Subcutaneous Transmitter (A3048)

© 2023, Kevan Hashemi, Open Source Instruments Inc.


Analog Inputs
Battery Life


[08-APR-23] The Subcutaneous Transmitter (A3048) is an implantable telemetry sensor for mice that provides amplification and filtering of one biopotential input. The A3028 is our smallest implantable telemetry sensor. When equipped with a CR1025 battery it displaces only 0.70 ml and runs for 33 days at 128 SPS. The A3048 operates with our Subcutaneous Transmitter system. The A3048 replaces the A3028P and A3028S.

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.

Volume of Transmitter Body0.89±0.1 ml
Mass of Transmitter Body1.8±0.1 g
Lead Dimensionsdiameter 0.5±0.1 mm, length 50±2 mm
Lead Terminationssteel coil, diameter 0.25 mm, length 1.0 mm
Maximum Dimensions21 mm × 13 mm × 3.7 mm
Minimum Operating Life42 days
Shelf Life5.5 years
Number of Inputs1
Input Impedance10 MΩ
Sample Rate256 SPS each channel
Sample Resolution16-bit
Input Dynamic Range27 mV
Input Bandwidth0.3-160 Hz
Input Noise≤8 μV rms
Total Harmonic Distortion<0.1%
Absolute Maximum Input Voltage±3 V
Table: Specifications of the A3048S2-AA-C50-D.

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. At the same time, the greater surface area makes the device more vulnerable to corrosion. We recommend the A3048 for implantation for no more than one hundred days. For longer implantations, consider the A3049.


[05-JUL-23] 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.

The following versions are defined already, but we are happy to define new ones to suit your needs.

Version Input Battery Capacity
Life (dy)
Life (yr)
A3048P1 0.2-20 Hz, 64 SPS, 27 mV 1250 (CR1025) 0.70 19 × 11 × 3.7 1.5 49 3.4
A3048P1 0.2-40 Hz, 128 SPS, 27 mV 1250 (CR1025) 0.70 19 × 11 × 3.7 1.5 38 3.4
A3048P2 0.2-80 Hz, 256 SPS, 27 mV 1250 (CR1025) 0.70 19 × 11 × 3.7 1.5 27 3.4
A3048S0 0.2-20 Hz, 64 SPS, 27 mV 2000 (CR1225) 0.89 21 × 13 × 3.7 1.8 79 5.5
A3048S1 0.2-40 Hz, 128 SPS, 27 mV 2000 (CR1225) 0.89 21 × 13 × 3.7 1.8 61 5.5
A3048S2 0.2-80 Hz, 256 SPS, 27 mV 2000 (CR1225) 0.89 21 × 13 × 3.7 1.8 42 5.5
Table: Versions of the A3048 Subcutaneous Transmitter. For each analog input we specify the bandwidth, sample rate, and input dynamic range in millivolts. Minimum operating life at 37°C in days. Shelf life for calculating fraction of battery capacity lost while on the shelf at 25°C.

For 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, VBAT, according to Z = 1.8 V × 65535 ÷ VBAT. The dynamic range is the battery voltage divided by the gain of the amplifier. When we specify dynamic range, we assume VBAT = 2.7 V, which is true for most of the life of a lithium primary call. When the amplifier gain is 100, the dynamic range is 27 mV.

The shelf life of a transmitter is how long it takes to exhaust the battery permanently when we leave it inactive on the shelf. 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.


We define the lead names and provide links to photographs and drawings of the leads in the A3028 Manual's Leads section. We present the various antennas we have used for implants in the Antenna section. The best antenna for rat implantation is the 50-mm A-Antenna.


We define the electrode names and provide links to photographs and drawings of the electrodes in the A3028 Manual's Electrodes section.

Analog Inputs

[19-JUL-23] The A3048 input usually consists of a 100-nF capacitor in series with a 10 MΩ resistor. These together form a high-pass filter with cut-off frequency 0.16 Hz. The A3048 amplifier provides 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 twenty-two A3028S2 transmitters as recorded during our Quality Control Two (QC2) tests.

Figure: Frequency Response of a Batch of A3048S2s. These devices provide a bandwidth of 0.7-80 Hz with sample rate 256 SPS.

The amplifier is powered by the battery voltage, VB, which is typically 2.7 V, but will be 2.9 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 27 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 the X is dominated by random electronic noise. There are no significant peaks in the spectrum outside the fundamenta.

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.


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.

S3048A_1.gif: Schematic of A3048AV1.
A304801A: Gerber files for A304801AR1 PCB.
A3048AV1: Top side component view of A3048AV1.
A3048AV1: Bottom side component view of A3048AV1.
S3048B_1.gif: Schematic of A3048BV1
A304801B: Gerber files for A304801BR1 PCB.
A304801BR1_Top.svg: Drawing of A304801BR1 top side.
A304801BR1_Bottom.svg: Drawing of A304801BR1 bottom side.
A3048BV1_Top: Top side component view of A3048BV1, spark-protected.
A3048BV1_Bottom: Bottom side component view of A3048BV1.
Code: Compiled firmware, test scripts.

The A3048AV1 circuit comes with a programming extension that provides the programming connector, a power plug, and test pins. 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.

[20-JUL-23] The following table lists versions of the assembled A3049 electronic circuit, out of which we make the A3049-series transmitters.

A3048AV10.5-80Hz, U5=MAX4471, L1=2n0, C6=OC
A3048BV10.2-80Hz, U5=OPA2369, C12=C13=15pF, R14=200R
Table: Versions of the A3049 Electronic Circuit.

The BV1 is equipped with an effective antenna protection network, is equipped with an op-amp with maximum offset 0.2 mV, and has the gain of the amplifier spread between the two op-amps.


[04-AUG-23] Here we list the electronic circuits we can use to assemble the various types of A3049 transmitter, and the modifications required by that circuit prior to assembly.

C7C8C9, C10, C11R6
0.2-40 Hz, 27 mVA3049BV1samesame3.9 nFsame
0.2-80 Hz, 27 mVA3049BV1samesamesamesame
0.2-160 Hz, 27 mVA3049BV1samesame1.0 nFsame
Table: Modifications to the Circuit Assemblies for Various Transmitter Versions.

The A3048BV1 produced by build B119305 have R3 loaded with 100 kΩ instead of 4.02 kΩ, so all these boards must be modified before calibration. At time of writing, we have 200 on the shelf. The next batch will have this error corrected.


[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.

Battery Life

[19-SEP-23] We equip all our subcutaneous transmitters with CR-series lithium primary cells. The voltage produced by these batteries begins at around 3.0, drops to 2.8 V for most of the battery's life, and drops rapidly towards the end of life, as shown below for CR1025 batteries.

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

To obtain the minimum 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.

Ia = 18 μA + (R × 0.12 μA/SPS)

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 0.11 μA/SPS (microamps per sample per second), or we could say that each sample requires 0.11 μC of charge from the battery. The above formula predicts 46 μA at 256 SPS. 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 A3049 circuits is roughly 5% lower than the maximum.

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

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 A3049. 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 A3048 replaces the long-running A3028P and A3028S transmitters. When we compare the A3048 with the A3028 transmitters, the most significant difference between the two is in their operating life. The minimum operating life of the A3028S2, for example, was 39 days, while that of the A3048S2 is 42 days.


[19-DEC-22] All versions of the A3048 are encapsulated in black epoxy with a coating of silicone. The silicone is "unrestricted medical grade", meaning it is approved for implants of unlimited duration in any animal, humans included.


[30-MAR-23] Start circuit design.

[06-APR-23] We remove the low-pass filter that lies between VD and VA on an A3047A1A. That is: we remove R1 and replace with 0 Ω, leaving the 10-μF capacitor in place on VA. We see no increase in noise, no switching noise. The logic on the A3047 uses only 1V8. The only things using VD and VA are the VCO and the amplifiers. We resolve to remove this resistor from the A3048. We start with A3028PV3 circuit, replace LTC1865L with ADS8600. We now have enough free space to add a two-component antenna matching network. We are unable to fit two SC-353 single op-amps on the board. We must stick with the SOT-23-8 dual op-amp. We cannot purchase the OPA2349. The MAX4471 is available. It is a drop-in replacement with the advantage that it is rail-to-rail input and output, its input offset is only 0.5 mV, and its quiescent current is slightly lower. But its gain-bandwidth is only 9 kHz compared to the OPA2349's 70 kHz. But the only pup transmitters we have been making are 0.3-40 Hz and 0.3-80 Hz. The first amplifier has a gain of 40, so we need 3.2 kHz gain-bandwidth product. The MAX4471 will do the job easily, and will do okay with 160 Hz as well. We rotate the VCO so we can add our matching network. The RF signal propagates diagonally across the circuit board. We exchange the power and ground planes for two additional signal planes, making six signal copper planes in all. Re-name components so they are contiguous.

[08-APR-23] Layout A304801A complete, schematic S3048_1.gif.

[11-APR-23] Printed circuit board submitted for fabrication.

[14-APR-23] Panel Gerber files received. Note that logic chip LC4064ZE-7MN64I now in-stock at DigiKey.

[09-MAY-23] Receive 100 of A3048AV1. These are equipped with 2-nF capacitors for 80-Hz bandwidth, and resistors for ×100 gain. After correcting one constraint error in the code, the circuit works perfectly. It does have an , with the one idiosyncracy: when we power it on through our multimeter set to microamps or milliamps, the resistance of the meter causes the circuit to become stuck in a state where it consumes several milliamps and does not complete its turn-on. When powered by a battery, the circuit always turns on and off correctly. We program and test three circuits at 128 SPS, 256 SPS, and 512 SPS and find the average current consumption is 29, 42, and 69 μA respectively, which is 10% lower than the consumption of the A3028KV2. We compare the saturation behavior of the A3040D2 amplifiers, which are identical to the A3028S2 amplifiers, to that of the A3048S2.

Figure: Saturation of the A3048S2 Input Compared to that of the A3040D2. Blue: A3048S2 saturating at the extremes. Others: A3040D2 saturating and reversing.

[12-MAY-23] The A3041AV1 amplfier is equipped with a three-pole low-pass filter and MAX4471 9-kHz dual op-amp. If we remove this filter, we see the maximum bandwidth the amplifier can deliver. With 1.0 nF capacitors, the amplifier has a corner frequency of 160 Hz, and with 2.0 nF capacitors the corner frequency is 80 Hz. The AV1 assembly comes with 2.0 nF capacitors by default.

Figure: Amplifier Bandwidth. Green: The amplifier with no low-pass filter. Blue: The amplifier with 160-Hz low-pass filter.

The A3048AV1 can provide a gain of ×100 with corner frequencies of 40 Hz, 80 Hz, and 160 Hz, but no higher. Its amplifier is not fast enough to provide a gain of ×100 at 320 Hz. The A3048BV1 will provide a faster amplifier.

[06-JUN-23] We add another passive component to the A304801A to make a T-network between the VCO and the antenna. We convert the amplifier to provide gain ×21 in first stage and x5 in second stage, for total of x105, as we did in A3049. The OPA2369, with its 12-kHz gain-bandwidth product, will provide gain ×21 up to roughly 500 Hz, while at the same time guaranteeing offset less than 0.75 mV, making the circuit suitable for amplifying biopotentials down to 0.0 Hz. We make some other adjustments to tracks and silk screen, generating A304801BR1, which we submit for fabrication, and new schematic S3048B_1. Assembly BV1 will be equipped with T-network C12=C13=15pF and R14=200Ω. This network gives complete protection against sparks from our plasma ball. It attenuates the transmit power no more than 1 dB. The BV1 will be loaded with 2.0-nF filter capacitors for 80-Hz corner frequency.

[28-JUN-23] We have two A3048S2 that failed in the same way during poaching, each after roughly twenty days. Prior to failure, sweep response is perfect every day. On the day of failure, the device won't turn on. Dissect both. Battery voltage is 1.5 V until we disconnect, then rises to 2.7 V. Connect battery to circuit, voltage drops to 1.5 V again. Jump start by connecting 2.7 V across battery briefly. Circuit powers up and transmits. Disconnect from battery. Connect external 2.7 V. Ater initial burst of current to power up the circuit, current consumption is ≈40 μA. Connect 10 kΩ to battery, voltage drops to 1.5 V. Connect 10 kΩ to fresh battery, voltage remains 3.22 V. If we connect either battery to an A3028KV2 circuit, one that consumes 75 μA, the battery can turn on the circuit, and battery voltage is around 1.9 V.

In a batch of 23 A3048P2, equipped with CR1025 battery, we see switching noise up to 2 μV rms, which consists of pulses of around 20 ms and height up to 30 μV at room temperature. For spectrum see here. We see no sign of such noise in the A3048S2 equipped with the CR1225 battery.

[30-JUN-23] Receive 120 of A3048AV1. Measure current consumption versus sample rate, add to our existing measurements, slope 0.106 μA/SPS, intercept 16.1 μA.

[19-JUL-23] Firmware P3048A05 provides uniform sampling with transmission scatter. The uniform sampling is achieved by always sampling at the end of each sample period, by asserting CSS for one CK period. The active CK period, when we read out the sample, takes place 1 to 16 CK periods later. We assert CSS only during the ADC readout, not for the full CK period. Applied to an AV1 assembly at 256 SPS we have current consumption 43.1 μA with scattered sampling and transmission. We have 43.4 with uniform sampling and scattered transmission. Distortion at 50 Hz drops from 40,000 ppm to 4.3 ppm.

[01-AUG-23] We have 200 of A3048BV1. First problem we discover is we specified 100 kΩ for R3. We must swap for 4.02 kΩ on all boards.

[18-AUG-23] We check the RF power emitted by the A3048AV1, with no antenna protection network, and the A3048BV1, with three-component T-protection network. Nathan reports. "We measured the RF power output of the A3048BV1 and compared it to that of the A3048AV1. We programmed and calibrated both boards. We then placed each one separately in a faraday enclosure and measured its power output using the spectrometer tool. They had comparable power output. We then tested its static protection by shocking the antenna of the transmitter with a spark from a plasma ball with a washer on top. The A3048BV1 survived the shocks from the plasma ball and operated perfectly fine afterward. The 3048AV1 lacked protection and would stop working after a couple sparks. The VCO would need to be replaced, indicated by the 18mA current consumption."