similar to C22, but on LT1797 IN-.  This better balances the diff inputs, and should reduce noise.  Can always depop if it causes issues.

Add LPF to opamp output, to filter noise sent to MEGA2560 ADC.  10k+100n should be good (tau= 1 ms)

Also note that input resistance isn't identical on opamp inputs.

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Solution:
Added Differential capacitor
Added LPF