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High Input Impedance op amp

Dear all,

I am measuring the resistance of a large resistor (up to several Mohms) using 4-wire terminal. The resistance is so large that it surpasses the input impedance of my voltmeter (100MOhm). So I am looking for an differential operational amplifier that offers a large input impedance (>2GOhm), that could take input spans several microvolts up to 0.5 volt.

Could you offer me some products that fits into our application?

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  • You will also need to consider the effect of bias currents in the opamps (and offsets in these) , if you can reverse the polarity of the current source, then averaging forward and reverse resistance values will remove these effects.

    You may also need to consider

    (a) using teflon wiring

    (b) using teflon PCB's

    (c) using driven guards around the input signals

    (d) careful washing of PCB to remove flux residue, and conformal coating of PCB

    The above measures help with the effects of (surface) leakage currents.

    Traditionally electrometer front ends used a flying capacitor , and a servo-ed voltage supply. The servo adjusts the voltage supply until there is no capacitor current when it is switched from (your resistor) to (voltage supply) .  This circuit produces a single sided voltage that can be easily measured in the presence of large CM noise.

    An EDN article by guru Jim Williams is interesting reading, and has a flying capacitor bridge in fig 26.11 of http://www.edn.com/Home/PrintView?contentItemId=4390838   The advent of high performance AD instrumentation amplifiers has pretty much relegated the flying capacitor approach to the realm of the dodo. I think HP (Agilent) used to make a hybrid flying capacitor module.

    Google " KeithleyLowLevelHandbook_7Ed.pdf  " for a useful handbook on low level measurements, it describes guarding.

    There's some interesting reading about error terms scattered through here http://www.analog.com/media/en/technical-documentation/technical-articles/489168400sscsect3.PDF

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  • You will also need to consider the effect of bias currents in the opamps (and offsets in these) , if you can reverse the polarity of the current source, then averaging forward and reverse resistance values will remove these effects.

    You may also need to consider

    (a) using teflon wiring

    (b) using teflon PCB's

    (c) using driven guards around the input signals

    (d) careful washing of PCB to remove flux residue, and conformal coating of PCB

    The above measures help with the effects of (surface) leakage currents.

    Traditionally electrometer front ends used a flying capacitor , and a servo-ed voltage supply. The servo adjusts the voltage supply until there is no capacitor current when it is switched from (your resistor) to (voltage supply) .  This circuit produces a single sided voltage that can be easily measured in the presence of large CM noise.

    An EDN article by guru Jim Williams is interesting reading, and has a flying capacitor bridge in fig 26.11 of http://www.edn.com/Home/PrintView?contentItemId=4390838   The advent of high performance AD instrumentation amplifiers has pretty much relegated the flying capacitor approach to the realm of the dodo. I think HP (Agilent) used to make a hybrid flying capacitor module.

    Google " KeithleyLowLevelHandbook_7Ed.pdf  " for a useful handbook on low level measurements, it describes guarding.

    There's some interesting reading about error terms scattered through here http://www.analog.com/media/en/technical-documentation/technical-articles/489168400sscsect3.PDF

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