In my quest for premature optimization, I tried out yet another instrumentation amplifier chip: The Microchip MCP6N11-100. At 1.55€, this chip is about 4 times cheaper than the INA128 instrumentational amplifier (which is currently the cost bottleneck), and would make a 16-electrode-PsyLink about half as expensive. It also works with as little as 1.8V (while INA128 requires 4.5V), eliminating the need for power conversion if a 3.7V lithium battery is used.
Unfortunately the pin configuration is different from the INA128 that the Electrode Module 3.1 normally uses, so I had to jury-rig the board a little:
But it worked, and the signal (the first one) is similar enough to those of INA128s (the last 4):
These were recorded with a Prototype 9.1 with two Electrode Module 3.1, one with 1x MCP6N11-100 at a gain of x501, and one with 4x INA128 with gains of x501 as well. I'm wearing it much like on the P8 demo video, except that there's two electrode modules side by side, not just one.
In the top center of the first picture (light blue signal), you can see that there is some clipping in the negative peaks of the wavelet, which is not great, but other than that, it does seem to show very similar signals. (They shouldn't be identical, of course, because each signal measures electrodes at different locations on the arm)
There's also a similar chip, the MCP6N16, which has a smaller footprint size and better performance, but has slightly higher cost and worse availability. It's more difficult to solder, too. But it's good to know that there's the option to switch to that one if we ever need to.
I'm quite happy with this result, and perhaps the next iteration of the electrode module will be designed for this chip rather than the INA128. :)