Subcutaneous Transmitter (A3048)

Contents

Description

[14-JUN-24] The Subcutaneous Transmitter (A3048) is an implantable telemetry sensor for mice that provides amplification and filtering of one biopotential input. The A3048R is our smallest implantable telemetry sensor. When equipped with a CR1216 battery its mass is only 1.6 g and it runs for 38 days at 128 SPS. The A3048 operates with our Telemetry System. The A3048 circuit mounts beside the battery rather than on top of the battery, which reduces its total volume and distributes its mass more evenly, making it a more comfortable fit in smaller animals. We turn the A3048 on and off with a magnet.

Figure: Subcutaneous Transmitter A3048S.

The A3048 amplifier can provide gain of ×100 for frequencies up to 160 Hz. The logic may be programmed to sample at 64, 128, 256, or 512 SPS. The low-pass filter may be configured for a corner frequency of 20 Hz, 40 Hz, 80 Hz, or 160 Hz. The input high-pass filter provides a corner frequency of 0.3 Hz, but may be removed to give gain all the way down to 0.0 Hz. All versions of the A3048 are equipped with 0.5-mm diameter red and blue leads, and a clear-coated loop antenna. The length of the leads, the battery loaded next to the circuit, the operating life, the termination of the leads, the sample rate, the gain of the amplifier, and the bandwidth of the amplifier all vary from one version to the next. The red lead is X+ and the blue lead is Xn−. The antenna is a 30-mm thin loop. The table below gives the specification of a particular transmitter version.

All versions of the A3048 are covered by a one-year warranty against corrosion and manufacturing defect.

Ordering

[26-FEB-25] There are many possible configurations of the SCT. Each configuration that someone has ordered, or asked us to quote on, graduates from being a mere "configuration" to a "version". We describe and list the available SCT versions in the section below. At the time of writing, the minimum order quantity for any particular version is one piece, but the price drops significantly if you order six or more of the same exact version. To get a quotation or to discuss which transmitters will best meet your study's needs, please email us at info@opensouresintruments.com.

Versions

[07-FEB-25] The Subcutaneous Transmitter (A3048) can be equipped with a three, leads up to 130 mm, and a range of bandwidths, gains, and sample rates. You specify which transmitter you want with a full SCT part number. The part number begins with A2048 and is followed by the primary version letter that tells us the battery we load on the circuit. Following the letter we have one or two more numbers and letters that specify the sample rate of the inputs. We use the numbers 1-5 to indicate 128, 256, 512, 1014, and 2048 SPS respectively. We use the letter "Z" to indicate that the low end of the frequency response reaches all the way down to 0.0 Hz. After a dash we have a number and letter to specify the length and type of the leads. After a second dash we have letters specifying the electrodes, and after a final dash we have a letter specifying the antenna.

Figure: A2048 Part Numbering Scheme. Click on the large boxes to jump to tables listing letter codes and options.

See our Electrode Catalog for a list of terminations and of depth electrodes to which our terminations can be attached. See our Leads Table for a description of our several types of insulated, helical steel leads. See our Antennas table for a description of the various types of antenna we can deploy on our implants. The following versions are defined already, but we are happy to define new ones to suit your needs. The operating life is the minimum time for which a newly-made transmitter will operate continuously. The shelf life is the time the transmitter can remain turned off in storage and still retain 90% of its operating life.

For analog input we specify the bandwidth, sample rate, input dynamic range, and channel number offset. In terms of ADC counts, the dynamic range is always 0-65535, as produced by a sixteen-bit ADC. The zero-value of an input is the sample we obtain when we short the two inputs together. The zero-value depends upon the battery voltage, VB, according to zero-value = 1.8 V × 65535 ÷ VB. The dynamic range is the battery voltage divided by the gain of the amplifier. When we specify dynamic range, we assume VB = 3.0 V, which is true for the first half of the life of a CR-series lithium battery at 37°C. When the amplifier gain is 100, the dynamic range is 30 mV, spanning −18 mV to +12 mV.

Figure: Subcutaneous Transmitter A3048R. Equipped with 36 mm × 0.7 mm leads. D-Pin on red, bare wire on blue.

See below for details of current consumption and how to calculate battery life of new versions of the A3048. By default, we set the top of the frequency range at one third the sample rate. The A3048'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 distortion is insignificant.

Analog Inputs

[07-FEB-25] When configured for AC recording, the A3048 consists of a capacitor, typically 100-nF, in series with a resistor, typically 10 MΩ. These together form a high-pass filter with corner frequency 0.16 Hz. The most common versions of the A3048 provide a gain of ×100, another high-pass filter, and a three-pole low-pass filter. We can remove the two high-pass filters by replacing the 100-nF input capacitor and another 10-μF capacitor in the amplifier with resistors. With no high-pass filter, the amplifier's pass-band extends down to 0.0 Hz. We configure the low-pass filter with corner frequency 20, 40, 80, 160, 320, or 620 Hz. These frequencies are matched with sampling rates 64, 128, 256, 512, 1024, and 2048 SPS respectively. The figure below shows the frequency response of a batch of twenty-two A3028S2 transmitters recorded during Quality Control Two (QC2). You will find a database of such plots here. We send such plots along with each batch of transmitters we ship.

Figure: Frequency Response of a Batch of A3048R2s. These devices provide a bandwidth of 0.3-80 Hz at sample rate 256 SPS.

In the response shown above, we see both the high-pass filter corner frequency and the low-pass corner frequency. If we define "corner frequency" as the frequency at which the gain drops to 70% of the gain in the pass-band of the amplifier, the corner frequencies for this batch of transmitters are 0.2 Hz and 90 Hz. That is: the bandwidth is slightly greater than the nominal 0-80 Hz. The most important function of the low-pass filter is to reduce the amplifier voltage gain by at least a factor of ten at a frequency that is one half the sample rate. These transmitters sample at 256 SPS, so we want the gain of the amplifier to drop be ten times smaller at 128 Hz than at 10 Hz. We see that this is indeed the case for our batch of R2s. The gain at 128 Hz is more than twenty times lower than the gain at 10 Hz.

Figure: Absolute Dynamic Range with Low and High Extremes.

The amplifier is powered by the battery voltage, VB, which is typically 3.0 V at 37°C, but will be 3.1 V for the first 5% of the battery's life and drop below 2.6 V in the final 5%. The amplifier saturates within 20 mV of 0V and VB. The following saturating sweep response shows how well the amplifiers handle large inputs. For a comparison of the A3048S2 saturation behavior and that of its predecessor the A3028S2, see here.

Figure: Saturation of the A3048S2 Input.

We measure the electrical noise on the A3048 input by placing the entire transmitter in water and letting it settle for a few minutes. Typical noise for an A3048S2 with 80-Hz bandwidth is 5 μV rms. The figure below shows the spectrum of electrical noise for a batch of A3048S2s.

Figure: Spectrum of Electrical Noise on Inputs of a Batch of A3048S2s. Vertical: 0.4 μV/div. Horizontal: 10 Hz/div.

The A3048P-series transmitters are equipped with a CR1025 coin cell. The CR1025 is 10-mm in diameter and 2.5 mm thick. When loaded with the CR1025, some transmitters will exhibit switching noise of amplitude up to 2 μV rms. This noise is caused by an interaction between transmitter's magnetic switch, which turns on and off at around 5 Hz, and the source impedance of the battery, which is larger for smaller batteries. Here is the electrical noise spectrum of a batch of A3048P2s.

Figure: Spectrum of Electrical Noise on Inputs of a Batch of A3048P2s. Vertical: 0.4 μV/div. Horizontal: 10 Hz/div.

The switching noise we see in the A3048P-series transmitters consists of 10-ms pulses at roughly 5 Hz. The height of these pulses decreases with temperature. At 37°C, they will be no more than 10 μVpp, but at room temperature they can be as large as 30 μVpp. A typical EEG signal from a bare wire electrode in a mouse is 40 μV rms, 160 μVpp. Switching noise pulses of 10-μV are hard to see.

The distortion of a signal by our telemetry system is the extent to which it changes the shape of a signal. We apply a sinusoid to the X inputs of an A3048AV1. The AV1 is equipped with a 0.5-80 Hz amplifier with gain ×100. Input dynamic range is 30 mV. We increase the frequency from 1/8 Hz to 100 Hz. For each frequency, we obtain the spectrum of the signal and measure the power outside the sinusoidal frequency as a fraction of the sinusoidal power using this script. We express the result in parts per million.

Figure: Distortion of Sinusoid versus Frequency. Blue: 10 mVpp. Orange: 1 mVpp. Non-sinusoidal power as a fraction of sinusoidal power in parts per million. Sine wave generated by BK Precision 4053B, specified total harmonic distortion <1 ppm.

The distortion of X is dominated by random electronic noise. There are no significant peaks in the spectrum outside the fundamental.

Figure: Spectrum with 50-Hz, 10-mVpp Sinusoid. Horizonal: 10 Hz/div. Vertical: 0.4 μV/div. The peak is 4000 μV.

The distortion generated by the A3048 is hundreds of time less powerful than that of its predecessor, the A3028P and A3028S. The A3048 samples the signal uniformly, thus eliminating the scatter noise present in the A3028 signal.

Battery Voltage

[16-MAY-25] If the offset in our amplifier is zero, and the on-board 1.8-V voltage regulator produces exactly 1.80 V, then we can deduce the transmitter's battery voltage using the following formula.

Where VB is the battery voltage and AVE is the average value of either X or Y in ADC counts. In practice, this formula is accurate to ±0.1 V provided the battery voltage is ≥2.4 V. The plot below shows how the average X and Y vary as we decrease battery voltage from 3.4 V down to the minimum operating voltage of the transmitter, which is 1.8 V.

Figure: Average Value of X and Y versus Applied Battery Voltage. As observed for an A3049AV3 assembly with MAX4474 in X amplifier, OPA369 in Y amplifier. The A3048's behavior is similar.

As the battery voltage drops below 2.4 V, the X signal drops towards zero, while the Y signal continues to rise.

Battery Life

[16-MAY-25] We equip all our subcutaneous transmitters with CR-series lithium primary cells. The voltage produced by these batteries begins at around 3.2 V, drops rapidly to 3.0 V, remains around 2.9 V for most of the battery's life, and drops rapidly towards the end of life.

Figure: Discharge of CR1025 Batteries. Discharge current is 75 μA, battery capacity is nominally 30 mAhr.

The inactive current consumption of the A3048, which is its current consumption when it is turned off, is roughly 0.8 μA at room temperature. When we calculate shelf life, however, we use 1.0 μA for the inactive current consumption, so as to arrive at a conservative estimate of the time it will take for the A3048 to use 10% of its battery while sitting on the shelf. The CR1225 battery has capacity 50 mAhr ≈ 2000 μAdy, so its shelf life is 200 dy = 7 mo. To obtain the operating life of an A3048 transmitter, we divide the battery capacity in μA-days by the maximum current consumption in μA, and then subtract one day. The subtraction of one day accounts for extended tests we perform during quality control. To obtain the maximum current consumption of an A3048 transmitter, we use the following relation.

Where R is the sample rate in Samples per Second (SPS) and I_a is the active current. We have 18 μA (eighteen microamps) base current consumption, which powers the logic chip (15 μA), amplifiers (1 μA), and miscellaneous circuits (2 μA). Additional current consumption for sample transmission is 0.12 μA/SPS (microamps per sample per second), or we could say that each sample requires 0.12 μC of charge from the battery. The above formula predicts 49 μA at 256 SPS. In the table below, we use our formula for maximum current consumption and combine it with the nominal capacity of the batteries we might use with the A3048. The CR1025 is the smallest CR-series coin cell available. The CR1620 is the largest coin cell we can load onto the A3048.

Figure: Operating Life in Days for Various Batteries.

The formula above is the maximum current consumption of an SCT in order to pass our quality control tests. The average current consumption of the A3048 circuits is roughly 5% lower than the maximum.

Figure: Measured Current Consumption versus Sample Rate. For A3048AV1, slope 0.106 μA/SPS, intercept 16.1 μA.

In practice, the current consumption of a batch of A3048S2s, running at 256 SPS, ranges from 42-47 μA.

Synchronization

[19-DEC-22] 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 auxiliary SCT to record a synchronizing signal along with the signals received from implanted SCTs. See the Synchronization section of the A3028 manual for details.

Encapsulation

[29-NOV-23] All versions of the A3048 are encapsulated in black epoxy and coated with silicone. The silicone is "unrestricted medical grade" MED-6607, meaning it is approved for implants of unlimited duration in any animal, humans included. The A3048's leads and antenna are encapsulated with dyed silicone, then coated with the same unrestricted medical grade silicone. The only materials the transmitter and its leads present to the subject animal's body are either unrestricted medical grade silicone or stainless steel.

Design

[26-FEB-25] The A3048 circuit board comes with a programming extension that provides the programming connector, a power plug, test pins, and a built-in antenna. Two test pins allow us to connect a signal to the amplifier input. Another allows us to see the telemetry transmission bits. The extension is connected to the SCT circuit by a 2.6-mm wide, 10-mm long neck. We use the extension as a way to hold the SCT during encapsulation. At some point during encapsulation, we clip the neck, leaving the SCT circuit on its own.

Figure: A3048AV1 Assembly. For closeup of the SCT circuit see A3049AV1_Top_SCT.

Each time we update the A3048 for automatic assembly, or require modification in-house to incorporate improvements, we issue a new version number to reflect the changes in the bill of materials.

The BV1 is equipped with an effective antenna protection network, precision op-amps, and balanced gain between the two stages of the amplifier. The BV2 introduces precision capacitors better control of the gain peak before the cut-off frequency, drops the the value of two decoupling capacitors so as to reduce the turn-on current burst, eliminates the unecessary blocking capacitor in the amplifier feedback loop, and exchanges the ADS8860 for the slower and less expensive ADS8866. The ADS8866 does not latch up as easily as the ADS8860 when it powers up.

Note that our designs are open-source and protected by the GNU Public Licence V3.0.

Modifications

[31-JUL-24] Here we list the electronic circuits we can use to assemble the various types of A3048 transmitter, and the modifications required by that circuit prior to assembly.

The BV1 produced by build B119305 have R3 loaded with 100 kΩ instead of 4.02 kΩ, so all these boards had to be modified before calibration. We have roughly 140 BV1 assemblies on the shelf, which we are converting to BV2 by replacing C9, C10, and C11 with 1800 pF 1% and C2 and C3 with 10 μF.

Development

[27-NOV-24] For details of the development and production of the A3048, see its Developement page.