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TINA/Spice/TINA-TI: Discrepancy between Oscilloscope and AC Transfer Characteristic for some op-amps

R Pearson
Prodigy 20 points
Part Number: TINA-TI
Other Parts Discussed in Thread: TLV2461, LMV321

Tool/software: TINA-TI or Spice Models

I am investigating supply decoupling of signal related currents in op-amp circuits driving a low impedance load. I have noticed that the AC transfer characteristic gives implausible results for some op-amps, and these implausible results disagree with the oscilloscope view of the power supply current waveforms. For example, comparing TLV2461 and LMV321 in the same circuit driving series 50R + 220U to 0V, the supply current waveforms at 10kHz are identical but the AC transfer characteristic says the signal current in the power supply is -40dB (i.e. 10mA) for TLV2461 versus -34dB (i.e. 20mA) for LMV321. The peak current measured on the 'scope is 20mA in both cases. This is shown in the screenshot below.

I have obtained more bizarre results with other TI op-amps, some of which give completely implausible current waveforms on the 'scope, e.g AC signals of nano amps magnitude, or sinusoidal current waveforms that are different in each power supply feed (thereby violating Kirchoff). It seems the op-amp models or TINA-TI (or both) do not always simulate power supply currents correctly. Any solutions?

  • 0
    TI__Genius 9775 points
    Hi R Pearson,

    Tracking power supply current to follow the actual IC silicon proves to be quite bit challenging for opamp macromodels. All being said, most of our recent models have made that attempt especially to track the transient power supply current of the output buffer of the opamp.

    On majority of our past models, we focused much on the signal path behavior itself instead of the detail behavior of the power path (e.g, transient power supply currents).

    Herman
  • 0 in reply to Herman Theodorus
    Prodigy 20 points

    Hi Herman,

    Thanks for your reply. So I think you are saying it is an issue with some of the op-amp models, rather than TINA-TI software? Does that mean I am out of luck with respect to any sort of solution to this issue? It is not just op-amp models from TI that have this issue. If I use a personal favourite from LT (over a decade old), that also gets the supply currents completely wrong in TINA-TI. Is it too much to hope that TI will update its older models to better represent the performance of current product offerings?

  • 0 in reply to R Pearson
    TI__Genius 9775 points
    Yes, the macro model itself and not the software.

    My speculative educated guess is that yes, any old opamp models most likely have similar issues unless inside the macro model they don't use ideal components, rather they use transistor level models that mimic the actual circuit architecture of the actual silicon IC. I think the best way is simply to try the model, unfortunately.

    We are upgrading some of our older models discreetly depending on the priority.