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    Offset correction of current to voltage amp configuration

    I have a classic I/V setup with 1 megohm feedback using an ADA4177.

    Normally the non-inverting is to ground but I would like to inject my offset bias into this input. Is this an ok approach or is there a better method for this configuration.

    I'm only interested in DC current conversion, no bandwidth requirements.

    I would tie the + input to gnd with low resistance (<50 ohms) and apply offset voltage direct to + input via a high value resistor.

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    Re: Offset correction of current to voltage amp configuration

    Why not just use a zero offset, stabilized op amp instead?
    The typically have only a few microvolts of offset.

    How low an offset do you need?

    The problem with applying a correction offset is that it doesn't track changes in offset due to temperature or time.
    Also you need a stable voltage source to generate the offset.
    But you can apply the correction at the plus input as you propose.
    Zapper
    Curmudgeon Elektroniker


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    Re: Offset correction of current to voltage amp configuration

    Your proposal should work fine. Using a low resistance resistor is generally a wise decision..... But I ignore whether the input bias current of the ADA4177 is low enough, such that it can be ignored with such large-valued feedback resistor.
    Perhaps you may have to add an equivalent series impedance to the non-inverting input.
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    Re: Offset correction of current to voltage amp configuration

    Thank you for the reply..

    I'm using this amp due to it's built in over voltage protection as it will be used to measure the leakage current of a 15KVDC hi-pot tester. The transients produced during breakdown have thwarted many OVP methods so far..

    The simulated offset is only about 350uV but the resulting "current" is displayed on a 200mV meter so I want to tweak that error out.



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    Re: Offset correction of current to voltage amp configuration

    Thank you for the response..

    The simulation in LTspice seems ok but I will try it out (bias cancellation resistor).

    Ib is about 1nA for this op amp and as I mentioned above it's a robust amp with built in OVP. This is for a high voltage test application and I have lost plenty of amps :(



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    Re: Offset correction of current to voltage amp configuration

    First, what value resistor do you have in series with your measurement? What's breaking down?

    Second, I'm wondering when and how you're going to use this offset circuit you're proposing?
    -When the system is initially built?
    -During re-calibration?
    -Every time the product is powered on?
    -Right before making a measurement?

    These all determine what type of amplifier is best (Absolute offset versus offset drift)

    Finally are you sure you need analog offset cancellation rather than doing it digitally? Instead of tweaking your analog offset circuit simply record the measurement with no input and subtract that amount out in the future (it can be stored in an eeprom for example).



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    Re: Offset correction of current to voltage amp configuration

    Thank for the response.

    The resistance in series with measurement node is in the tera-ohm range, unless it breaks down of course. It's a 15KV hipot so 75G will give me full scale 200nA = 200mV in my configuration.

    The offset will be adjusted at build and re-calibration if needed.

    Digital would be nice but I'm not that sophisticated nor is this device. It really just needs to be robust as we are testing long lengths of HV coax. The breakdown transients in coax can be horrendous.

    Though maybe not the best amp for I to V it is intended for use in harsh environments like this. The simulated offset is very small (350uV) but enough to put me out of tolerance.



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    Re: Offset correction of current to voltage amp configuration

    To amplify tiny volt levels across a resistor, there is also common-base operation of a transistor. It produces high voltage gain, from a low input resistance.
    Changes are applied at the emitter terminal, which achieves a change in current through the base, although it occurs indirectly (whereas a conventional amplifier applies the signal directly to the base).

    It has an offset problem however the offset is toward one polarity (referenced to the supply rail). Thus the offset may be more predictable, and simpler to deal with, as compared to the op amp offset.

    Temperature affects the transistor, which needs be adjusted or compensated somehow.

    Here is a simulation demonstrating the concept. It appears to work with a darlington arrangement. It may or may not be adaptable for your project.

    Click image for larger version. 

Name:	measure microAmperes via common-base darlington NPN operation (experimental).png 
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    The signal comes from a source ranging between 0-200V. Notice it sends max 200 uA through the sense resistor, creating 227 mV across it. This is amplified to 3.64 v.



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