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Measuring Input Impedance with Multimeter

Jay He
Intellectual 280 points
Other Parts Discussed in Thread: OPA657

Hi, we're building a fully differential amplifier circuit using two OPA657 as the front buffers. It has some problems and while checking things out, we decided to look into the input impedance of the OPA657. The specs say it should be 10e12 ohm which means "open" on our Fluke 115 multimeter. However, out of 10 chips we have, 7 showed "open" while 3 showed ~ 30Mohm. Why is that? Does that mean there is some problem with the 3 chips showing 30MOhm?

 

Thanks,

 

Jay

  • 0
    TI__Genius 17905 points

    Jay,

    Do you find your circuit performance differs between the boards where the OPA657 input resistance measures as open versus 30 Mohm? Looking at the datasheet, the input resistance number is a C level test specification, meaning that it is most likely based on simulation and not tested in production. I have not tried measuring the input impedance of an op amp buffer circuit using a multimeter.

    Are you testing the 10 different chips on the same physical board (i.e. switching the part out), or are these 10 different boards?

    If you would like to discuss the issues of your full circuit and want to share schematics, etc, please e-mail me at k.flores@ti.com.

  • 0 in reply to Kris F
    Intellectual 280 points

    Kris,

     

    We're working on one board a time. So it will be on same board. We just picked up a few OPA657 and played with it. It will not be a scientific study anyway. So far what we used are the ones with "open" input resistance with the multimeter and they seem to work. We haven't tried any of those with 30MOhm impedance. So I guess this question is more a knowledge type. I'm hoping there is a definitive answer for this since this kind of FET opamp has been out for so long.

  • 0 in reply to Jay He
    TI__Genius 17905 points

    Jay,

    Using a multimeter to measure an op amp's input resistance will not give accurate results. A multimeter applies a current and measures the voltage to take a resistance measurement. Tenths of microamps of leakage current, whether in the board or in the chip, for example, if the multimeter current is going through the input pin ESD diodes, could easily give a reading of 30-Mohm. I would not take a multimeter reading of 30-Mohms as an indication of anything wrong in the part.

  • 0
    TI__Genius 13935 points

    I measured a Fluke 85 in resistance mode and the applied voltage on the test probes was about 0.6V, but it wasn't very consistent.  I agree that using a multi meter to measure input impedance is somewhat error prone.  When you get resistance on the order of 10^12 it takes pretty carefully controlled conditions to make accurate measurements.  Even flux on the PCB or humidity levels of the air can impact these measurements. 

    According to one article the surface resistance of FR4 is only around 3x10^12.  This makes the PCB a potential contributor to the measurement, especially in high humidity environments. 

    http://www.pcbdesign007.com/pages/zone.cgi?a=21182