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Original question:

OPA4197: INPUT IMPEDANCE

OPA4197: OUTPUT NOT STABLE

meriton
Prodigy 140 points
Part Number: OPA4197

Hi There,

   Now we use OPA4197 and ADC IC for voltage output, but we find the output waveform is not stable. Please help to check. Not sure why this happens? 

  ADC 441 generate a 2.5V reference voltage for OPA4197. 

   Could I add a 10uf cap (connect to GND) on the OUTPUT B, C D for getting the stable outputs?  Thanks, 

   

   

  • +1
    Guru 140200 points

    Hi Meriton,

    connecting a big capacitance directly from the output of an OPAmp to signal ground is no good idea. This will erode the phase margin and can cause stability issues. Is there already a capacitive load connected to the outputs of OPAmp, by the way? Can you show what is connected to the outputs of OPAmps?

    I understand the second picture in that way, that the output voltage is drifting with time? Is that what you mean by "not stable"? What is the time scale? If it is seconds, minutes or even more a low pass filter will not help anyway.

    I don't think that it has to do with the OPAmp itself. I think it is the imbalance of gain determining resistors and the imbalance of resistor drifts. What resistors do you use? I would take +/-0.1% or +/-0.05%  toleranced resistors with +/-25ppm/°C or +/-10ppm/°C temperature drift.

    Kai

  • 0
    TI__Guru* 93105 points

    Hello Meriton,

    The voltage drift you are observing at each OPA4197 output doesn't look anything like what would be expected from this precision op amp. The OPA4197 has very low Input offset voltage drift, on the order of a few µV/°C or less, and the graphs appear to show high drift over the time period shown. As Kai mentioned we don't know what the time scale is? Kai's questioning of the resistor temperature coefficients is a valid concern.

    My thought is the op amps you are evaluating need to be confirmed as being true TI OPA4197 product. Was the product purchased directly from TI, or one of TI's authorized distributors? There are a lot of gray market op amp products being sold as legitimate product, but in reality are low grade products that provide much lower performance. This has been more of a problem because of the worldwide semiconductor shortages.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to kai klaas69
    Prodigy 140 points

    Hi Kai,

         Please refer to below information, and help to check ,

         1.  Is there already a capacitive load connected to the outputs of OPAmp, by the way?  ---- No, for the waveforms I showed, no cap connect to the output of OPAmp.

       2.   Can you show what is connected to the outputs of OPAmps?  ---- a relay on ETS88 test machine.

    3. the output voltage is drifting with time? Is that what you mean by "not stable"? ---- yes.  

    What is the time scale?  ----  400ms/point.  total 2000 point. every 400ms meaure voltage.

    4.  What resistors do you use?  ---- resistors with 0.1% accuracy. But not sure the temp drift, we will check.

     

  • 0 in reply to Thomas Kuehl
    Prodigy 140 points

    Hi Thomas,

        You could refer to my response below. Thanks. 

  • +1 in reply to meriton
    Guru 140200 points

    Hi Meriton,

    thanks for the feedback.

    Thomas has made a very good point: Is it possible that you have a fake OPA4197 in your circuit? Don't laugh, this can happen faster than you may think.

    To find out whether it has to do with the OPAmp, you can run for a test all the OPAmps as voltage follower with a gain = 1. For doing this, carefully (!) remove R11, R14 and R17 and replace R15, R20 and R21 by short circuits. Please try to avoid any overheating of the OPAmp during this procedure! Or by other words, don't directly touch any of the pins of OPAmp with the soldering iron.

    After making these modifications you can do another measurement and show us the results Relaxed

    Kai

  • +1 in reply to kai klaas69
    TI__Guru* 93105 points

    Hi Meriton,

    Kai's idea of testing the OPA4197 op amp sections in a unity gain follower configuration is a very good one because it takes any resistor drift out of the test circuits. Then you should only observe the op amp voltage offset drift which should be very low for a real OPA4197.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to kai klaas69
    Prodigy 140 points

    Hi Kai,

        HAHAHHA, got it.  we will check. I will check if this is a fake one.  Thanks.

  • 0 in reply to Thomas Kuehl
    Prodigy 140 points

    Thanks, Got it, I will check.

  • 0 in reply to kai klaas69
    Prodigy 140 points

    Hi Kai,

      Sorry for reply late.  we guess we buy the fake OPA4197. Thanks a lot for your comments.

  • 0 in reply to Thomas Kuehl
    Prodigy 140 points

    Hi Thomas,

      Sorry for reply late.  we guess we buy the fake OPA4197. Thanks a lot for your comments.

  • 0 in reply to meriton
    Guru 140200 points

    Thanks for the feedback and good luck Relaxed

    Kai