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OPA855-Q1: Why the recovery time of the OpAmp becomes longer?

Zhiyao Liu
Prodigy 80 points
Part Number: OPA855-Q1
Other Parts Discussed in Thread: OPA858-Q1, OPA858

Hi together,

figure1 is my circuit schematic. My circuit is having a problem. After switching the channel of the switch chip, The output recovery time of the OpAmp (OPA855-Q1)  becomes very long，I tried a new op amp chip. The recovery time is much shorter. It seems that there is a problem with the chip. May I ask which performance indicators will affect the recovery time?

figure2 shows the input signal and output signal of the abnormal chip.

figure3 shows the input signal and output signal of the normal chip.

Thanks!

Zhiyao

  • 0
    Prodigy 80 points

    Oh ，there may be some question with my description, the OpAmp is OPA858-Q1 not OPA855-Q1. I'm sorry for my carelessness.

    Thanks 

    Zhiyao

  • 0 in reply to Zhiyao Liu
    Guru 140200 points

    Hi Zhiyao,

    leaving the common mode input voltage range and the linear output voltage range will result in abnormal settling times. Too high output loads can also result in abnormal settling times. Because of that I would increase R7.

    Also keep in mind that a decompensated OPAmp may not like C4 in the feedback loop.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi Kai，

    Will a larger resistance value of R7 increase the recovery time? We have produced many PCBAs for this circuit, And there has been only one failure.

     this 'failed' chip can reproduce the problem when placed on any PCB, so I doubt that this chip fails.

    And witch performance may cause this problem.

    Zhiyao

  • 0 in reply to Zhiyao Liu
    Guru 140200 points

    Hi Zhiyao,

    Zhiyao Liu said:
    Will a larger resistance value of R7 increase the recovery time?

    No, you misunderstood me. For very high frequencies C5 presents a short circuit. This means that the output of OPA858 sees a 33R impedance connected to signal ground, which will overload the output of OPA858. And this overload can abnormally increase the settling time. Then an increase or R7 (and/or a decrease of C5) can help to avoid an abnormal increase of settling time.

    Zhiyao Liu said:
    We have produced many PCBAs for this circuit, And there has been only one failure.

    Yes, but components and chips show manufacturing tolerances and there can be one outlier showing an abnormal performance.

    By the way, do you have carried out a phase stability analysis in the meantime to see whether C4 causes instability? Remember, installing a feedback cap in the feedback loop of a decompensated OPAmp is usually no good idea.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi Kai,

    Thank you for your explanation, I agree with you on question 1. About question 2 ,Yes I have carried out a phase stability analysis,.For some special reason, some of the capacitance/resistance in the picture have been modified, 

    Alao I agree there maybe manufacturing tolerances, but can you list some  manufacturing tolerances that affect this problem.

    Thanks

    Zhiyao

     

  • +1 in reply to Zhiyao Liu
    Guru 140200 points

    Hi Zhiyao,

    after running a simulation I realize that you massively drive the output of OPA858 into saturation. Remember that R5 and R6 give a gain of 61V/V !

    zhiyao_opa858.TSC

    With a much smaller input amplitude and without driving the output into hard saturation the settling time heavily decreases. Keep in mind., though, that because of the long tail of input signal and the very high gain, the output voltage cannot immediately go to zero but must follow the input signal. So the output signal must also show a long tail. Because of this, the long tail of output signal is not the effect of a long settling time:

    Here the settling without long tailing input signal:

    Here without output low pass filter:

    And -finally- with a much lower gain:

    So, what you assume to be ultra slow settling is simply the consequence of a long taling input signal in combination with a very high gain and a hard saturation of output stage.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi kai,

    Thank you for your explanation. I will try to adjust my circuit.

    Zhiyao