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OPA690: distortion in output

Minghao Yin
Prodigy 365 points
Part Number: OPA690
Other Parts Discussed in Thread: OPA695,

Hi team, 

this is our customer schematic diagram. U15 is a analog switch. 

This is waveform, the above waveform is output, the below waveform is input.(R54-200ohm, R60-39.2ohm, R53-22ohm)

Then customer change the resistance.(R54-2000ohm, R60-374ohm, R53-22ohm), the distortion is disappear. But customer complain consistency is not good. 

Then customer change OPA695 but there is a small oscillation.

Is there any recommend resistance with the near amplify times?

And why the distortion happened in OPA690? Why oscillation happened in OPA695?

  • 0
    TI__Genius 13190 points

    Hi Minghao,

    From looking at the input signal and the output in the gain configuration the device it is in, it does not seem like it is a distortion problem as the device is not really being forced into any saturation state. From seeing the circuit and the circuits' behavior it seems like it might be a stability issue. As a quick debug step is it possible to remove the mux and short the circuit so the R60 resistor is being implemented. The concern is that the mux at the inverting input of the device might be causing some stability issues by adding parasitic capacitance to the circuit. For the OPA695, Table 1 in the datasheet highlights recommended feedback network resistor values for different gains.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Prodigy 365 points

    Hi Ignacio,

    I short circuit the analog switch, and change the amplifier to OPA695, it seems some board OPA output is better.

    But customer give a feedback that some board output performance better. While some are not very well.

    Is there any other ways could cause that problem? How to change the board ?

  • 0 in reply to Minghao Yin
    TI__Genius 13190 points

    Hi Minghao,

    It is good the customer is seeing better results by removing the switch. I have a couple debug steps you could try for the OPA695. When you shorted the switch do you still have the switch in place? This could still be adding parasitics to the inverting node and causing stability problems, is it possible to completely remove the switch completely as a quick test? In the same configuration with the switch removed, I would try changing the feedback network to resistors closer to the recommended values for the OPA695. In a gain of around 6V/V, this means implementing a feedback resistor around 430ohms or 470ohms. I would also debug the device first by removing the next stage after the OPA695, therefore removing R53 and directly probing at the output. By doing so you can narrow down where the sources of error are coming from in the design and debug the circuit one step at a time.

    Best Regards,

    Ignacio