Please note: Although this question mentions an electronic circuit depicting an EEG, I believe this is purely a bio-electrical question that is appropriate for this site, and should be answered by any battle-weary Biomedical Engineers that have had experience with EEG.
I stumbled across this DIY EEG Instructable, which is interesting and novel, but which has a number of drawbacks (from an EE perspective).
What surprises me most about this circuit is the constant amount of correction/filtering that the author is doing at each step:
- Two 60Hz notch filters one at each "end" of the circuit, to reduce noise at a particular frequency (do we really need two?)
- High pass filter for circumventing galvanic skin response
- Low pass filter for waves > 30Hz
- Yet another high pass filter
- Op-Amp
To me, it makes sense to digitize the circuit upfront (by running the analog brain waves through an analog-to-digital converter, or ADC).
This leaves me feeling that perhaps there is some intrinsic/inherent value to EEG evaluation of analog (as opposed to digital) waves. So I ask: is there a compelling reason to tune/correct/filter raw analog brainwave/analog signals for EEG, or is it perfectly fine to digitize the waves and then process them?
Answer
You MUST do analog processing to make the signal suitable for sampling by an ADC. Do the math. What happens when you have a 12 bit ADC covering 3 volts? Is there sufficient resolution to handle 0.1 mv signals? – No. The LSB in such a case is about 0.7mV. At 16 bits, your signal would span a bit more than 2 LSBs. There must be some amplification, probably by a factor of about 500, to give you enough LSB's to sample effectively, and you need to condition the signal to remove offsets to be able to get a gain that high.
So, attempting to sample without analog preconditioning is probably misguided here.
There is some argument to be made that sampling unconditioned with 24-bit or higher ADC's is OK, but I think that an instrumentation amplifier input stage with modest gain, which is well optimized for high common mode rejection ratio, is a fine thing.
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