Determine capacitor value gives amplifier characteristic independent of frequency
Resistors alone make a voltage impedance divider.

Capacitors alone make a voltage impedance divider.  


(common practice i scope 10:1 probes and 100k:1 HVAC voltage sensing bushings.)


Z(f) for RC source therefore must match Z(f) load ratio for RC net impedance divider for matched flat attenuator.


I'll let you do the simple math.  Normally R2>>R1


-  for added bonus


estimate tolerances on R1//C1 and for a fixed   R2, what is tolerance error on C2's effect on attenuation error vs f?
assume average saline-presoaked needles for 15 minutes are 1.4 megohms  at 10Hz to 6.6 kolohms  at 10kHz.  
What is optimal C2 R2 and tolerance sensitivity at 10kHz? from R1C1?  per unit  or per % error on BW?