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Original question:

OPA2171: AOL

OPA2171: OPA2171 Application Question

Qiang Ren
Intellectual 1010 points
Part Number: OPA2171
Other Parts Discussed in Thread: OPA2991, TINA-TI

Team, 

My customer finds one OPA2171 output vibrate issue, due to phase margin(Rout=30Ohm, C=1000pF):

We look into other competitor's D/S, when R=50Ohm, C_Load higher than 1000pF, the phase margin is optimized. Take this pic as example:

Please help to give:

1. OPA2171's similar test result as above.

2. Any other suggestions to optimize the output vibrate issue.

Thanks!

  • 0
    Guru 140200 points

    Hi Qiang,

    this cannot be answered without schematic. Please post a schematic.

    Kai

  • 0 in reply to kai klaas69
    TI__Intellectual 1010 points

    Hello,

    Thank you very much for your support!

    Please see the SCH as below and help to give some suggestions. By the way, we don't have the phase vs cap load image, right? I'm asking because we want to see if it's possible to increase the 1000pF cap value to optimize the phase margin.

  • 0 in reply to Qiang Ren
    TI__Intellectual 1010 points

    And add one more comments here: It's production board and it was competitor device before. So it's hard for customer to do layout change. 

    Thanks!

  • 0 in reply to Qiang Ren
    TI__Genius 17637 points

    Hello Qiang, 

    The stability of the op amp is dependent on the output impedance of the op amp. Therefore to drive a capacitive load, different op amps will require different stability compensations. There is a TIPL Stability curriculum linked here.

    For the OPA2171, there will be a significant decrease in % overshoot if the the 30 ohm is increased to a 50 ohm, as seen in Figure 38 & 39 of the datasheet:

    Qiang Ren said:
    1. OPA2171's similar test result as above.

    For your original question, the answer is found in Figure 45. 

    I recommend the customer also try the OPA2991. The performance driving 1000pF capacitive load is better than the OPA2171: 

    All the best,
    Carolina

  • 0 in reply to Qiang Ren
    Guru 140200 points

    Hi Qiang,

    additionally to what Carolina said, reduce the 15k resistance to zero and minimize the resistance between voltage source and the +input of OPAmp. Or add a cap from the +input to ground and a cap in parallel to the 15k resistor.

    In any case both inputs of OPAmp must see a low impedance, otherwise your circuit will remain instable. Run a transient analysis (TINA-TI e.g.) to see the impact of these two resistors on the ringing and overshot.

    Kai