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OPA177: Will the OPA177 have issues with gains much less than 1, say 1/20 and suffer from phase reversal?

mark long
Prodigy 20 points
Part Number: OPA177
Other Parts Discussed in Thread: OPA191

Hello:

I’m considering using the Texas Instruments (Burr-Brown) OPA177 for some simple precision inverting operational amplifier designs. The applications will be a differential amplifier, inverting amplifier summer, and a 2-pole Multiple Feedback Low Pass Filter (LPF).

My frequencies range from DC to 20 KHz. I’m concerned about using the OPA177 for it being unity-gain stable, I’m concerned it may oscillate when used at gains much less than 1. Is this true? We now use the legacy OP-400 quad precision op-amp. The OP-400 is bulletproof being stable at ALL gains and we have NEVER seen it exhibit phase reversal. We routinely use the OP-400 for these inverting amplifier circuits to attenuate the input by a factor of 20 (Gain = 1/20).

Will the OPA177 have issues with gains much less than 1, say 1/20 and suffer from phase reversal? In general, I’m looking for a new/improved precision op-amp version of the OP-400 in both single and quad forms that retains the bulletproof nature of the OP-400 supporting very small gains, e.g. 1/20, and does not exhibit phase reversal.

  

Thank-you

Mark Long

NAWCAD

Mark.long@navy.mil

 

  • 0
    Guru 48395 points

    Well Mark, I think you meant to say low phase margin as an attenuator, not phase reversal - if the op amp is unity gain stable, that will be fine unless you introduce a too low feedback pole at the inverting input C. 

    This comes up enough that I just published this discussion on inverting attenuators - this goes on to cover things beyond what you might care about, but the front end gives you some sim approaches, 

    https://www.planetanalog.com/are-inverting-attenuators-with-voltage-feedback-op-amps-inherently-unstable-and-why-might-you-want-to-apply-them-insight-11/

    In mid August Planet analog broke the links to expand the figures, not fixed yet, so here is the original pdf, 

    3225.Designing Inverting Attenuators Aug2_2019.pdf

  • 0 in reply to Michael Steffes
    Guru 48395 points

    If you did mean phase reversal, that implies an input overdrive concern - most modern devices are tested thoroughly in design and characterization to not show overdrive phase reversal - usually that shows up as an overdrive plot. 

  • 0 in reply to Michael Steffes
    TI__Guru* 93105 points

    Hi Mark,

    It looks like Michael covered all the important points regarding op amp stability concerns. His Planet Analog article should help you evaluate your concerns. The OPA177 is unity gain stable so the stability or instability attained very much comes down to the circuit in which you apply it.

    Do note that the OPA177 is a very good precision op amp, but what we would consider a legacy op amp. You will note that the original date on the datasheet is 1990. Op amp technology has progressed and there are newer, even higher performance op amps that you might want to consider. For example, the OPA191 is a modern, e-trim CMOS op amp that meets or exceeds the OPA177 performance. It is unity-gain stable and built-in antiphase reversal built-in.

    You can find the OPA191 datasheet here:

    http://www.ti.com/product/OPA191

    If you have any questions, let us know.

    Regards, Thomas

    Precision Amplifiers Applications Engineering