The input stage is the most critical for common-mode rejection and noise. We used a Burr-Brown INA121 differential amplifier, set to a gain of 10. The input stage is DC coupled. With a 9-volt supply, a differential DC offset of up to about a volt is tolerable. Offsets of a few hundred millivolts are typical, assuming similar metals for the differe
PREAMP - schematic

ntial electrodes. The common-mode rejection of the circuit is as good as the Burr-Brown head-stage. Any one of the opamps in the circuit can be removed (along with its supporting resistors and capacitors) if the function is not needed. For instance, if the 60 Hz notch filter is not needed, output may be taken from the output of the third op-amp.

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