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So this would be something to measure brain waves, similar to an EKG?
If so, the circuit requirements are then very demanding. Very high gain, fully differential amps, low noise.
Designing such a high performance circuit was done, but the complexity was astronomical. Back in the 1980s I saw an all transistor EKG unit, it contained over 500 discrete transistors. To prevent board leakage, it had Teflon inserts where the critical discrete components were mounted.
Also, it required about half an hour of power up for thermal stabilization. Followed by a nulling procedure.
Opamps, integrate very high performance, thermally matched components. Leakage and parasitics constrained within the package, orders of magnitude better than what a discrete design can achieve.
If a microcontroller is included, then self-nulling and tuning are possible.
Nowadays, but I should mention that I've not yet seen one personally, there is an analog front end, which feeds the signal into a precision ADC.
All the conditioning and analysis is performed in the digital domain.
Yes, the trend in electronic signal processing is to do as much of the processing in the digital domain as possible.
So there is just enough analog processing of the analog signal, such as a front-end preamp for amplifying, filtering, etc., so that the signal can then be A/D converted for further digital processing.
For example, this can be seen in HDTV receivers, where there is an analog signal tuner (digitally controlled) to select and amplify the desired digitally modulated analog RF signal, which is then demodulated for digital processing of the signal the rest of the way to the screen and speakers.