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THS3001: Gain and phase margin

steve smith1
Prodigy 220 points
Part Number: THS3001

On the datasheet it describes increasing the feedback resistor value to decrease the bandwidth thus improving the gain and phase margin. My question is how do you know what the gain and phase margin is for this part or any CFA. Are you talking closed loop gain/phase margin or are you talking open loop gain/phase margin? Come to think is it even possible to measure open loop gain/phase margin because if the feedback resistor is required to set the bandwidth & gain/phase then how can it be open loop ? Does TI measure open loop gain phase on CFA's if so how ? So that i can replicate on the bench for my design. This question is aimed at THS3001 but is equally vallid for any TI CFA 3001, 3491 etc.

  • 0
    Guru 47565 points

    They are talking open loop gain and phase margin, not a bench measurement, usually done like this where it is imperitive to have the correct open loop inverting input impedance modeled, Faster parts like this need an inductive element in that model, 

    https://www.planetanalog.com/stability-issues-and-resolutions-for-high-speed-current-feedback-op-amps-insight-7/

  • 0 in reply to Michael Steffes
    Prodigy 220 points

    Thanks Mike, I've done simulations this way but as we've previously discussed THS3001 is an old part/model so not sure how valid the results are.Follow on question then is when we talk about gain and phase margin it is the Open Loop response, the only practical bench measurement I've seen is adding a small (10-50R) resistor in the feedback loop then injecting a swept signal via broadband transformer but this is somewhat invasive and still only a closed loop measurement. Is there any way to use this to predict stability? Or is the only useful bench-level predictor to inject a fast pulse and look at the step response ? Or do you have a better suggestion ?

  • 0 in reply to steve smith1
    Guru 47565 points

    We will let the TI folks take it from here

  • 0 in reply to steve smith1
    TI__Expert 6035 points

    Hi Steve,

    We do not have bench-level measurements for open loop transimpedance (Zol) gain and phase. Data presented in the datasheet comes from simulation at a design level. As you mentioned, there is no straight-forward method for measuring these values. The best bench-level predictor of stability is to input a pulse signal and check the output response. You can determine the phase margin (and hence stability) of your circuit based on the overshoot observed at the output of the op amp.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 220 points

    Thanks Nick,

    Understood. So if derived from simulation doyou have a plot of gain and phase margin Vs different feedback resistor values for this device then? My customer wants to know the gain and phase margin for an output stage using this device. Steve

  • 0 in reply to steve smith1
    TI__Expert 6035 points

    Hi Steve,

    We have Figures 25-29 and Figures 31-34 which show closed loop output response for varying feedback resistors. We do not have phase margin values plotted in the datasheet, however, you can calculate the phase margin based on the peaking of the frequency response of the overshoot of the pulse response.

    Thanks,

    Nick