# input capacitance opamp non inverting buffer

#### jdavis6809

Jul 9, 2014
2
hello

using 2 non inverting opamp buffers (AD8620) to read the voltage across 2 resistors connected in series

1 reference resistor and 1 resistor DUT via reed relays (reed relays are switched in turn to read each resistor)

Output of opamp buffers feed an instrument amplifier AD8211

The input of the buffer opamps are wired to the reed relays via 1k resistors in the air to minimize leakage

All of the input circuit from the bnc's to the non invering inupts of the opamps are wired off board to minimize leakage

I know how to offset the input cable capacitance by using a guard opamp

the plan was to be able measure the impedance from 1m ohm to 100M ohm driven by 1vrms sine wave 100Hz to 100Khz

I could not fine much information on the internet on how to neutralize the input capacitance for a non inverting opamp buffer

impedance measurement up to 100K ohm ok

thanks

john

#### Arouse1973

Dec 18, 2013
5,178
You don't actually say what your problem is, but I am guessing you are not getting the desired results you were after. It's really difficult to comment without seeing a circuit diagram. Please supply a clearly drawn circuit diagram with the following:

1) List any measured voltages and label point where measured or print them on the drawing linked to the circuit point.
2) List what you think they should be.
3) Clearly define component indent's, i.e R1 C2 etc.
4) Clearly define component name and value, example. 100R, 200uF, LM**** PIC18**** that sort of thing.
5) List any formulas and workings you might have used to calculate component values if any.

This really speeds up the response of someone trying to help.

Cheers

#### jdavis6809

Jul 9, 2014
2
hello

uploaded a circuit diagram (not complete just shows the front end)

this from my simulator

prototype has been built

problem I have is when range resistor is above 1M ohm and DUT is same value

output voltage should be the same but it is not due opamp input capacitance

I have compensated for the bnc kelvin probe cable capacitance (guard amps)

I want to know how to compensate for the opamp input capacitance

frequency range 100Hz to 100Khz

input voltage is 1v rms sinewave

#### Attachments

• LCR Meter Front End 1.pdf
156.8 KB · Views: 118

#### Arouse1973

Dec 18, 2013
5,178
Great, let me have a look.

#### Arouse1973

Dec 18, 2013
5,178
You could try and place a small capacitor across each sense resistor, similar to how a scope probe works. You might only need these with the larger values of resistors. Also run the resistance measurement part of the LCR at a much lower frequency.

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