Instrumentation amplifier protection from high voltage transients with minimal signal degradation
The correct way:
R1 and R2 are the same, tight tollerance are used to limit the input current to the IA, if the last already has built-in protection ESD diodes. If your IA doesn't have the built in diodes, you have to add them externally. NOTE that diodes have quite large leak current, so probably you won't find any suitable (best option is Interfet Picoamp, FJH1100, BAV199 ). However you can replace the diodes wit JFET transistors and you can get leaking current as low as hundred femto amps.
The C1,C2 with R1,R2 form a lowpass filter. C1 and C2 have to have a tight tolerance and low temp. coef. like C0G/NPO. C3 with R1 and R2 forms a filter to reduce the mismatch of components R1,R2,C1,C2. C3 = 10 x C1/C2. Source figure 5-24



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