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OPA992: TIA: Can differential input impedance of OP-AMP impact Vo?

DeWei Lan
Prodigy 250 points
Part Number: OPA992
Other Parts Discussed in Thread: OPA928, ADS7953, OPA994, OPA991

Recently, I design Hi-Z TIA as this picture:


The feedback resistor is 100Meg ohm

I read datasheet for OPA992 which mentioned its *differential input impedance* is 100Meg same to my feedback resistor



I also read the OPA928 data sheet and I found this application circuit:



For this case, feedback resistor up to 10G ohm, but OPA928's *differential input impedance* still be 100Meg ohm.

Is there no problem when the differential input impedance is the same as the feedback resistor?

  • +1
    TI__Mastermind 23310 points

    Hi DeWei,

    DeWei Lan said:
    Is there no problem when the differential input impedance is the same as the feedback resistor?

    Well it depends. I don't believe there's anything inherently wrong with doing that. Do remember that the larger the values your resistors are, the more that your input bias current will affect the op amp's output voltage. However, for TIA applications that's a given since you need to have a larger feedback resistor. 

    Have you run this in simulation to verify stability of the op amp? I see a potential problem with this op amp driving a 4.7uF capacitor and only having a 27ohm isolation resistor. 

    Best Regards,

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 250 points

    Hi :

    Thank you for your reply!

    Regarding 4.7uF capacitor,

    I've test it on the previous board and obtained the signal I wanted.

    Since you mentioned the 4.7uF capacitor, I'm interested in it.

    I've read datasheet:

    Higher capacitance result in a higher ration of overshoot.

    However, my system only requires a 100Hz bandwidth for analog signals (even 10Hz I though would be enough)

    , so I can't identify any issue about overshoot.

    Is there another potential problem if I put 4.7uF?

    Best Regards!

    De Wei

  • 0 in reply to DeWei Lan
    TI__Mastermind 23310 points

    Hi DeWei,

    Overshoot is a symptom of the op amp's instability. We measure that in phase margin. We have a whole series talking about op amp stability located here: https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html. I highly recommend watching through the videos to get a better understanding.

    For the abridged version, you want to make sure that your op amp is stable enough not to distort your signal beyond what your system needs. We generally recommend a phase margin of 45 degrees to account for process variation. 

    Best Regards,

    Robert Clifton

  • 0 in reply to Robert Clifton56
    Prodigy 250 points

    Hi :

    Thank you for your suggestion.

    Now I understand the issue of stability.

    Actually, after OP-AMP, it would be connect to ADC: ADS7953.

    This circuit is a front-end of ADC.

    Now I know the phase margin limit capacitor about <70pF as the datasheet said:

  • 0 in reply to DeWei Lan
    TI__Mastermind 23310 points

    Hi De Wei,

    Glad I could help you better understand stability. It's the most common thing that customers deal with when they are designing with op amps. 

    That datasheet curve is only showing the phase margin of the op amp when it's directly connected to the capacitive load. There are ways of improving the phase margin of the device by including an isolation resistor between the feedback network and the capacitive load. This will push the pole (delay introduced) away from the feedback network, thus improving it's stability. 

    There is a device that we have that is an unlimited cap load driving part. We just released it. It's caused the OPA994. I would highly recommend taking a look at it to see if it would be a good fit in your system since you are designing with a large capacitive load. 

    Best Regards, 

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 250 points

    Hi :

    Thank you for recommend OPA994.

    I've read datasheet of OPA994.

    Bias current too high that does not correspond my necessary.

    For driver higher capacitor load, is OPA991 a good choice?

  • 0 in reply to DeWei Lan
    TI__Mastermind 23310 points

    Hi De Wei,

    OPA991 might be a good choice depending on the isolation resistor that you use. I recommend running simulation on it to verify. 

    Best Regards,

    Robert Clifton