Figure: Encapsulated Subcutaneous Transmitters. The smaller A3019A is on the left. Its volume is 1 ml without the antenna and leads. The A3019B is on the right, with a larger battery. Its volume is 1.8 ml, but its battery life is over twice as long.
The Subcutaneous Transmitter (A3019) is an implantable telemetric sensor for monitoring EEG. We are currently developing the sensor with the help of the Test Transmitter (A3020). We expect the volume of the A3019 to be less than 1 ml. Its performance and design will be similar to that of the A3013.
The assembled A3019A with battery, but without encapsulant or leads, is entirely enclosed within a rectangular volume 6 mm × 12 mm × 13 mm, or 0.94 ml.
| Property | Value |
|---|---|
| Volume (without leads) | 1.0 ml |
| Volume (with leads) | 1.5 ml |
| Operating Life | 4 wks |
| Shelf Life | 60 wks |
| Number of Inputs | 1 |
| Type of Input | Differential |
| Input Impedance | 10 MΩ || 2 pF |
| Samples Per Second | 512 |
| Sample Resolution | 16-bit |
| Input Dynamic Range | 20 mV |
| Input Bandwidth | 0.7 Hz − 160 Hz |
| Input Noise | 12 μV |
| Input Mains Hum | <1 μV |
The A3019 has one antenna and two leads. The two leads are X+ and X−. These are helical springs coated in silicone. Their tips are bare and tinned, ready for soldering to an electrode. The antenna is folded into a loop.
| Name | Function | Comment |
|---|---|---|
| X+ | X signal input | connected to 100 nF and 10 MΩ |
| X− | X ground input | connected to VCOM |
| Antenna | radio-frequency antenna | a flexible wire |
The A3019A-P is the transmitter with programming extension, prior to encapsulation. The photograph below shows the A3019A-P with its programming connector, external battery connection, and test pins. The photograph shows the parts on the top side of the board.
When we receive an order for a new A3019, we start with the circuit shown above, minus the antenna and leads. We replace any capacitors and resistors required by the gain and bandwidth of the new transmitter. We add the leads and antenna. We encapsulate in epoxy and silicone and test.
S3019_1.gif: Subcutaneous Transmitter Circuit Diagram.
Code: Logic Chip Firmware Library.
A301901A.zip: PCB Gerber Files.
A301901A_Panel.zip: PCB 2×5 Panel Gerber Files.
A3019A.xls: BOM, PIK, KIT, and Cost for A3019A Production.
A3019A_SMT.pdf: Component placement drawing.
A3019A_Labels.pdf: Kit bag labels.
An A3019 comes in the following versions, each of which we specify with A3019f, where f is the function code. All versions are encapsulated in epoxy and silicone, and supplied with silicone-insulated leads.
| Function Code |
X | Battery Capacity (mA-hr) |
Volume (ml) |
Operating Life (weeks) |
Shelf Life (weeks) |
|---|---|---|---|---|---|
| A | 20-mV range, 0.7-Hz HPF, 160-Hz 3-pole LPF, 512 SPS | 48 | 1.0 | 4 | 60 |
| B | 20-mV range, 0.7-Hz HPF, 160-Hz 3-pole LPF, 512 SPS | 120 | 1.8 | 10 | 150 |
We set the function of a transmitter during assembly and programming. We change cacitors on the board to set the filter frequencies. We change resistors to set the gain. We program the logic chip to set the sample frequency for each channel.
Last updated 14-MAY-10. Start with A3019 circuit with all SMT and TH parts loaded with the exception of the battery. We receive the A3019 in this state from our assembly company. For a layout of the components on the A301901A circuit board see here.
[07-APR-10] We receive our A301901A circuit boards and start building four boards by hand. We have difficulty with U1, the A1171 in a DFN-6 package. We can solder the package by applying no-clean BGA flux to the board, tinning the footprint pads, placing the DFN-6 on the footprint, and heating the chip from above with a soldering iron and a blob of solder as a conductor. With the soldering iron at 350°C we find that the A1171 magnetic switch provides erratic performance. We become confused between the quality of the soldering and the operation of the chip. All chips that functioned reliably afterdwards, through heating and cooling from further assembly and cleaning, we applied with the soldering iron at 300°C. One mode of failure was excessive current consumption, bringing down the junction of R1 and C1 to the point where the chip was improperly supplied. We suspect, but we are not certain, that the A1171 is particularly sensitive to heat, in the same way that we previously observed with the LC4064ZC in a BGA-56 package.
Having figured out how to get the DFN-6 on the board, we now worked on the BGA-5 of U2. We used roughly five chips for every one we mounted correctly. It is easy to smear the balls under the chip. But these chips do not appear to suffer from damage by excessive heat, so once they are on the board, they are reliable.
On 29-MAR-10, with 200 of our A301901A printed circuit boards in fabrications, we received notification from Digikey that the ASHK oscillator by Abracon had been discontinued by the manufacturer. We immediately attempted to buy 100 of the parts to tide us over, but we were too late. The only drop-in replacement we could find is the ECS-327KO by ECS, which will operate between 1.3 V and 5.5 V, although it is specified at 3.3 V. In our circuit it receives 1.8-V power. We bought 100 of these. They worked fine in the A3020A circuit, where we tied the tristate input U4-1 to the power supply U4-4. In the A301901A layout, we connect U4-1 to U6-A3. Pin U6-A3 is in the 3-V I/O bank of U6. It provides only 3-V or 0-V for its output. The ECS-327KO data sheet assures us that we can apply 3 V to the tristate input even when the power supply is 1.8 V. We confirmed this with ECS. So we figured all should be well.
We applied 2.7 V to U4-1 with U6-A3. This 2.7 V is the actual voltage on the 3VD power supply in our battery-powered tests. Current consumption increased by roughly 15 μA. With U6 programmed to HOLD its input levels, which is necessary for low current consumption, U4 does not oscillate. When we switch to PULL-UP inputs, oscillations start again. We conclude that U4 cannot drive a HOLD input when its tristate input is being driven with a voltage greater than its supply.
We cut the track from U4-1 to U6-A3 on three A3019A circuits. After a thorough cleaning, and assuming we don't short U4-1 to 0V with a probe, U4 provides 32.768 KHz. We conclude that we must cut this track on every A301901A circuit board.
Current consumption when OFF is 4.0 μA. We find that we can reduce current consumption when ON by 1.3 μA if we disable the test pin outputs.
By end of day, we have three working A3019As without amplifiers. All are programmed to digitize VCOM, with ID numbers 1, 2, and 3. No3 has a curious problem whereby we must short R2 or else current consumption jumps to 500 μA.
During the course of the day we replaced all logic chips at least once. We applied new chips by heating the board from underneath for two minutes, then blowing hot air from above for thirty seconds. Current consumption when active is between 70 μA and 80 μA. We recall problems with varying current consumption in the A3009A, where we were damaging the logic chip with our heat gun. We left two of the transmitters to run overnight.
[08-APR-10] Current consumption of our three transmitters is No1: 73 μA, No2: 80 μA, and No3: 69 μA. Noise on the analog signal is No1: 13 counts, No2: 15 counts, No3: 57 counts. We encounter no problems with U4 starting up when we turn on power. We add analog amplifiers to all three circuits. Current consumption is now No1: 73 μA, No2: 83 μA, and No3: 72 μA. Reception from all three is robust. We leave them transmitting.
[22-APR-10] We encapsulate two dummy circuits, an A3019A with the 48-mA-hr battery and an A3019B with the 120-mA-hr battery. Both require care to cover the protruding battery leads, but encapsulation proceeds well and the result is two transmitters with three coats of silicone each and volume 1.0 ml and 1.8 ml respectively.
[07-MAY-10] We receive nine A3019A from Advanced Assembly in Colorado. We expected a full panel of ten, but one board was "burned in re-work". The printed circuit around the missing board in the panel was scorched. The remaining boards had problems, as we describe in this e-mail. The A1171 chips on eight of the boards did not work. We re-mounted them and six of them worked erratically. So we think they were damaged by heat, but we cannot be certain because we have no fresh chips with which to replace them. The other two worked reliably. We ended up with three working circuits out of nine. We discovered that we could re-mount the BGA-5, U2, after taking its balls off, which we found much easier than mounting it with the solder balls in place.
During the course of our tests, we found that U4 can fail to start up properly when its enable pin, U4-1, is floating and we have the programming jumper across R2 removed. With the programming jumper, the chip starts up reliably, provided that you have not dragged the enable pin down to 0V with a scope probe in the previous two minutes, and provided that the board has been well-cleaned and dried. We now understand why our previous hand-made board No3 consumed excessive current with R2 un-shorted. The oscillator consumes excessive current when it starts up, pulling down the 1V8 supply by the voltage drop through R2. With U4-1 floating, the oscillator does not start properly and enters an unstable state that continues to consume current. We connect U4-1 to U4-4 so as to pull-up the enable line, and now we find that U4 always starts up, even with the initial voltage drop across R2, and futhermore we can look at U4-4 with a scope probe and have no effect upon the voltage on the enable pin. We have a choice of solutions to the start-up problem: replace R2 with 0 &Ohms; or apply a wire link to U4. We prefer to add the wire link. If we remove R2 we are left with the concern that noise from the 1V8 supply will contaminate VCOM, which we don't notice now with a gain of 100, but we may notice later with a gain of a lower gain. And with U4-1 floating, we have an unpleasant problem with the pin staying low even after we cycle the power.
[13-MAY-10] We receive new A1171 chips and replace those that we thought were damaged by heat. We see precisely the same erratic behavior. The voltage at U1-6 is around 0.8 V and U1 is unresponsive to a magnet. When we short R1 with tweezers, U1 starts up and works fine. We conclude that most A1171 chips will fail to start up most of the time when they are powered through a 50-kΩ resistor. We can either drop the value of R1 or short it when we connect the battery. If we drop R1, we will introduce more asynchronous noise onto VCOM from the A1171's internal switching clock. If we rely upon shorting R1, then we must trust that the battery power will be continuous during the life of the encapsulated circuit.
After three days of drying, two circuits with power consumption >130 μA are now consuming less than 90 μA. Another circuit that showed excessive consumption has some kind of printed circuit board error. We removed all parts from the board except the magnetic switch and still we see 300 μA consumption.
We retrieved transmitter No3 and found that its U4 was having the same start-up problem we observed elsewhere. But connecting U4-1 to U4-4 did not solve the excessive power consumption problem with R2 = 1kΩ. We see a step down of 0.5 V on 1.8V during transmission. The rising and falling edges of this step have time constant 1 μs, which suggesets R1 with C5 but in the absence of C4. We replaced C4. For some reason C4 is not connecting to the 1.8V supply. Consumption drops from 350 μA to 75 μA when we short R2 and we see no increase in input noise.
We now have 7 working boards out of the 9 sent to us by AAPCB. We decide to proceed with production with the circuit as-is. We will start the circuit by shorting R1 when we install the battery. We will apply a wire link to U4, which takes less than 5 minutes per board, including cleaning. The result will be a robust circuit with the minimum noise injected by the logic and switch circuits into the analog channel.
The following modifications are required by the A301901A circuit board.
