CN-0350 is designed for a single piezo sensor input. I need to change the design so it is able to sample from two piezo sensors in parallel.
A simple solution would be to duplicate the analog part of the circuit and use a 2-channel ADC (e.g. AD7091R-2 instead of AD7091R). However I would prefer to keep that number of parts low.
The ADC AD7091R-2 exposes the output pin of its input multiplexer and the input pin of the actual adc and thus allows to place analog circuitry between them. So I am guessing if I can just place the whole analog circuit of the CN-0350 behind the input multiplexer and connect the two piezos directly to the ADC input pins. The output signals of the Piezo sensors are quite delicate (small charges) so I am wondering if the ADCs input multiplexer would somehow introduce problems.
If that does not work, would it be sufficient to use one charge amplifier circuit (U1A, R3, R4, C2) per piezo and place the second amplifier stage (U1D, R5, R8, R7, R6) behind the ADC input multiplexer? In that case I would require 5 opamps (-> so one more IC on the board), can't the reference voltage be buffered with a single opamp?
Side-question: whats the purpose of R5?
Your second suggestion is the better option.
Using your first option, the input multiplexer of the AD7091R-2 will be able to switch only one end of your piezo sensors. And you are correct…
Using your first option, the input multiplexer of the AD7091R-2 will be able to switch only one end of your piezo sensors. And you are correct the multiplexer on resistance and leakage current will affect the stability and high corner frequency for the input signal.
The AD7091R-2 has an internal reference buffer but it is recommended to use an external buffer. If you remove U1C, the reference output will drive the R1 and R2 divider. Figure 37 of the datasheet shows that it is only specified up to 100uA current load. You will have to change R1 and R2 values and consider the input bias current of the amplifier.