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OPA2132 Oscillation

Michael Huang3
Intellectual 2460 points
Other Parts Discussed in Thread: OPA2132, OPA1688, OPA2192

Hi team,

I use OPA2132 with ±5V supply, but oscillates.. with 3M+ Hz frequency. Rf =2.5k, Rb=400k. Power is filtered by caps and bead.

I remove the Rb, it is still oscillating. I reduce Rf as 0ohm, Rb not installed, oscillating...

I reduce the components around it and it can't work. finally, I lift the pins up and make it leave the PCB pads to make parasitic capacitor.  So now the pins VDD, VSS, IN+ are connected on the board, the IN- and OUT1 are up and floating. Then connect IN- and OUT1 with 2.5k resistor. Still oscillating.

I am confused why it oscillates with such small resistor. I try Rf=250k, it stops oscillating. I change to other opamp like OPA2192, OPA1688, it is not oscillating.

Could you kindly help look into it?

Michael

  • 0
    TI__Guru 62965 points

    Hi Michael,


    Please attach the schematic so we can properly diagnose the customer issue.

  • 0 in reply to Collin Wells
    TI__Intellectual 2460 points

    Colin,

    Above is the schematics that I describe in words. Totally delete the input circuit, but still oscillating.

    Regards

    Michael

  • 0 in reply to Michael Huang3
    TI__Guru 64530 points
    Micheal,
    Please add two 1uF decoupling caps between ground and each power supply pin and replace RF 2.5k feedback resistor with a short.
  • 0 in reply to Marek Lis
    TI__Intellectual 2460 points

    Lis,

    Thanks for your reply.

    According to the former post, the VDD/VSS is decoupled with bead and 0.1uF caps. And BTW, the cap is very close to it.  There are many components in the original circuit, I remove one by one and find even there is only a Rf 2.5k, there is still issue there.  Replace to other opamp, this issue doesn't happen any more.

    And I post this question just want to know why a 2.5k Rf will cause the oscillation but 250k will not. Could you kindly help do a experiment to reproduce it in your lab.  In common sense, a larger Rf will cause instability easier.

    Regards

    Michael

  • 0 in reply to Michael Huang3
    TI__Guru 62965 points

    Hi Michael,

    You're correct and a larger feedback resistor will cause worse stability issues in an op amp circuit. I wonder if based on the amount of flux and other PCB contaminants present if a parasitic capacitor is being formed in parallel to the feedback resistor. When the resistor is increased to 250k, the capacitor interacts with it to create a pole that add some phase margin to the system. It's probably still not stable, but won't maintain a sustained oscillation either.

    In either case, add a capacitor in parallel to the feedback resistor to solve the issue with all of the op amps. The capacitor will limit the bandwidth at a frequency of 1/(2*pi*RF*CF).

  • 0 in reply to Collin Wells
    TI__Intellectual 2460 points
    Hi Collin,

    Thanks for your reply. You are right, a small cap in parallel with the Rf 2.4k can easily make it stable. In my experiment, to eliminate the flux or other factor affect the stability, I raise up the pins of the IC. Pins 1 2 3 are raised and leave the pads. The resistors are 0603 size and can be placed between OUT and IN- without a wire. IN+ is wired to GND.

    Regards
    Michael
  • 0 in reply to Collin Wells
    TI__Intellectual 2460 points

    Hi Collin,

    I made a small board, and solder OPA2132 on it. No copper pour, no flux, only a buffer with Rf=2.4k. VDD/VSS are decoupled by 0.1uF. IN+ is grounded.

    Strange! Output is a 4M sinewave, but IN- is a 3M sinwave. If this is a new sample, I will think this chip is broken...

    I know the cap can make it stable, but why the waveform between the 2.4k resistor are different???

    Regards

    Michael