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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
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

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