Head-Mounting Transmitter (A3040)

© 2021, Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Versions
Connector
Antenna
Design
Modifications
Analog Inputs
Synchronization
Input Noise
Power Measurement
Battery Life
Wrapping
Development

Description

[20-DEC-21] The Head-Mounting Transmitter (HMT, A3040) is a head-mounting telemetry sensor for rats and mice that provides high-fidelity amplification and recording of up to four biopotential inputs. The A3040 is currently in development in collaboration with ION/UCL. The experimenter cements a connector to the animal's head, and wires biopotential and reference signals to the connector. The HMT mates with this connector, which serves both to hold the HMT to the head and to deliver the signals to its amplifiers. The connector carries up to six voltages: four signals and two reference potentials. The HMT transmits its wireless signals using its integrated flex-circuit antenna, which will hover over the back of the animal. The HMT uses the same telemetry system as our Subcutaneous Transmitters (SCT). Its design is almost identical to that of the A3028 subcutaneous transmitter. We intend for the HMT to replace the crude head-mounting versions of the A3028.


Figure: Head-Mounting Transmitter Sketch. The transmit antenna protrudes behind the body of the device. The mouse's nose will be to the left in the drawing, and its tail to the right. The connector axis is parallel to the mouse body axis.

The HMT needs to be splash-proof and dirt-resistant, but need not be water-proof or corrosion-resistant. We wrap the circuit in self-fusing silicone tape so as to cover the electronics, but allow the connector and antenna to protrude. When the HMT's battery runs down, we disconnect it from the animal's head, remove the silicone tape, remove the battery from its retainer, load a new battery, and wrap the HMT in fresh tape before re-mounting it on the animal. By this means, the HMT can be used for continuous recordings of indefinite length, so long as we can tolerate five-minute interruptions for battery replacement every two or three weeks.

Unlike the SCT, the HMT has no magnetic switch. When we slide the battery into the retainer, it starts running and it keeps going until either the battery runs down or we remove the battery. We can deduce the state of the battery from the telemetry signals, so we will always have a day or two warning before we need to replace the battery if we want to maintain a continuous recording.

PropertyValue
Volume of Transmitter Body1.6 ml
Mass of Transmitter Body with Wrapping≤2.0 g
Maximum Dimensions14 mm × 14 mm × 8 mm
Maximum Operating Current145 μA
Battery TypeCR1225 Coin Cell
Battery Capacity2000 μA-days
Minimum Operating Life14 days
On-Off Switchload or remove battery
Number of Inputs2
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
Total Harmonic Distortion<0.1%
Absolute Maximum Input Voltage±4.3 V
Table: Specifications of the A3040B Dual-Channel 160-Hz HMT.

The HMT provides four amplifiers, with either one or two reference potentials. Current consumption is linear with total sample rate.

Versions

All currently-planned A3040 versions have the same profile: 14 mm × 14 mm × 8 mm with 14-mm rear flex antenna. But they may be programmed to enable one to four input channels at various sample rates and bandwidths. We can equip them with CR1025, CR1220, or CR1225 batteries. The larger the battery the greter the mass of the device and the greater its operating life. The table below gives example versions with their signal bandwidths and operating life.

Version W X Y Z Battery
Capacity
(μA-dy)
Mass
(g)
Minumum
Operating
Life (dy)
A3040A1 0.3-40 Hz, 128 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.0 48
A3040A2 0.3-80 Hz, 256 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.0 35
A3040A3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.0 23
A3040A4 0.3-320 Hz, 1024 SPS, 0.41 μV/cnt Disabled Disabled Disabled 2000 (CR1225) 2.0 13
A3040B3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled Disabled 2000 (CR1225) 2.0 13
A3040C3 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt 0.3-160 Hz, 512 SPS, 0.41 μV/cnt Disabled 2000 (CR1225) 2.0 9.5
A3040D2 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 0.3-40 Hz, 128 SPS, 0.41 μV/cnt 2000 (CR1225) 2.0 23
A3040P1 0.3-40 Hz, 128 SPS, 0.41 μV/cnt Disabled Disabled Disabled 1250 (CR1025) 1.7 30
Table: Primary Version Codes of A3040 Head-Mounting Transmitters. Minimum operating life at 25°C in days. Typical operating life is 10% higher.

Connector

[05-JAN-22] The A3040A provides a nine-way, dual-row, 0.025" pitch, surface-mount socket with a single guide post for connection to the electrode interface. We use custom Omnetics NSD-09-VV-G, which we order with part number A78538-001.


Figure: Drawing of NSD-09-VV-G Socket by Omnetics Connector Corporation.

Our numbering of the contacts on the NSD-09-VV-G differs from the one given by the manufacturer's drawing. The figure below is an exerpt from the S3040A_1 schematic, showing our numbering as seen looking down on a soldered socket.


Figure: Pinout of NSD-09-VV-G Socket Showing Our Pin Numbering. View as seen when holding the A3040A and looking down on the connector. .

The NSD-09-VV-G socket mates with any NSD-09-G plug, provided the plug's guide post hole is in position one. We designed the A3040A to mate with Neualynx's EIB-8 electrode interface board. In the figure below, we see the top of the EIB-8's nine-way plug, with the guide post hole in position one, on the top-right.


Figure: EIB-8 Interface Board, Showing Connector P1. Pin numbers are ours, as used in our S3040A_1 schematic, not numbers assigned by the manufacturer.

The EIB-8 provides ten connection pads. The two GND pads are connected to P1-1, which the A2040A connects to VC, the common voltage of the amplifiers. The eight electrode connections A1-A8 are connected to P1 like this: A1/P1-4, A2/P1-6, A3/P1-8, A4/P1-10, A5/P1-9, A6/P1-7, A7/P1-5, A8/P1-3.

Antenna

[06-JAN-22] The antenna on the A3040 is a flexible circuit with a rounded end, holding a zig-zag antenna tuned to 915 MHz using an inductor and a capacitor loaded on the circuit at the antenna base.

Analog Inputs

Synchronization

For advice on how to synchronize video, stimuli, and environmental phenomena to the HMT signals, see the Synchronization section of the A3028 manual.

Input Noise

Power Measurement

For advice on how to calculate the power of a biometric signal, see the Power Measurement section of the A3028 manual.

Battery Life

The HMT current consumption from a 3-V Lithium Primary cell will be no greater than:

Ia = 26 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.

Wrapping

The A3040A has no permanent epoxy or silicone encapsulation. Before loading the transmitter on the animal, we fold the circuit as shown in our sketch. We take a 40-mm long, 25-mm wide piece of self-fusing silicone tape, such as that made by XFasten. We make a perpendicular cut 10-mm deep, half-way along the 40-mm length of the tape. We fit the end of this cut over the connector on the HMT and wrap the two 20-mm sides of the tape around the HMT and over its battery. We pinch the free edges of the tape together. The tape fuses immediately to itself. We trim the excess edges with scissors. The resulting wrap adds approximately 0.4 g to the mass of the HMT, so the mass of the HMT itself must be no more than 1.6 g to meet our total weight target of ≤2 g. When we replace the battery, we cut the tape with scissors, remove the tape, remove the battery, and now we have the opportunity to wash the circuit if we like. We rinse in hot water and clean with a tooth brush. We soak the circuit and in ethanol for a few minutes along with the new silicone wrap, blow dry again, load new battery, wrap in silicone with sterile gloves, and install on the animal.

Design

The following files define the A3040 design. Note that we distribute all these files under the GNU Public License. Any design that incorporates any part of our work must itself be distributed under the same GNU Public License.

S3040A_1.gif: 4×0.3-160 Hz, BGA-64, flex antenna.
A304001A: Gerber files for A3040A PCB.
A304001A_Top: Top view of A304001A.
A304001A_Bottom: Bottom view of A304001A.
Code: Logic chip firmware library.
CR1225: Diameter 12 mm battery electrical data sheet.
A78538: NSD-09-VV-G, nine-way, dual-row, 0.025" pitch, surface mount connector with guide post.
BC-2009: Battery retainer for CR1025, CR1220, and CR122, showing cuts we make before loading.

Modifications

Development

[19-DEC-21] Sketch of prototype here.

[03-JAN-22] Finish first draft of A304001A rigid-flex circuit board, equipped with four amplifiers. We are using the ADG804 four-to-one analog multiplexer to select one of four signals for the ADC. We are using the LTC1865L in SOP-8 package because we want to conserve our stock of MSOP-10 packages for our smaller transmitters. We find that the op-amp and passives for each amplifier weigh 34 mg.

[06-JAN-22] We receive battery retainers and modify them by cutting off end flanges and solder to an existing 12.5-mm square transmitter board. The corners of the retainer protrude by 0.5 mm. We prefer to bend the retaining spring so that it pushes down on the center of the battery. After these modifications, the battery is secure in the retainer, with its negative tab pressed on the center of the circuit board, where we expect to have our gold-plated negative battery pad.


Figure: CR1225 Mounted in Battery Retainer.