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Re opamp input capacitance AD8620 AD8211

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

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

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