Piezo input stage for drum triggering
C1, R1 are a high-pass, meaning the circuit is an edge detector. D2, C2, R4 in contrary is a charge collector / timer circuit, which is meant to transform the height of the input peak into a length. The u*t area being the same on input and output.

That is done to help the sample&amp;hold circuit in the following ADC to have a constant input value rather than an arbitrary one depending on whether it is synced to the beat of the drummer or not. For your ADC control, that means you have to sample each input circuit a number of times in a row and add up the values to get a better reasoning on the height of the input peak.

This limits the number of channels you can multiplex for a given ADC.