i am using ad8620 configured as a differential unity gain
buffer feeding a ad8221 instrumentation amplifier
the ac signal (100hz to 100khz) is fed to two resistors in
series
aim is to measure 1m ohm to 100M ohm
sense resistors are 100R, 1k, 10k , 100k, 1M and 10M ohm
switched via reed relays
1 resistor is reference 0.1% and the other is DUT
reed relays select which resistance is selected to be
measured by the differential buffer
all input components from the bncs to the buffer are in the
air to reduce leakage
guard opamps are used to eliminate input cable capacitance
the problem is when sensing high resistance > 100k
input capacitance is shunting the signal
do not know how to configure
buffers to compensate the opamp
input capacitance
also looked for dual fet with very low input capacitance in
dip8 or so8 input capacitance to high or
not specified
see circuit attached
thanks john