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Re opamp input capacitance AD8620 AD8211

Question asked by jdavis6809 on Jul 11, 2014
Latest reply on Jul 11, 2014 by jdavis6809

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