With the headset printed and assembled I think I might've actually acquired a motor rhythm. Headset shown in image below.
At this point I'm going for the minimum demonstration of functionality, so my PCB is just secured to the headset with twisty-ties, and the ESP32 is mounted with an adhesive breadboard. Eventually I'll want a 3D-printed enclosure at the back to hold a battery, and a board with a charging circuit and breakout headers to mount the ESP32 dev board to.
So in this configuration I had the driven ground clipped to my left ear, the reference channel going to the top of my head (Cz in the 10-20 system), and the recording channel positive input going to the left motor cortex (C3). The two scalp electrodes are these shorter spiky Ag/AgCl electrodes, although I have the longer ones too that I will use in the future. Also worth mentioning I'm just holding a usb battery in my hand and powering the whole system through the ESP dev board's usb jack. I also had the openbci comfort insert thingies (printed in TPU because I'm worth it) in the C4 and Oz(?) positions to get a pretty snug fit.
So I wouldn't be providing this update unless I had a recording that looked like an EEG signal, so here it is:
For this recording I was imagining playing piano with my right hand, since that would in theory produce a motor signal in the left motor cortex. I had to run the signal through a 60 hz notch filter to see anything other than noise, but after the filtering it looks like there is a ~500 uV peak-to-peak rhythm. That may be too strong to be EEG and could just be from motion or scalp EMG though, hard to say. I'm also confused by the DC offset, I'm not sure how that got through the amplification stage. But either way, it's reasonable enough to say that the amplifier circuit and communication with the ESP32 are hunky dory, so I'll probably start putting together the impedance measurement circuit now and completing the rest of the amplifier channels on this board. Ideally I'll be able to plop a second MCP3912 module on the same SPI bus and get 8 channels of data without much extra effort, but I'm still a pretty long way from taking that step.