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OPA380: Composite op-amp theory, mechanism used for precision

Andrew Trico
Prodigy 40 points
Part Number: OPA380
Other Parts Discussed in Thread: OPA188, THS3125, , OPA659, OPA656

Hi,

I would like to understand a little better the mechanism that the OP380 uses in order to have a low output DC offset while being a current feedback op-amp. In the datasheet it mentions it has another internal op-amp which is auto zeroing connected into the non inverting pin like this:

Could the same strategy be used with any current feedback op-amp? For example if I wanted to create a composite op-amp with the THS3125 and OPA188. 

Thank you!

  • 0
    TI__Genius 15975 points

    Hi Andrew,

    I don't think the OPA380 is a current feedback op-amp. It is rather a voltage feedback op-amp that is used for current-to-voltage conversion or trans-impedance application and also performs the DC offset correction.

    I don't think the same strategy can be used with current feedback op-amp, as it becomes unstable if you add a capacitor across its feedback network. So, using the THS3125 and OPA188 to build the trans-impedance circuit similar to the OPA380 would not work. However, you should be able to use a voltage feedback amplifier like the OPA656 or OPA659 along-with the OPA188 to build the trans-impedance circuit similar to the OPA380 with auto-zero DC offset correction.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 40 points

    Hi Rohit,

    Thanks for the answer. I went through the datasheet of the OPA380 and you are right, there is no mention of current feedback anywhere. I think I assumed it was current feedback since it has high bandwidth and capability of delivering high output current too.

    As you mentioned, the THS3125 would become unstable if there was a capacitor on the feedback loop. Nevertheless, if it was connected with the OPA188 as the OPA380 is internally, there would only be a capacitor across the OPA188 if the 75pF was omitted. Would this not work?

    Thanks again!

    Andrew

  • 0 in reply to Andrew Trico
    TI__Genius 15975 points

    Hi Andrew,

    Yes. There is a possibility that you might be able to make this work without the feedback capacitor across the THS3125. I don't think the 75pF is part of the trans-impedance feedback loop, because it acts as positive feedback for the DC offset correction circuit (OPA188 circuit).

    One thing to note is that I would be careful with the THS3125 trans-impedance gain to not go below the Rf value mentioned in the datasheet for stability purposes.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 40 points

    Hi Rohit,

    Thanks for the answer. To what bandwidth would you tune the OPA188 circuit? Since it is doing mostly DC offset correction, should I tune it to a low frequency such as 100Hz? Also I will definitely look into the right resistor value for the THS3125, thanks for the note.

    Andrew 

  • 0 in reply to Andrew Trico
    TI__Genius 15975 points
    Hi Andrew,

    Yes. A bandwidth of around 100Hz would be sufficient to tune the OPA188 circuit for DC offset.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 40 points
    Thank you Rohit