Capacitor connected not ADC

[gjoo]

I'm a little confused on why c21 is connected to the negative terminal of the adc and not grounded and how does it work with c21. What is c21 purpose in this circuit.

 

[gjoo]

How do C21 and c22 work together?

 

[Harald Kapp]

C22 + R23 + R27 form a differential RC low pass filter for the differential signal from U4A and U4B.
In addition R27 +C22 form a single ended low pass filter from U4A to GND.

 

[gjoo]

Thank you