Subcutaneous Transmitter (A3047)

The GND lead is the low-impedance ground potential for the amplifier inputs. Amplifiers X1, X2, and X3 use GND as their reference potential if we leave their inverting inputs (X1−, X2−, and X−) disconnected. The A3047A2C-B is an example of a configuration in which all four amplifiers use GND as their reference, so it is equipped with only five leads. We solder the end of each lead to one of the pads on the top side of the board before loading the battery over the leads and burying their solder joints in the epoxy. Each amplifier is includes its own three-pole low-pass filter whose cut-off frequency we set with three capacitors. Each amplifier may be either AC-coupled or DC-coupled to its input leads by means of inserting a capacitor or resistor in series with the input leads. The gain of each amplifier is also adjustable by means of a resistor.

Figure: Frequency Response of a single A3047A3D.

During our final quality control, we measure the frequency response of every A3047 amplifier. We apply a sinusoidal to all inputs and plot the amplitude of the transmitted signal versus frequency. You will find a database of such plots here.

Figure: Frequency Response of the A3047A1B. The amplifiers in the A1B are: X2 ×50 2-80 Hz, X3 ×25 0.0-40 Hz, X4 ×25 0.0-160 Hz. The X1 amplifier is not used.

When we drop a transmitter in water, so that its electrodes are immersed, and we allow the water to settle until it is still, we will see on our analog inputs the sum of two electrical signals. One is electrical noise generated by the transmitter circuit. The other is chemical noise generated by the electrode metals reacting with the water. With 316SS electrodes, no solder joints, the chemical noise is negligible. Analog inputs with dynamic range 30 mV will see roughly 5 μV of electrical noise in 1-160 Hz.

Figure: Spectrum of Electrical Noise Generated by Channel X4 in A3047A3D. These channels are 0.0-160 Hz, 120 mV range. We take the spectrum of a 32-s interval and plot in the Neuroplayer. Total noise is 5 μV rms.

We connect a 0.2-Hz square wave of amplitude 50 mVpp with 10-MΩ source impedance. We expect a 25-mVpp signal on all three inputs. In the caption below we give the dynamic range of each input. Note that the X2 input relaxes after each square wave edge, while X3 and X4 do not, because their response extends to 0.0 Hz.

Figure: Step Response of A3047A3D to 15-mVpp Square Wave. Duration: 1 s. Range: 0-65535 counts. Brown: X1, 30 mV, 2-80 Hz. Salmon: X2, 60 mV, 2-80 Hz. Blue-Gray: X3, 120 mV, 0.0-20 Hz. Black: X5, 120 mV, 0.0-160 Hz. Gray: T, 64 SPS.

We apply a 100-mV triangle wave through 10 MΩ, so we expect 50-mV at the inputs. The triangle wave drives the X3 input into saturation on both sides of the dynamic range, and almost drives the other inputs to their limits as well. Note that the amplifiers are able to drive their outputs right to the positive and negative supply voltages. In X2 we see non-linearity in the ramp when emerging from saturation. This non-linearity arises only when the amplifier output is driven into saturation at the edges of the dynamic range.

Figure: Response of A3047A1A to 50-mVpp Triangle Wave. Duration: 4 s. Range: 0-65535 counts. Magenta: X2 60 mV 0.16-80 Hz, Brown: X3 range 30 mV 0.0-40 Hz, Salmon: X4 60 mV 0.0-160 Hz.

Each of the amplifiers X1-X4 are equipped with non-inverting and inverting inputs so as to permit a differential voltage measurment. The low-impedance ground pad must be connected to some part of the subject body. If we make no connection to the inverting input of an amplifier, the amplifier will use GND as its inverting input. If we connect both inverting and non-inverting inputs to the same signal, their difference is zero, and the ideal amplifier output will be equal to the zero-value. Our amplifiers are not ideal, as we demonstrate with the following experiment. We connect X4+ and GND of an A3047A1A to the ground of our signal generator. We connect X2+, X2−, X3+, and X3− to 20 mVpp sine wave to measure the common mode gain response. We connect X2− and X3− to GND to measure the differential gain. We divide the differential by the common mode amplitudes to obtain the common mode rejection ratio, which we plot in decibels.

Figure: Common Mode Rejection Ratio of X2 and X3 in A3047A1A as a Function of Frequency. We apply a 20-mVpp common-mode and differential sweep.

The CMRR for frequencies below 7 Hz is >40 dB. A common-mode signal on the low-impedance ground will appear on differential amplifier outputs with amplitude more than one hundred times smaller for frequencies less than 7 Hz. Common mode signals will be ten times smaller for frequencies up to 70 Hz. In the recording below, we see a 70-mV pulse recorded on X4 by an A3047A1B-A immersed in saline, but not by X3 immersed in the same saline. The drop in X3 is no more than 10 μV, suggesting CMRR of roughly 80 dB at low frequencies.

Figure: Chemical Artifact on X4 (Brown) but Not X3 (Pink). Disturbance of X3 is <10 μV while X4 drops 70 mV.

The distortion of a signal by our telemetry system is the extent to which it changes the shape of a signal. We apply a 20 mVpp sinusoid to the X1 (×50, 0.0-160 Hz) and X3 (×25, 0.0-40 Hz) inputs of an A3047BV2. We increase the frequency from 1/8 Hz to 200 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 10-mVpp Sinusoid versus Frequency. For A3047BV2 inputs X1 and X3. Distortion power as a fraction of input power in parts per million. Sine wave generated by BK Precision 4053B, specified total harmonic distortion <1 ppm.

The distortion of the X is dominated by random electronic noise. As we leave the pass-band of each amplifier, we see the apparant distortion increase only because the amplifier's low-pass filter is attenuating the fundamental harmonic of the input. We see a second harmonic in the spectrum of a sixteen-second interval, as shown below, but the amplitude of this harmonic is below the harmonic amplitude specified by our function generator, so it could be a genuine feature of our input signal.

Figure: Spectrum with 10-Hz, 10-mVpp Sinusoid. Horizonal: 5 Hz/div. Vertical: 0.4 μV/div. The fundamental harmonic amplitude is 4000 μV. The second harmonic amplitude at 20 Hz is 1.6 μV

We note that the A3047 samples its signals uniformly, thus eliminating the scatter noise present in earlier transmitters such as the A3013, A3019, and A3028.

Thermometer

[11-NOV-24] The A3047 thermometer is on the bottom side of the board, on the opposite side from the battery. When we implant the A3047, the most natural orientation of the device is to place the battery facing outwards. The thermometer faces inwards, with a thin layer of epoxy and silicone between it and the animal's body temperature.

Figure: Subcutaneous Transmitter A3047A1A-A Before Encapsulation.

The absolute accuracy of the A3047's thermometer is ±1°C. Its stability and precision are better than ±0.1°C. When recording from an A3047, the NDF archive will contain the sixteen-bit temperature values in their own signal channel. We can translate these values into Centigrade using an interval processor. We can export the temperature signal to an EDF file, and in doing so perform the translation to Centigrade using the EDF header.

Figure: Interpolation Table for Use With Thermometer Samples. We assume VC = 1.803 V and nominal values from the LMT70 data sheet.

The measurement value transmitted by the A3047 is a sixteen bit number with the lower two bits always zero. Nominal measurement at 25.0°C is 35226. The slope of its response is −190.5 cnt/K, or −5.25 mK/cnt. The noise is 5 cnt rms or 25 mK. The figure below gives sample value versus temperature, assuming nominal values for the temperature sensor itself and the power supply voltage to the analog to digital converter.

Figure: Typical Sample Value versus Temperature. We take the values from the data sheet, assume VC = 1.803 V, and digitize to sixteen bits.

To transform the sample value into a temperature, we use the tabulated values above, which we obtain by combining the manufacturer's data sheet, our measurement of the on-board reference potential VC = 1.803 V, and the linearity of the converter. In Tcl code we can assign the tabulated values to a table string and use our lwdaq linear interpolation routine to obtain the temperature, like so:

set T_table "41767 -10 39910 0 38044 10 36168 20 34285 30 32393 40 30492 50 28583 60"
lwdaq linear_interpolate 34970 $T_table
26.362188

If we find that the value obtained by the above calculation is offset from the correct value at our operating temeprature, we can subtract the offset from our calculated value and be confident that our adjusted measurement will be accurate to ±0.1°C within ±5°C of our operating temperature.

Figure: European Data Format (EDF) Export Configuration.

When exporting from NDF to EDF in the Neuroplayer, we write into the EDF header a minimum and maximum value for temperature that directs our EDF viewer to provide an accurate translation of the raw measurements in the temperature range we are most interested in studying. In the case of animal body temperature, we set the temperature at the minimum measurement value to be 211.9°C and the temperature at the maximum measurement value to be −136°C.

Figure: European Data Format (EDF) Export of Voltages, Temperature, and Activity. These are taken from our EDF_Demo.zip recording. Magenta: X1 voltage. Brown: X2 voltage. Green: X3 voltages. Red: Temperature. Purple: Activity.

The above plots are derived from our EDF_Demo.zip example recording. To view EDF files, we use the free software EDF Browser.

Figure: Thermometer Moved into Warm Water. T1-T4: RTDs. Ts: SCT. Stirring 0-1 min, no stirring after.

The thermometer on the A3047 is the LMT70, a 0.9-mm square component on the bottom side of the transmitter circuit. The battery is loaded on the top side. When implanted, the thermometer is separated from the animal's body by approximately one millimeter thickness of epoxy and silicone. The A3047A1 transmits 128 SPS of temperature measurements. Measurement noise 25 mK rms. The speed with which the thermometer responds to changes in ambient temperature is dominated by the heat capacity of the transmitter. For the A3037A, with its CR2330 battery, the time constant is one minute in water.

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.

Body Capacitance

[19-DEC-22] See Body Capacitance in the A3019 manual.

Battery Life

[02-FEB-24] 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 A3047, 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 A3047 to use 10% its battery while merely sitting on the shelf. The CR2330 battery has capacity 165 mAhr = 11000 μAdy, so its shelf life is 1100 dy = 36 mo.

To obtain the operating life of an A3047 transmitter, we divide the battery capacity in μA-days by the maximum current consumption in μA given by the formula below to obtain the guaranteed battery life in days.

In the above relation, we have 30 μA base current consumption, which powers the logic chip (15 μA), amplifiers (6 μA), temperature sensor (6 μA), and miscellaneous circuits (8 μA). Additional current consumption by digitization and transmission is 0.12 μA per sample per second, or we could say that each sample requires 0.12 μC of charge drawn from the battery. The above formula predicts 275 μA for 2048 SPS. We measure 225 μA. At 960 SPS the formula predicts 145 μA, we measure 126 μA, 136 μ, and 137 μA in our first three devices.

Figure: Operating Life in Days for Various Batteries.

In the table above, we use our formula for maximum current consumption and combine it with the nominal capacity of the batteries we might use with the A3047. The CR1620 is the smallest battery we believe we can load onto the 20-mm diameter circuit. The CR2477 is the largest battery we believe a rat can tolerate.

Encapsulation

[29-NOV-23] All versions of the A3047 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 A3047'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. When we solder screws or pins to the ends of the leads, there is also solder. Solder reacts slowly with saline, so solder joints must be protected from body fluids by an insulating layer of cement during implantation.

Design

[20-DEC-24] The A3047 provides four analog inputs, each of which may be configured either to use the common ground potential VC, or to use its own reference potential. Each amplifier is equipped with a three-pole low-pass filter and a single-pole high-pass filter. The high-pass filter can be disable to permit recording down to 0.0 Hz. The A3047 also provides a temperature sensor accurate to ±0.1%deg;C at animal body temperature. Aside from its more numerous and versatile analog inputs and temperature sensor, the design of the A3047 is similar to the A3028: a micropower logic chip communicates with a sixteen-bit analog-to-digital converter (ADC), drives a five-bit digital-to-analog converter (DAC) to produce a frequency-modulation output, which we connect to a voltage controlled oscillator (VCO) to produce our telemetry tranmission. A magnetic sensor drives a flip-flop that turns on and off power to the circuit. The inactive power is the power delivered to the sensor and flip-flop. Sample rate is controlled by a 32.768 kHz precision clock. Transmission timing is controlled by a ring oscillator that we calibrate during assembly. Active current consumption is dominated by the ring oscillator, analog-to-digital conversion, and the voltage-controlled oscillator, which turn on and operate for approximately 10 μs in order to generate each sample.

Figure: A3047BV1 Assembly Showing Pads and Eight Leads. This particular assembly has a pad for X− that we later eliminated.

The A3047's four amplifiers have identical component layout. By changing the values of their resistors and capacitors, we can configure each amplifier with its own gain and frequency response. In order to give the amplifiers a high-pass filter, we exchange resistors for capacitors, so that component R101 might, for example, be a 100-nF capacitor rather than a resistor. The circuits we receiver from our assembly house have their amplifiers configured in a particular way that we hope we can use with minimum further adjustment.

The BV2 provides four amplifiers. We use the BV2 to make the A3047AB1, which uses only three of the amplifiers. The plot below shows the frequency response we obtain when we transmit the outputs of all four amplifiers at 512 SPS.

Figure: Response of A3047BV1 to Sinusoidal Sweep of 40 mVpp. All four amplifiers sampled at 512 SPS. Sweep 1-500 Hz in 2 s. Light Green: X1, ×50, 0.0-160 Hz. Brown: X2, ×50, 0.2-80 Hz. Purple: X3, ×25, 0.0-40 Hz. Dark Green: ×25, 0.0-160 Hz.

The 0.0-Hz response of X1, X3, and X4 on the BV2 allows it to amplify a low-frequency square wave. In the plot below, we see three intact square waves for 0.0 Hz response and a distorted square wave for the 0.2-Hz high-pass filter of the X2 amplifier. The 0.0-40 Hz amplifier rounds the edges of the square wave. All amplifiers introduce some bounce in response to an instantaneous step. The bounce is a feature of the sharp low-pass filters we use in the A3047 amplifiers.

Figure: Response of A3047BV1 to 40-mV Square Wave. All four amplifiers sampled at 512 SPS. Square wave 1-Hz. Light Green: X1, ×50, 0.0-160 Hz. Brown: X2, ×50, 0.2-80 Hz. Purple: X3, ×25, 0.0-40 Hz. Dark Green: ×25, 0.0-160 Hz.

Details of the design are available in the following library of design files. Note that all our designs are protected by the GNU General Public Lisence.

Modifications

[25-SEP-23] To create the A3047BV2 out of the A3047BV1, we do the following.

1. Link 0V pad to 0V with wire.
2. Remove R111, R211, R311, and R411.
3. Replace R112, R202, R204, R212, R312, and R412 with 1.0 MΩ.
4. Replace U101, U201, U301, and U401 with OPA369.
5. Replace R307 and R308 with 200 kΩ.
6. Replace R407 and R408 with 100 kΩ.
7. Replace R1 with 10 Ω.

As a result of these modifications, we reduce the gain of the two DC amplifiers to ×25 and we reduce the maximum input offset voltage of each channel to a few hundred microvolts. We leave the frequency response the same. The BV2 is used in the A3047A1B transmitter. For a map of modifications, see BV2_Mods.

[13-AUG-24] To create the A3047A3D circuit from the A3047BV2, we do the following.

1. Load 2.0 MΩ for R103 and R106.
2. Load 1.0 MΩ for R104.
3. Load 1.8 nF, 1%, for C101, C102, C103.
4. Load 8.2 nF, 1%, for C301, C302, C303, R101.

[08-OCT-24] When we create the A304701B from the A304701A PCB:

1. Connect 0V pad to 0V.
2. Add antenna protection circuit: 15 pF, 200 Ω, 15 pF.
3. Eliminate R111, R211, R311, R411.
4. Eliminate R1 and C6.
5. Eliminate U402, R404, and R405.
6. Eliminate X− pad, connect X4 to VC.

Development

[18-OCT-24] For details of the development and production of the A3047, see its Developement page.