop will perform almost as well as the 50-mm loop. When we immerse an A3028E with 50-mm antenna in water, the radio-frequency power it emits remains constant to within a factor of two as we half-immerse it in water, completely immerse it in water, or press it up agains the wall of a water-filled beaker. The center-frequency of its transmission increases by 1 MHz from air to total immersion in water. The dielectric constant of water is ninety times greater than that of air. Water affects the propagation of radio waves along the antenna, slowing them down, so that the effective length of the antenna is greater. The water also affects the oscillator within the transmitter circuit, changing its resonant frequency. When we immerse an A3028A with 30-mm antenna in water, the power it emits rises by a factor of ten (10 dB) and the center-frequency rises by 2 MHz. The figure below is the power spectrum we measure when we hang an A3028A by its leads in air, then surround it with a beaker of water.


Figure: Power Received from A3028A with 30-mm Antenna In Air and Water versus Frequency. When half-immersed or pressed up against the wall of a beaker of water, the spectrum lies somewhere between these two extremes.

We make our helical antenna out of the same MDC13867A helical spring insulated with silicone that we use for EEG leads. We measured transmitted power versus antenna length in water and in air for these helical antennas by cutting back the antenna in stages. The graph below shows three plots: one for air, one for water with the tip of the antenna insulated from the water, and another in water with the tip of the antenna in contact with the water.


Figure: Power Received from A3028A with Helical Antenna. Each 10 mm of MDC13867A helix contains 60 mm of wire.

With the tip un-insulated, we are using the entire 50-ml beaker as an antenna. Even with a 0-mm antenna we get enough power. But if we drop the entire transmitter into the water, we get no power transmitted. We cannot use the entire animal body as an antenna because the body encloses our implanted transmitter. We must insulate the tip of the antenna. We perform similar experiments with the A3028P equipped with a helical antenna, cutting back the antenna and insulating its tip as we go. We obtain the best performance at the edge of a beaker of water with a 5-mm antenna. This 5-mm antenna contains 30-mm of wire. But the performance of the 5-mm antenna in air is so poor as to be impractical. We must be able to tell if the transmitter is on or off before we put it on the shelf. We settle upon 13 mm as the best compromise between air, water, and implanted.

[05-JAN-19] The following plot shows reception from seven A3028P2 devices with 13-mm helical antennas implanted in mice living in an IVC rack with isolation chamber.


Figure: Reception from Seven A3028P2 Implanted in Mice in IVC Rack versus Time. For detail of eight days see here.

Average reception is 97%. Short drops to 80% or below are shared by all seven devices, suggesting that the curtains of the chamber were open for cage cleaning and animal feeding.

[29-MAR-16] We calibrate the A3028 center frequency to lie in the range 913-918 MHz at room temperature, which is around 22°C. The MAX2624 oscillator that provides the radio-frequency signal has a temperature coefficient of −0.2 MHz/°C. In an animal body at 37°C, the center frequency will drop to 910-915 MHz. Our Octal Data Receivers (A3027) and the older Data Receiver (A3018) are designed to provide reliable message reception in for center frequencies in the range 908-918 MHz. The spectrum of the entire signal spans a ±5 MHz range about the center.

Design

The following files define the A3028 design. Note that we distribute all these files under the GNU Public License.

S3028A_1.gif: 2×0.3-160 Hz, A3028AV1 and A3028AV2.
S3028B_1.gif: 2×0.3-160 Hz, A3028AV3.
S3028C_1.gif: 2×0.3-160 Hz, with stimulus protection
S3028D_1.gif: 2×0.3-160 Hz, A3028AV4.
S3028E_1.gif: 1×0.3-160 Hz, 1×0.3-80 Hz, A3028AV5.
S3028F_1.gif: 1×0.3-160 Hz, 1×0.3-80 Hz, no U1, A3028GV1.
S3028H_1.gif: 1×0.3-160 Hz, 1×0.3-80 Hz, with charging diodes, A3028HV1.
S3028J_1.gif: 3-200 Hz, ×31 Differential Amplifier for Y input.
S3028K_1.gif: 1×0.3-160 Hz, 1×0.3-80 Hz, with charging diodes, A3028KV1.
S3028P_1.gif: 1×0.3-40 Hz, with charging diodes, A3028PV1.
S3028Y_1.gif: BND to SCT Interface.
A3028HV1.ods: Bill of materials for A3028HV1.
A3028GV1.ods: Bill of materials for A3028GV1.
A302801G.zip: PCB for A3028GV1.
A3028PV1.ods: Bill of materials for A3028PV1.
A3028PV1_Top.gif: Component map of A3028PV1, top side.
A3028PV1_Bottom.gif: Component map of A3028PV1, bottom.
A302801QR1.zip: PCB for A3028PV1.
A302801QR1_Top.svg: View of top side of A302801QR1 PCB.
A302801QR1_Bottom.svg: View of bottom side of A302801QR1 PCB.
A3028PV2.ods: Bill of materials for A3028PV2.
A302801QR2.zip: PCB for A3028PV2.
A302801QR2_Top.svg: View of top side of A302801QR2 PCB.
A302801QR2_Bottom.svg: View of bottom side of A302801QR2 PCB.
A302801H.zip: PCB for A3028HV1.
A3028HV1_Top.gif: Component map of A3028HV1, top side.
A3028HV1_Bottom.gif: Component map of A3028HV1, bottom.
A302801H_Top.svg: View of top side of A302801H PCB.
A302801H_Bottom.svg: View of bottom side of A302801H PCB.
A302801H_Panel.pdf: Drawing of 2 × 5 panel of A302801H.
A3028GV1_Top.gif: Component map of A3028GV1, top side.
A3028GV1_Bottom.gif: Component map of A3028GV1, bottom.
A3028KV1.ods: Bill of materials for A3028KV1.
A302801KR1.zip: PCB for A3028KV1.
A302801KR1_Top.svg: View of top side of A302801KR1 PCB.
A302801KR1_Bottom.svg: View of bottom side of A302801KR1 PCB.
A3028KV2.ods: Bill of materials for A3028KV2.
A302801KR2.zip: PCB for A3028KV2.
A302801KR2_Top.svg: View of top side of A302801KR2 PCB.
A302801KR2_Bottom.svg: View of bottom side of A302801KR2 PCB.
Code: Logic chip firmware library.
BR1225: Diameter 12 mm battery electrical data sheet.
BR1225AHB: Diameter 12 mm battery mechanical drawing.
BR2330: Diameter 23 mm battery electrical data sheet.
BR2330AHD: Diameter 23 mm battery mechanical drawing.

The A3028 provides two amplifiers for recording two channels of EEG, or simultaneous EEG/EMG recording. The photographs below shows the A3028HV1, the latest in a series of similar circuits, each of which provides a slight improvement upon the last in terms of reliability, noise, or current consumption.


Figure: Top Side of A3028HV1 Assembly. Once the two necks are cut off, the circuit is 12.5 mm square. For bottom side here. Solder mask colors for assembly versions are as follows: H Black, G Red, R Blue, A Green.

The A3028HV1's circuit board is black. Previous versions of the A3028 circuit were red, blue, and green. The A3028PV1 is the most recent and only pup transmitter assembly, as shown below.


Figure: Top Side of A3028PV1 Configure for A3028TR Showing ML621-SDN Battery Before Encapsulation. The battery is 6.8 mm in diameter.

Thew A3028PV1's circuit board is red, and a notch to accommodate side-mounting of a coin cell. The A3028T, A3028P and A3028S transmitters made with this circuit board are long and thin rather than short and fat.

Modifications

We purchase boards fully-loaded with parts from our assembly house. We change various capacitors and resistors to create new versions of the assembly. The table below shows how we modify the A3028KV2 assembly to create the listed versions. We modify only those components that are necessary for the new version to function correctly.

VersionC7C8C9, C10, C11C12C13C14, C15, C16
A3028HV1samesamesamesamesamesame
A3028Asamesamesamesamesame1.0 nF
A3028Bsamesamesamesamesamesame
A3028Csamesamesamesamesamesame
A3028Dsamesamesamesamesame1.0 nF
A3028Esamesamesamesamesamesame
A3028Fsamesame510 pFsamesame510 pF
A3028Hsamesame2.0 nFsamesamesame
A3028Ksamesamesamesamesame3.9 nF
A3028Lsamesame510 pFsamesame510 pF
A3028Msamesame240 pFsamesame240 pF
A3028Q3samesamesamesamesame1.0 nF
A3028Q4samesame510 pFsamesame510 pF
A3028U1.0 kΩ620 kΩsame1.0 kΩ620 kΩ1.0 nF
A3028Wsamesame3.9 nFsamesame3.9 nF
A3028WZ1.0 kΩ620 kΩ3.9 nF1.0 kΩ620 kΩ3.9 nF
Table: Modifications to the A3028KV2 Assembly for Listed Versions of A3028. Entries marked "same" mean no modification is required. For the locations of components see A3028KV2_Top and A3028KV2_Bottom.

The following table gives modifications to the A3028PV2 assembly to create the listed devices.

VersionC7C8C9, C10, C11
A3028PV2samesamesame
A3028P1samesame3.9 nF
A3028P2samesamesame
A3028P3samesame1.0 nF
A3028T1samesame3.9 nF
A3028T2samesamesame
A3028T3samesame1.0 nF
A3028S1samesame3.9 nF
A3028S2samesamesame
A3028S3samesame1.0 nF
A3028S1Z1.0 kΩ620 kΩ3.9 nF
A3028S2Z1.0 kΩ620 kΩsame
A3028S3Z1.0 kΩ620 kΩ1.0 nF
Table: Modifications to the A3028PV2 Assembly for Listed Versions of the A3028. Entries marked "same" mean no modification is required. For the locations of components see A3028PV2_Top and A3028PV2_Bottom.

The most complex modification is that required by the A3028J and A3028V versions. These are combined EEG/EMG transmitters with four input leads. We start with the two-channel A3028KV2 circuit. We leave the X amplifier intact, but modify the Y amplifier on the top side. We remove C13, C14, and C16. We replace C12, R12, R13, and C15 with 2.0 MΩ. We replace R15 with 100 nF. We replace R16 and R18 with 50 kΩ. We load 510 pF for C18. We solder the green Y− lead to the pad of the C13 footprint that is nearest to C12. The result is the S3028J differential amplifier, in which output Y is equal to 31 × (Y+Y) with bandwidth 3-200 Hz.

Analog Inputs

[05-FEB-20] Most dual-channel versions of the A3028 have three leads. On the circuit diagram, these are named X, Y, and C. They are red, yellow, and blue respectively. Single-channel versions omit either the red or yellow lead. The A3028P provides one input channel only. Its inputs are X and C, and the leads are red and blue. The red and blue leads are connected to the battery with diodes. When a transmitter is equipped with a rechargeable battery, the diodes permit us to charge the battery through the red and blue leads.

NameFunctionComment
XX positive inputRed, connected to VC by 100 nF in series with 10 MΩ, connected to VBAT by diode.
YY positive inputYellow, connected to VC by 100 nF in series with 10 MΩ.
Cshared negative inputBlue, connected directly to VC, connected to 0V by diode.
OUTradio-frequency antennaClear, flexible steel antenna
Table: Connections to Devices with 0.3 Hz High-Pass Filter. When we eliminate the high-pass filter, the 100 nF capacitor is replaced by 0 Ω.

The A3028J is an unusual dual-channel transmitter: it has four input leads. They are red, yellow, blue, and green. The two signals it transmits are Red − Blue and Yellow − Green. We discuss this unusual version later in this section

NameFunctionComment
X+X positive inputRed, connected to VC by 100 nF in series with 10 MΩ.
X−X negative inputBlue, Connected directly to VC
Y+Y positive inputYellow, connected to VC by 4 MΩ.
Y−Y negative inputGreen, connected to VC by 4 MΩ.
OUTradio-frequency antennaClear flexible steel antenna
Table: Connections to the A302J.

The A3028's on-board amplifiers and filters take the X and Y inputs and produce the XA and YA signals. The gain of the A3028A amplifiers is ×100, while Its high-pass filter provides a cut-off frequency of 0.3 Hz, and its low-pass filter provides a cut-off frequency of 160-Hz. The gain of the A3028U amplifiers, on the other hand, is ×10, with no high-pass filter, and a 160-Hz low-pass filter. In all versions of the A3028, XA and YA are applied to a sixteen-bit analog-to-digital converter (16-Bit ADC). In the A3028A and A3028U, the ADC samples XA and YA alternately at 512 SPS per channel, for a combined sample rate of 1024 SPS. In the A3028B, sampling from YA is disabled, and XA is sampled at 512 SPS. In the A3028C, sampling from XA is disabled, while YA is produced by a 0.3-40 Hz filter sampled at 128 SPS.


Figure: Epileptic EEG in Adult Mouse. Recorded with an A3028P providing 0.3-40 Hz and 128 SPS. Seizure provoked by picrotoxin.

The 16-bit ADC produces numerical values 0-65535, and these values are what we see plotted in the Receiver Instrument and Neuroarchiver Tool. A value of 0 for means XA = 0V, and 65535 means XA = VBAT. A fresh lithium battery, such as the BR1225 in the A3028A, provides VBAT = 2.8 V during the first day of operation. With amplifier gain ×100, each sixteen-bit count represents 435 nV at the amplifier input. In the last few hours of a transmitter's life, VBAT drops to 2.2 V, at which point one ADC count is 350 nV. For most of the life of the transmitter, however, the battery voltage will be just below 2.7 V. When working with the A3028A, we assume a conversion factor of 400 nV/cnt. When we set the range of our signal plot to 5000 counts, this will be close to 2.0 mV. A fresh lithium-polymer battery, such as the PP031012 in the A3028B-R, provides VBAT = 4.2 V when fully charged, and 3.0 V when almost exhausted. But for most of the battery's life, its voltage is close to 3.7 V. When working with the A3028B-R signal, we assume a conversion factor of 560 nV/cnt. Consult the version table for the conversion factors other versions.

In theory, the dynamic range of the A3028A is 2.7 V ÷ 100 = 27 mV. But in practice, the amplifier is not able to drive XA all the way up to VBAT nor all the way down to 0 V. The actual dynamic range is closer to 24 mV. We specify a dynamic range that is 90% of the range we get when we multiply the sixteen-bit count conversion factor by 65536. Thus for the A3028U, with conversion factor 4.1 μV/cnt, the dynamic range is 90% of 270 mV = 240 mV.

Most versions of the A3028 provide a DC-blocking capacitor at each input. These are C7 and C12 in the schematic. Together with input resistors R5 and R12, they each form a high-pass filters with time constant 1.0 s. There is another high-pass filter in each amplifier, consisting of R6/C8 for X and R13/C13 for Y. In most versions of the A3028, these provide a time constant 0.5 s. The figure below shows the theoretical step response of the A3028A inputs, and compares it to that of other devices we have manufactured.


Figure: Step Response of A3028 and Other Circuits. The A3028 plot applies to any A3028 input with 0.3-Hz high-pass filter.

The common-mode reference voltage in the amplifier is VC, and VC is derived from the battery voltage by a 1.8-V regulator. Thus VC is 1.8 V, and the average value of both XA and YA are also 1.8 V. Thus the average digital value of X and Y correspond to 1.8 V. This allows us to calculate the battery voltage from the average value of either channel. We have VBAT = 65535 / Xave × 1.8 V, where Xave is the average value of X. This relation applies to all versions of the A3028, and both inputs, regardless of the gain and frequency response of its amplifiers, and regardless of what battery we have installed.

The figure below shows the X signal from an un-encapsulated A3028E as we reduce VBAT from 4.2 V to 1.2 V over 300 s. Our formula for VBAT from the average value of X works down to VBAT = 1.9 V. After that, the X decreases, suggesting an increase in VBAT, when in fact the actual VBAT drops to 1.8 V and the transmitter turns off.


Figure: X versus Time. We plot the sixteen bit value of X, with each vertical division being 6553.5 counts. Time is 15 s per division. Power supply voltage is 4.2 V on the left and 1.2 V on the right, so dropping 150 mV per horizontal division.

The analog input impedance of X and Y is 10 MΩ in the pass-band of the amplifier. Most versions of the A3028 provide a high-pass filter at 0.3 Hz to remove slowly-varying electrochemical potentials so that the gain of the amplifier can be ×100 and the dynamic range is the battery voltage divided by 100. For non-rechargeable devices, the battery voltage is 2.7 V for most of the operating life of the device, so the dynamic range is 27 mA. The A3028U amplifier, however operates all the way down to 0.0 Hz and accommodates electrochemical potentials by reducing the amplifier gain to ×10 so that the input dynamic range is 270 mV. Transmitters that are sensitive down to 0.0 Hz are able to record cortical spreading depressions (CSDs), as illustrated here.

We set the cut-off frequency of the A3028 three-pole, 3-dB ripple, Chebyshev, low-pass filters by our choice of filter capacitance, CF. Most often, we set the cut-off frequency at one third the sample rate. There are three capacitors in each filter that must have the same value for the filter to function properly. They are C9, C10, and C11 for the X filter and C14, C15, and C16 for the Y filter. With CF = 1.0 nF, the cut-off frequency is 160 Hz. The following graph shows how the gain of the two inputs of an A3028A varies with frequency.


Figure: Frequency Response of the A3028A.

The frequency response in the pass-band shows a slight decrease in gain from 10 Hz to 80 Hz, followed by a bump up at 130 Hz. These two features are the 3-dB ripple of the three-pole Chebyshev filter. By tolerating this non-uniformity of gain in the pass-band, we obtain a far sharper cut-off at the top of the pass-band. Amplitude drops by a factor of ten, or 20 dB from 160 Hz to 256 Hz. Frequencies above 256 Hz when sampled at 512 SPS will produce artificial sin-waves of lower frequency, in a process called aliasing. The purpose of our low-pass filter is to prevent aliasing. Even with the sharp cut-off of our Chebyshev filter, we might still see aliasing of high frequency components in EEG.


Figure: Aliasing of 500-Hz 10-mV Input for 512 SPS and 160-Hz Low-Pass Filter. We see 12 Hz of amplitude 200 μVpp.

The plot above shows how 10-mVpp of 500 Hz applied to the X input of an A3028A would appear after low-pass filtering and sampling. The signal has an apparent frequency of 12 Hz, which is 512 − 500. The EEG signal power decreases with frequency, so we can place our cut-off frequency at one third the sample rate and be confident we will see no significant distortion due to aliasing. The same is not true of EMG, in which the signal power density reaches a maximum somewhere between 100-300 Hz. When we record EMG at 512 SPS, we recommend placing the cut-off frequency at one sixth the sample rate, so that all EMG components above half the sample rate will be eliminated by the low-pass filter. The A3028J provides a 0.3-160 Hz EEG input and a separate differential 3-200 Hz EMG input, both sampled at 512 SPS.


Figure: Frequency Response of Two A3028Js. We apply a 16-mVpp sinusoidal sweep to the X and Y inputs and record the rms amplitude of the sixteen-bit transmitted signals. The X inputs, intended for EEG, have the higher gain and 0.3-160 Hz bandwidth. The Y inputs, intended for EMG, have the lower gain and the approximate 3-200 Hz bandwidth.

We expect some variation between amplifiers, because the three-pole filter is sensitive to the exact component values. The plot below shows the frequency response of a batch of fourteen A3028E transmitters stimulated with a 30-mV sinusoid through a 20 MΩ resistor.


Figure: Frequency Response of Fourteen A3028Es. For similar plot of response of four dual-channel A3028Ds, see here.

At 10 Hz, the fourteen channels agree to within ±0.5 dB. At 130 Hz, where we have the bump in the Chebyshev response, they agree to within ±1 dB. These variations are well within the range we expect with 5% capacitors and 1% resistors.

An A3028 with two inputs, such as the A3028A or A3028D, provides two separated amplifiers for the X and Y signals. Both these signals use the same reference electrode C, which we call the common input. When we apply a signal to X, we would like Y to be unaffected. That is to say: when Y is connected to C, we would like the recorded value of Y to be zero regardless of the voltage we apply to X. We define the crosstalk from X to Y as the amplitude of Y divided by the amplitude of X when Y is connected to C. The crosstalk from Y to X we define in a similar manner. The crosstalk in the A3028 circuit is less than 2% and varies with frequency.


Figure: Illustratio of Crosstalk from X (Green) to Y (Blue). We apply a 10-mV sinusoid of increasing frequency to X, and we leave Y connected to C.

The crosstalk reaches a maximum of 2% at 140 Hz, just below the cut-off frequency of the amplifiers. For 1-100 Hz, the crosstalk is 1%.

Aside: We have defined crosstalk in terms of a ratio of amplitudes, but it is more traditional to define it as a ratio of signal powers, or the ratio of the square of amplitudes, and then take the log to base ten and multiply by twenty to obtain the crosstalk in decibels. In these units, the A3028D crosstalk is −68 dB.

When we apply a pure sinusoidal waveform to the A3028, the sinusoid is amplified, filtered, digitized, transmitted, received, and recorded by the data receiver. The purpose of the A3028 is to provide high-fidelity EEG recordings, so we hope to see only a pure sinusoidal waveform emerge in the frequency spectrum of the recorded signal. We apply a 14-mVpp sinusoid of increasing frequency to the input of an A3028E transmitter providing 512 SPS and 0.3-160 Hz bandwidth. We measure the power of the recorded signal after subtracting its fundamental harmonic. This non-fundamental power is a combination of imperfection in the applied sinusoid, noise in the amplifier, and distortion in amplification and transmission of the signal. We divide the non-fundamental power by the total signal power to obtain a measure of the distortion of the sinusoid by the recording process.


Figure: Distortion in A3028 Recordings. We plot the ratio of non-fundamental power to total power for a sinusoidal input applied to a device with scattered sampling and another with uniform sampling. Most devices use scattered sampling to extend battery life.

For the scattered-sampling devices, distortion remains less than 1% in the 0.3-160 Hz pass-band of the A3028 amplifier. At 10 Hz and below, distortion is less than 0.01%. The distortion power for scattered-sampling devices has a white noise spectrum: it is not distortion of the original sinusoid, but noise added to the sinusoid by the power of the sinusoid. This noise is due to random variation in the sampling instants, which is a feature of the process by which A3028 transmitters avoid systematic collisions with other transmitters. We call it scattering noise. If we sample uniformly, we obtain the second plot shown above, in which distortion is less than 0.003%. With uniform sampling, the non-fundamental power is dominated by periodic signals that may be the result of distortion in the amplifier, or they may be imperfections in the signal produced by our function generator. The figures below show the spectrum of the non-fundamental power with uniform sampling.


Figure: Spectrum of Non-Fundamental Signal in A3028E 0.3-160 Hz, 512 SPS, 0.41 μV/cnt. Left: 5 Hz 7-mV fundamental, 700 nV/div, 5 Hz/div. Right: 100 Hz 7-mV fundamental, 700 nV/div, 25 Hz/div. Calculated from 8-s intervals with window function turned off. The peak that goes off the screen is the fundamental. Note the harmonics and offset harmonics.

Uniform sampling increases current by roughly 5%. In devices ×100 amplifiers (0.41 μV/cnt), scattered sampling does not introduce significant distortion of biopotential signals, and there is no practical reason to sample uniformly. But in devices with ×10 amplifiers, the distortion introduced by scattered sampling is more significant. All our zero-frequency devices, those with bandwidth extending down to 0.0 Hz, have ×10 gain (4.1 μV/cnt) so as to accommodated constant chemical potentials introduced by the metal interfaces in our signal path. For these devices, we enable uniform sampling.


Figure: Frequency Response of A3028U, X and Y, For 60-mVpp Sinusoid.

The A3028U provides a 200-mV dynamic range and 0.0-160 Hz bandwidth on both inputs. Its EEG amplifier gain is only ×10, compared to our usual ×100. The A3028S2Z provides a single 0.0-160Hz input with ×10 gain. Despite their low gain, the sixteen-bit resolution provides high-fidelity recording of seizures. The sensitivity to 0.0 Hz, combined with an X-Electrode bound by solderless crimp contacts, provides high-fidelity recordings of cortical spreading depresssions, provided we sample the signal uniformly.


Figure: One-Second Interval of Kainic Acid Seizure Recorded with A3028S2Z and X-Electrode. From ION/UCL.

The least significant bit of the 16-bit digitized samples is 4.1 μV for battery voltage 2.7 V. We enable uniform sampling for all zero-frequency devices and total harmonic distortion remains less than 25 ppm (0.0025%) from 0.03 Hz to 160 Hz. In the case of the A30238WZ, the uniform sampling increases current consumptin from 48.5 μA to 50.4 μA.


Figure: Distortion in A3028U Recordings, Measured for Both Inputs. Operating band 0.0-160 Hz.

The A3028V provides two channels: one running at 496 SPS and 0.3-160 Hz, one at 16 SPS and 30-640 Hz. The first is for EEG recording, the second for EMG amplitude measurement. The Neuroarchiver will reconstruct the 496 SPS signal into a 512 SPS signal by filling in the missing samples using the previous sample values, and the result will be an EEG signal almost identical to the one we would obtain with 512 SPS. The Neuroarchiver will leave the 16 SPS signal as it is, provided you enter "16 512" for the default frequency in the Neuroarchiver configuration. The figure below shows the gain versus frequency of the EEG and EMG inputs. Our measurement of the EMG gain between 200-500 Hz is erratic because of aliasing noise produced by under-sampling a non-random periodic sinusoid. With a random EMG input, we expect no such error in our measurement of amplitude.


Figure: Frequency Response of Three Early-Version A3028Vs, EEG and EMG Channels. These prototyhpes were not equipped with a true differential amplifier on the EMG channel, and had EMG bandwidth 30-640 Hz. Newer versions are fully-differential on the EMG input, with two dedicated leads, and 3-200 Hz bandwidth.

The figure below shows how the A3028E's 160-Hz low-pass filter responds to pulses applied to the X input.


Figure: Response of A3028E to 1-ms Pulses of 10 mV Delivered Through 50 Ω. The pulse frequency is 8 Hz. The vertical range is around 12 mV. For 5-ms pulses of 30 mV and 8 Hz delivered through 20 MΩ and the same vertical range, see here.

The low-pass filter introduces the ringing after the pulse, which consists of three maxima and minima in damped harmonic oscillation after the pulse has ended. The figure below shows the response of the A3028E to perforant pathway discharge spikes of amplitude roughly 10 mV.


Figure: Perforant Pathway Discharges, as Recorded with Wire Termination. The vertical range is around 12 mV.

The ringing we see after the discharge spikes is not generated by the brain itself, but rather by our low-pass filter. The slow, positive pulse following the negative spike is, however, produced by the brain, and the ringing is overlaid on top of this pulse.

Synchronization

[22-AUG-21] When we want to mark in our SCT recordings the time at which some event took place, such as the start of a video recording, the moment that a light was flashed, or when an noise commenced, we can use an auxilliary SCT to record a synchronizing signal along with the signals received from implanted SCTs. Most often, the synchronizing signal will take the form of a TTL voltage, where the TTL low and high voltages used to mean ON and OFF. A TTL low-level is any voltage in the range 0.0-0.2 V, while a TTL high-level is any voltage in the range 2.4-5.0 V. The BNC to SCT Interface (A3028BSI) allows us to connect an SCT to a synchronizing signal with a BNC cable.


Figure: The BNC to SCT Interface (A3028BSI-B). This version is equipped with all eight spade clips, and is designed to measure the frequency response of four SCTs simultaneously during quality control. Four standoffs are common to all versions of the BSI, and allow us to place the interface on a metallic surface. The SCTs will rest on the yellow regions of the board, where their antennas can operate efficiently.

The A3028BSI provides up to eight spade clips to connect four single-channel or two dual-channel SCTs to the same input signal. When we want to incorporate a synchronizing signal into our recording, we will use only one pair of clips. The four pairs of clips permit us to measure the frequency response of four SCTs simultaneously, which is useful to us during quality control and throughout accelerated ageing. The A3028BSI-A is intended for logic level synchronization. It provides attenuition of ×1000 so a 3.3-V logic transition will appear as a 3.3-mV step up or down at the SCT input. The BSI-A provides a time constant of 100 μs to remove unwanted high-frequency noise, while at the same time preserving 1-ms synchronization. Note that most SCTs have a 0.3-Hz high-pass filter on their input, so a step up comes out like this in the recorded signal, and a step down is the opposite. The average value of a high-pass filtered signal is zero.

Version Input
Impedance
Output
Impedance
Attenuation
(Input/Output)
R1, R2, R3, R4-R7, C1 Number
of Clips
Application
A3028BSI-A 10 kΩ 10 kΩ 1000 10K, 100K, 100R, 10K, 1u0 2 Logic Level Synchronization
A3028BSI-B 51 Ω 10 MΩ 100 51R, 100K, 1K0, 10M, 100p 8 Quad SCT Frequency Sweep Response
Table: Versions of the SCT to BNC Interface (A3028BSI). Schematic with component names S3028Y. The input impedance is the impedance seen by the incoming signal at the BNC socket. The output impedance is the impedance in series with the attenuated input signal. The attenuition divides the input signal so that it may be viewed in full by the SCT input.

To set up logic level synchronization, we connect the synchronizing signal to the BNC socket on the interface. We are going to record the signal transmitted by an SCT sitting on the interface circuit board, so must receive the SCT signal with an antenna. One way to record the SCT signal is to place an antenna outside our Faraday enclosures and place the interface next to the antenna, so as to obtain reception of the SCT signal even in the presence of local interference. If we operate outside an enclosure, we must make sure that the antenna we use for the synchronizing SCT does not share a receiver input with an antenna inside a Faraday enclosure. An external antenna sharing a receiver input with an internal antenna will introduce interference into the Faraday enclosure, compromising reception within the enclosure. So we will need a dedicated receiver input for our synchronizing SCT signal. Another way to record the SCT signal is to place the interface in one of our Faraday enclosures, in which case we must bring the synchronizing signal into the enclosure with a coaxial feethrough. The one-to-one BNC feedthroughs we provide in the back of our bench-top enclosures will serve this purpose, but we may have to remove an antenna from the enclosure in order to free such a feedthrough. If we don't want to reduce the number of antennas in the bench-top enclosure, we can add another feedthrough to the back wall for the synchronizing signal. When we are recording in a Faraday canopy, our Eight-to-Four Coaxial Feedthroughs (A3039C) may not be used to carry a synchronizing logic level into the enclosure. Instead, we can either tape a single BNC feedthrough connector to the floor of the Faraday canopy, or we can use one port on an Eight-to-Eight Coaxial Feedthrough (A3039A).

To act as the synchronizing transmitter, we select an SCT that we do not intend to implant in an animal. We turn it on and connect the ends of its two leads to the two spade clips. If the ends of the leads have no termination, we remove insulation from the helical wire of the lead, and fasten the exposed wire into the spade clip. As the logic level changes, we will see steps up and down in the SCT recording, or order 4 mV.

Input Noise

[22-MAR-21] By input noise we mean the signal recorded by the transmitter when it is sitting stationary in water. The A3028 has three significant sources of input noise: soldered terminations, its magnetic sensor, and its amplifier input. If we use bare wire terminations, we eliminate the first source. If we choose the correct battery, we eliminate the second source. The figure below shows the noise spectrum of twelve A3028B-AA-B45-B transmitters sitting in 37°C water. Total noise in the band 0.3-160 Hz for these devices is 4-6 μV rms.


Figure: Input Noise of Twelve A3028B-AA-B45-B Transmitters in 37°C Water. Devices made with A3028HV1 circuit, introduced FEB-21, and CR1225AH battery, introduced MAR-21. Horizontal scale 25 Hz/div, vertical scale 200 nV/div. Spectrum obtained with 8-s interval.

If we make the same device with an inferior choice of battery, we see noise generated by the circuit's magnetic switch. The figure below shows the noise spectrum of ten A3028B-AA-B45-B transmitters manufactured in FEB-18 with our original mouse-transmitter battery.


Figure: Input Noise of Ten A3028B-AA Transmitters in 37°C Water. Devices made with the A3028GV1 circuit, introduced FEB-17, and BR1225 battery. Horizontal scale 20 Hz/div, vertical scale 400 nV/div. Spectrum obtained from an 8-second interval.

Currently, we make our P and S version transmitters with the A3028PV1 circuit and all others with the A3028HV1. Both are equipped with a magnetic sensor that turns itself on and off at 5 Hz. Switching noise, if present, will appear as a fundamental at 5 Hz accompanied by strong harmonics all the way up to 100 Hz. Provided we use the correct battery with either circuit


Figure: Input Noise of Twelve A3028P1-AA-C33-C Transmitters in 37°C Water. Devices made with A3028PV1 circuit, introduced FEB-18, and CR1025 battery. Horizontal scale 5 Hz/div, vertical scale is 200 nV/div. Spectrum obtained from a 32-second interval. Maximum total noise in 0.5-40 Hz is 3.5 μV rms.

Maximum total noise in 0.5-40 Hz for these A3028P1 devices is 3.5 μV rms. We see no sign of switching noise either, although we certainly do see switching noise when we equip this same circuit with the BR1225 battery.

[21-AUG-18] The total noise on each input in the plot above is 5±1 μV rms. The peaks near 20 Hz, along with their harmonics at 40 Hz and 60 Hz, are generated by the magnetic sensor on the A3028GV1 circuit board. This sensor turns itself on and off at roughly 20 Hz. We call these peaks switching noise. The switching noise amplitude is the amplitude of the largest harmonic of the switching frequency. In the above signal, the largest harmonic is the fundamental harmonic, and its amplitude is 3.2 μV. When we present measurements of input noise, we specify the version of the A3028 electronic circuit, the battery loaded on the circuit, and the type of termination at the end of the leads. The electronic circuit has evolved over the years, and the noise it generates has slowly reduced. The lower the source resistance of the battery, the lower the noise. Terminations with solder joints generate low-frequency noise.


Figure: Input Noise for Three A3028C-CC Transmitters in 37°C Water. Horizontal scale is 10 Hz/div. Vertical scale is 400 nV/div. Spectrum obtained from a 32-second interval. These devices were made with the A3028GV1 circuit and BR1225 battery, and M0.5 screw terminations. Total noise in 2-80 Hz is ≤6.5 μV.

Switching noise is proportional to the battery's source resistance. The magnetic sensor switches itself on and off to reduce its current consumption. It draws current when it is on, and this current causes a potential drop across the source resistance of the battery. The battery voltage on the board contains bursts of noise at the magnetic sensor switching frequency, and this noise makes its way into the amplifiers. Of all the batteries we use with our transmitters, the BR1225 has the highest source resistance and produces the highest switching noise amplitude. The figure below compares the source resistance of various batteries. We find that lithium primary cells have higher resistance when they are fresh, so we discharge them by 0.1% before measuring their source resistance.


Figure: Battery Resistance vs. Diameter for Various Battery Types. For resistance versus thickness see here. Li_BR: BR-series lithium primary cells from Panasonic. Li_CR: CR-series lithium primary cells from Panasonic. MnLi: ML-series rechargeable batteries. LiPo: lithium polymer rechargeable batteries. When the battery is rectangular, we use the geometric mean of its length and width for its diameter. Resistance measured after 0.1% discharge.

Earlier versions of the A3028B-AA circuit suffered from switching noise up to 12 μV, which was visible in the Fourier transform of 32-second intervals of baseline mouse EEG. The A3028GV1 circuit reduced switching noise by increasing the decoupling capacitance in parallel with the battery by a factor of four. Now we reject any


Figure: Switching Noise of A3028B-AA Transmitters made with A3028RV2 Circuit and BR1225 Battery. The A3028RV2 was in use until JAN-18. We plot the spectrum of an 8-s interval recorded with ten transmitters in a beaker of water at 37°C. The switching noise is the peaks around 20 Hz and their harmonics. Vertical scale is 800 nV/div. and horizontal is 25.6 Hz/div. The largest peak is 6 μV, which is the maximum value we accepted during quality control.

[28-MAY-19] When we equip the A3028 with gold-plated pins, we solder these pins to the ends of the stainless steel electrical leads. When we immerse these soler joints in water, they generate electrical potentials that we call chemical artifact, as we discuss in Chemical Artifact. Most of the power in chemical artifact is below 0.3 Hz, but the amplitude of the artifact above 0.3 Hz can be hundreds of microvolts. Our quality control procedure for each batch of transmitters is to soak them in water at room temperature for three days, remove them, measure their frequency response, and put them into water at 37°C to measure input noise. When we put transmitters with soldered terminations into water, we first scrub their solder joints with a brush, which greatly reduces the chemical artifact. When they are first immersed, we see input noise of 10-50 μV rms in the range 2.0-160 Hz. After a few minutes, the noise drops to below 12 μV rms. When we immerse a batch of transmitters with bare wire terminations, the input noise is immediately less than 12 μV rms, and after a few minutes is less than 8 μV rms.

[11-MAR-14] The A3028 leads pick up radio-frequency power from the antenna. If the blue lead is left open-circuit and the red and yellow leads are connected to some solid body, the blue lead acts like an antenna. The radio-frequency power it picks up is demodulated by parasitic diodes in the EEG amplifier. The result is noise visible on the EEG, as shown below. Its amplitude is roughly 100 μV rms. If you see such noise in an implanted transmitter, your blue lead has a broken conductor.


Figure: Broken C-Lead Noise. The broken lead is not connected to the animal body, and acts as a radio-frequency antenna.

[07-JUN-19] Here's how we can check if a wire is broken inside the silicone insulation of one of the leads. We put the ends of the leads in water, on top of an antenna, turn the transmitter on and examine its signal in the Neuroarchiver Tool or Receiver Instrument. If the leads are intact, the amplitude of the noise on the signal should be less than 10 μV rms.


Figure: Measuring Input Noise with Lead Tips in Water. The transmitter is above a loop antenna that has had its mounting brackets removed, allowing the antenna disk to sit directly on our work bench. The noise we see at the inputs of this A3028A transmitter should be less than 25 cnt rms, which is 25 cnt × 0.4 μV/cnt = 10 μV rms.

[01-DEC-16] We take a two-channel A3028D made with the A3028RV1 circuit and BR2330 battery and put it in water at 40°C. We use 8-s recording intervals to measure the amplitude and frequency of the switching noise in each of the two input channels No3 and No4 as the water cooled to 19°C.


Figure: Switching Noise Amplitude and Frequency versus Temperature. The transmitter has two channels. No3 is on the bottom side of the circuit board along with the hall effect sensor that generates the noise. No4 is on the top side of the circuit board.

The frequency of the switching noise increases with temperature. We perform the same experiment with four transmitters at once, using this processor to calculate the rms amplitude of the fundamental harmonic of the switching noise. Each transmitter is made with the A3028RV1 circuit, but we have two different types of battery.


Figure: Switching Noise Amplitude versus Temperature. No3/4 is an A3028D with BR2330 battery, No5 is an A3028B with BR1225 using the Red input, No10 is an A3028C with BR1225 using the Yellow input, No13/14 is an A3028A using BR1225.

In No13/14 the noise is far greater in Red than in Yellow. But in No10 the noise is as large as any we have seen, and appears on the Yellow input. The noise in the transmitter equipped with the larger, lower-resistance BR2330 battery is an order of magnitude smaller. We touch the tip of a ×1 probe to the package of the Hall Effect Switch on a battery-powered, un-encapsulated transmitter. We see no sign of 20 Hz, certainly less than 200 μV.

[21-MAR-21] Loading the CR1225AH onto the A3028HV1 circuit results in no observable switching noise in the encapsulated transmitters. Total noise is never greater than 6 μV in 1-160 Hz with bare wire terminations.

Power Measurement

Suppose we want to know the root mean square amplitude of a biometric signal, or the mean square amplitude, but we do not need to know its exact shape. When we monitor electromyography (EMG), our chief interest may be the power of the signal, to determine if an animal is awake or asleep, rather than examining the fluctuations in the signal itself.

As we mention above, when we try to represent a continuous, time-varying signal as a sequence of discrete samples, we must make sure that the variation in the time-varying signal between the sample points approximates a straight line. When we plot the sampled signal, we are going to display it by drawing straight lines between the samples, so these straight lines are supposed to represent what the signal actually did between the samples. If the signal varies greatly between samples, we will not see this variation. Failing to see rapid variations between samples may not concern us, but rapid variations can, through sampling, look like much slower variations when we join the samples with lines. If we sample a 100-Hz signal at 48 SPS, for example, our straight-line reconstruction of the signal will be a 2-Hz sinusoid. This generation of slow variation from rapid variation is aliasing. To avoid aliasing, we must filter the signal we want to sample so that it contains no changes of direction between sample points. With a perfect low-pass filter, which has gain 1.0 up to frequency fc and 0.0 above fc, we can sample at 2fc and avoid aliasing. In practice, low-pass filters are not perfect, and avoiding aliasing is not quite sufficient to provide adequate representation. In the A3028, we sample at 3.2fc. Our low-pass filter gain drops by a factor of 10 from fc to 1.6fc. For example, in our 512 SPS transmitter for EEG, we filter at 160 Hz. The filter gain drops by a factor of ten from 160 Hz to 256 Hz.

But aliasing does not change the power of a signal, not unless we somehow miss all the powerful moments of a signal by a spectacularly unfortunate choice of sample instants. For an irregular signal like EMG, sampling at 16 SPS and taking the standard deviation of the samples, will give us a good measure of the EMG signal amplitude. Most EMG power is in the range 40-300 Hz, so we can high-pass filter at 30 Hz and low-pass filter at 320 Hz to separate the EMG from any artifacts that may be generated by our EMG pick-up electrodes, sample at 16 SPS and obtain EMG power at a cost of only 1.8 μA in current consumption. Thus the A3028V monitors EMG on Yellow input and EEG on Red input and has the same battery life as the single-channel A3028B EEG monitor.

Body Capacitance

For a discusion of body capacitance, see Body Capacitance in the A3019 manual.

Battery Life

[30-SEP-21] The A3028 run down its battery sitting on the shelf in its inactive state and sampling and transmitting signals in its active state. The inactive current of the A3028 consists of the current consumption its magnetic sensor (U2) and its logic switch (U3). The shelf life of the transmitter is the time it takes the maximum inactive current to drain the battery. The maximum inactive current for temperatures −20°C to +80°C is 2.5 μA. At room temperature, the inactive current consumption of A3028 devices is only 0.8 μA. The A3028P1 battery has capacity 30-mAhr = 1250 μAdy, so its shelf life is 1562 dy = 52 mo = 4.3 yr. Transmitters equipped with larger batteries have shelf lives of ten years or more, so we can leave them inactive for six months and lose less than five percent of their operating life.


Figure: Example Discharge Curves for Lithium Primary Batteries. We discharge five CR1025, 30-mAhr, 3-V cells with five A3028P3 transmitters, each consuming ≈75 μA. The batteries are connected with a wire soldered directly to the negative terminal and a solder joint directly on the positive terminal. Expected life is 400 hr, observed life is 391-410 hr. We measure VBAT by taking the average of one second's worth of the signal. The average value of the signal corresponds to VCOM, which is 1.8 V. From this we deduce VBAT, which corresponds to a sample value 65535.

Rechargeable versions of the A3028 may be recharged to full capacity whenever they are not implanted. We connect the red and blue leads to a recharging circuit and wait until the recharge is complete. We describe how we recharge transmitter batteries in our Battery Recharger (A3033) manual.


Figure: Discharge of Manganese-Lithium Rechargeable Batteries. Discharges of three A3028T1 transmitters sitting in a beaker of water, nominal current consumption 32 μA. Battery ML621, nominal capacity 5 mA-hr. Nominal battery life is 160 hr. Suffix "1" is initial discharge, suffixes "2" and "3" are the second and third discharges.

The active current of the A3028 is its current consumption while transmitting its signal. This current depends upon the number of samples the transmitter takes per second. The A3028B provides one channel with 512 SPS. The active current consists of the quiescent current of the logic chip, which is independent of sample rate, and the consumption of the sample and transmit process, which increases linearly with sample rate. The figure below shows active current versus the total sample rate. To obtain the total sample rate, we add the sample rates of both channels.


Figure: Active Current versus Total Sample Rate for Various A3028 Circuits. We program the circuits with scattered sampling.

We have SCT circuits assembled in batches of one or two hundred at a time. Current consumption varies from one batch to the next as we buy new reels of parts to populate the boards. When we calculate the minimum operating life of an SCT, we use the following equation to obtain the maximum active current of the circuit when powered by a 2.7-V lithium primary cell.

Ia = 22 μA + (R × 0.12 μA/SPS)

We divide the nominal battery capacity by the maximum active current to obtain our minimum operating life. The typical operating life is 10% higher. The active current increases with battery voltage. The active current of an A3028 powered by a 3.7-V LiPo battery is 10% greater than that of the A3028 powered by a 2.7-V Li primary cell.


Figure: Active Current versus Battery Voltage for A3028Q. Total sample rate 2048 SPS.

The capacity of lithium primary cells decreases with active current and increases with temperature, as shown in the following plot taken from the BR1225 data sheet.


Figure: BR1225 Battery Capacity versus Load Current and Temperature.

The above plot suggests that a BR1225 battery delivering 250 μA at 15°C will provide 44 mAhr, or 176 hrs operation. Under these conditions, however, we observe closer to 190 mAhr, which implies the full 48 mAhr capacity.


Figure: Current Consumption versus Temperature for A3028P1. Sample rate 128 SPS.

Some transmitters implement a uniform sampling interval to reduce total harmonic distortion. These transmitters consume more current. A typical A3028U provides uniform sampling at 1024 SPS and consumes 144 μA, while a typical A3028D provides scattered sampling at 1024 SPS and consumes 137 μA. To determine the maximum current consumption of a transmitter with uniform sampling, we use the following equation.

Ia = 22 μA + (R × 0.13 μA/SPS)

The capacity of rechargeable batteries decreases with repeated drain and recharge cycles. A lithium-polymer (LiPo) battery drained to 5% of capacity and recharged to 95% will endure a hundred charge cycles before its capacity has dropped to 90% of its initial value. A LiPo battery drained even once to 0% will immediately lose 50% of its initial capacity. We drained several manganese-lithium (ML) batteries to 0% and were unable to observe any loss of capacity afterwards.


Figure: Example Discharge Curves for Manganese-Lithium Batteries. We charge the batteries through a 400-Ω resistor using various voltages 2.9-3.3 V. The ML621 is 6.8 mm dia, 2.1 mm thick, 5.0 mA-hr. The ML920 is 9.5 mm dia, 2.0 mm thick, 11 mA-hr.

Charging a magnesium-lithium battery is simple: connect it to a fixed voltage through a series resistor. We charged ours with voltages 2.9-3.3 V with a series resistor of 400 Ω. We recorded battery voltage as the battery discharged through a transmitter. The batteries take 24 hours to charge to 90% capacity and 48 hours to charge to 100%. Charging to 2.9 V gives 90% the capacity of charging to 3.3 V. We see no significant degradation of capacity over ten discharge cycles. Because of accumulated damage to their leads and encapsulation, we do not expect a transmitter to survive more than a ten implantations.


Figure: Example Discharge Curves for Lithium-Polymer Batteries. Each chart is C/I where C is nominal battery capacity in mA-hr and I is current drain through a transmitter in μA. The fraction thus expressed is the expected operating life in thousands of hours. We charge with a 4.2-V source current-limited to 20 mA for 19-mAhr and 150 mA for 190 mA-hr batteries.

We discharge LiPo batteries through transmitters over several months. The plots above show how the battery voltage, as reported by the average value of the red input, declines with time. We have one 190-mHr battery with a connector, one new 190-mAhr battery with a connector, and one 190-mAhr battery encapsulated with a transmitter circuit. The encapsulated battery had been poached at 60°C for several months. The 190-mAhr battery with connector delivered 150 mAhr when discharged at 150 μA and 156 mAhr when discharged at 520 μA. The encapsulated 190-mAhr battery delivered 190 mAhr when discharged at 90 μA. The 19-mAhr battery delivered between 19-21 mAhr when drained at 40-280 μA. We conclude that poaching an encapsulated battery does not reduce its capacity below nominal. We note that some batteries can deliver only 80% of their nominal capacity, while others can deliver 110%.

Batteries are vulnerable to heat. Lithium polymer batteries operate well at 60°C, but their capacity is halved at 80°C. Lithium primary cells operate well at 80°C, but they suffer damage if we heat them with a soldering iron. We never solder to batteries. We either use batteries with pre-loaded tabs, or we spot-weld tabs to the batteries ourselves.

Battery Recharging

[29-JUN-18] When an A3028 is equipped with a rechargeable battery, we recharge the battery after explantation. We deliver the charging current through the red and blue leads. Re-implanting requires that we either clean cement off the ends of our leads, or cut back the leads to release them from the cement of the head fixture. In either case we are going to have to expose fresh wire for the next implant. We use this fresh wire to make contact with the charging circuit. We provide recharging instructions in our Battery Recharger (A3033) manual. We provide battery charger circuits for some of our rechargeable transmitters, which will make it easier for you to recharge your own devices. You may also send your exhausted transmitters back to us and we will recharge them for you, as well astest the leads for damage to the silicone insulation, and check the gain and frequency response. We can repair or replaced damaged leads. Our recharging and repair services are included in our price list.


Figure: Rechargeable Subcutaneous Transmitter For Rats. Recharged to full capacity through EEG leads between implants. The body is 32 mm long, 21 mm wide and at 9 mm deep at its thickest point. Displacement volume is 4.0 ml. The leads are 150-mm long and tipped with bare steel wire terminations that can be stretched out, cut, and held in place with a skull screw.

[07-DEC-18] We run down the batteries of three A3028T1 rechargeable transmitters four times. These devices are equipped with the ML621 Manganese-Lithium Batteries, with nominal capacity 5 mA-hr. The current consumption of the A3028T1 128 SPS transmitter is 33±2 μA, so we expect the lifetime of the battery to be at least 140 hours.


Figure: Four A3028T1R Charge-Discharge Cycles. Charging with a constant voltage in series with 1-kΩ resistor connected to EEG leads. First and third charges with 4.2 V. Second with 4.1 V. Fourth with 4.3 V.

The first discharge of each battery provides the longest life. Charging with 4.3 V gives greater capacity than charging with 4.1 V. But all discharges give us more than our expected 140-hr life.

Encapsulation

[30-JAN-18] We encapsulate the A3028 in black epoxy and silicone. The outer layer of silicone is MED-6607, an unrestricted medical-grade silicone. The sensor leads are stainless steel springs coated with dyed silicone. The antenna is either a stranded stainless-steel wire coated with silicone, or a short sensor lead in the case of the smallest transmitter versions. We remove one in ten transmitters from our production line for accelerated aging. These transmitters run until they exhaust their batteries while fully immersed in water at 60°C or 80°C.

Corrosion causes the failures of electronic circuits in warm, humid environments. Both silicone and epoxy are permeable to water vapor. An implanted transmitter encapsulation becomes saturated with water vapor. Occasional drops in temperature cause condensation in any cavities that may exist within the transmitter, such as within a cracked ceramic capacitor or beneath a surface-mount component. Condensed water, the warmth of the animal body, flux residue, and electrical voltages combine to promote corrosion that leads initially to loss of performance in the EEG amplifier and ultimately to premature draining of the battery. At 60°C, corrosion occurs over ten times faster than at 37°C in an animal body.

We guarantee our transmitters against failure from corrosion while implanted. Our encapsulation procedure seeks to remove flux residue by repeated washing and scrubbing in hot water, and to eliminate cavities in the epoxy by use of vacuum and atmospheric pressure. The minimum operating life of an encapsulated A3028RV3 circuit in 100% humidity at 60°C is 100 days, with the first artifacts of corrosion appearing in the EEG signal after 50 days. Implanted in an animal, we expect no sign of corrosion in the EEG for 500 days, and no failure through corrosion for 1000 days. None of the popular versions of the A3028A run for 500 days. The A3028GV1 circuit we have just introduced for use in mouse transmitters. We poached ten devices at 80°C. Two stopped transmitting after 6 days, which is equivalent to 360 days implanted, and the remaining 8 lasted 13-14 days, which is their full operating life, and equivalent to 780 days implanted. Looking at our notes we find that we did not clean these circuits a second time before encapsulating, so we suspect the two early failures were caused by flux residue and moisture. But in any case, none of our mouse transmitters run for more than 100 days implanted, so we have no fear that these will fail prematurely by corrosion.

Development

We have moved our development to a separate page, Subcutaneous Transmitter Development.