I've been wondering about this for a while.... have you tried?

I'm not a EEG designer but I've experimenting and learning about it. Sounds like you make PiEEG? This is really cool and I appreciate the docs on the github.

After amplification, you could get the signal 0-100uV to ~0-Vanalog; so then the entire 8-bit range could ideally be used. Realistically, after filtering, due to roll-off / artifacts one could probably use 3/4ths of that space -- so ~ 192 voltage levels that can be determined by the ADC per sample. Does this sound right?

It'd probably depend on what level of control / reliability one would want from the EEG data; I imagine that you could probably cut a lot of corners if, say, you were only measuring for hemispheric coherence in a small set of frequency bands, or stages of REM sleep (and maybe ERPs like p300).

I don't know, this is in my upcoming experiments. Originally this was an attempt to build an EEG amplifier / filter circuit for an atmega328p 10-bit ADC but for my purposes I settled for a 12-bit (and possibly hardware oversampling) on an EFM32.

With an active electrode, it should be good enough. After all, we are talking about hobby-grade results.

The Olimex ones are 10-bit (supporting both active and passive electrodes), while it's claimed[0] that 12-bit resolution is sufficient for EEG.

However, it's worth noting that there are obvious advantages to the higher resolution.

[0] https://www.biosemi.com/faq/24bitsystem.htm

He mentions active probes. Active probes have amplification.

As his link makes clear, your ADC just needs enough resolution to measure the signal when the (amplified) offset voltages don't saturate it.

yes, I told about digital EEG device from this website. To clarify analog device, I need a little more time