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OPA314: Driving a 22 uF Coupling Cap with an OPA314

Taoufiq Bellamine
Prodigy 240 points
Part Number: OPA314

Hello,

I'm using the OPA314 to drive a 22 uF coupling cap at the output of my Audio stage. Unfortunately I get a slew type behavior when I hit the input with a super fast large signal transient (1 uS rise) and the output DC does not recover until after second or so. The inputs and outputs are biased at 2.5, unity gain configuration, and the swings are within the allowed range.

Thanks

  • 0
    TI__Mastermind 30600 points
    Hi Taoufiq,

    Welcome to TI's E2E forums and thanks for your question. Could you also:

    * Tell me what your supply voltages are?
    * Provide a schematic (or just the relevant portion of it)?

    These things would allow me to verify that my simulation model is correct.

    Thanks,
    Paul
  • 0 in reply to Paul Goedeke
    Prodigy 240 points

    Paul,

    Below is the basic schematic. The OPA314 is driving a 22 uF and a 100 K. The supply is single 5V. Out pin is biased at 2.5V. When I hit this circuit with an impulse like acoustic signal, sometimes the DC at the output is disturbed and takes a while to recover. Any ideas on how to improve this is welcome.

    Thank you

  • 0 in reply to Taoufiq Bellamine
    TI__Mastermind 30600 points
    Hi Taoufiq,

    I ran a few simulations (both AC and transient) on your circuit but didn't see any glaring stability issues. Can you please provide scope shots of the non-inverting pin of the op amp and the signal on both sides of C7?

    Could you also describe the fast transient in more detail? I am unclear on whether or not the signal is a pulse or sustained step.

    Thank you,
    Paul
  • 0 in reply to Paul Goedeke
    TI__Mastermind 30600 points
    Hi Taoufiq,

    After talking with some colleagues, I have some more input on your question.

    Looking at the output of the schematic you provided, you have a high pass filter with a corner frequency of ~72 milliHertz and an RC time constant of 0.220 seconds. The settling time of the RC circuit is driven by the time constant and can be approximated as 5*RC which is 1.1 seconds. This matches the behavior that you have described.

    To fix this issue, your best option is to reduce the size of one or both of the RC components. Given that you're in an audio application, you have a little bit of room to move your high pass corner frequency up from 72 mHz. Maybe you could try something around 10 or 15 Hz? This would reduce your settling time and improve the pulse response.

    Hope that helps,
    Paul