Circuit diagram of the 1st stage bio amplifier. We use the INA126 instrumentation amplifier to measure the differential voltages between two electrodes. Also recommended is the pricier INA128 which has built in input protection and has a lower noise floor. Note that they have different ways of setting their gains, mainly because the INA126 has internally two OP-amps and the INA128 three OPamps. These amplifiers need a minimum amount of components: In the simplest case just the two blocking capacitors.