Connect with us

input capacitance opamp non inverting buffer

Discussion in 'General Electronics Discussion' started by jdavis6809, Jul 9, 2014.

Scroll to continue with content
  1. jdavis6809

    jdavis6809

    2
    0
    Jul 9, 2014
    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
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    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
    Adam
     
  3. jdavis6809

    jdavis6809

    2
    0
    Jul 9, 2014
    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
     

    Attached Files:

  4. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    Great, let me have a look.
     
  5. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    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.
    Adam
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-