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OPA1671: output noise

Allan Fan
Expert 5730 points
Part Number: OPA1671

Hi Expert,

Customer use OPA1671 and see output a 62Hz carrier on 1KHz tone(even in without 1KHz tone condition). We though it might be came from 5V supply but didn't see that frequency. Do you have any idea about it and how to remove it?

Mic input level.docx

Thanks,

Allan

  • 0
    TI__Guru* 93105 points

    Hello Allan,

    The OPA1671 circuit design itself shown does not show anything that would explain the generation of a 62 Hz sine wave. However, the OPA1671 is operating in a high gain of +101 V/V. If not properly shielded from external fields they can add to, or modulate the intended 1 kHz signal. I find on line that the standard Taiwan line voltage is 110 V at 60 Hz, very similar to the US. From the DSO images you provided it appears a field from the power line may be getting coupled into the circuit. Sixty Hertz is very close to 62 Hz and the difference may just be a measurement difference.

    Is the PC board protected from external fields during the testing? Is it an open circuit on a test bench, or is it inside a shielded enclosure? If the board is out in the open, one quick check you can make for 60 Hz field ingress is to move the board away from the bench while the circuit is operating. If you find the amplitude of the 60 or 62 Hz signal changes as the board is moved about the lab space, then that is a strong indicator that it is line signal being coupling into the OPA1671 circuit.

    Regards, Thomas

    Precision Amplifiers Applications Engineering