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PCM1681: Mitigate audible noise when 56kHz LPF is added

Koyo Asaishi
Intellectual 1710 points
Part Number: PCM1681

Hi team,

Customer added 2pole active filter at the output of PCM1681 and the cutoff freq is 56kHz.
But audible noise was mitigated though the fc is 56kHz which is out of range of audible freq.

When they use simple RC filter, audible noise was mitigated as well.

Can you estimate why it is happened?

Best regards
Koyo

  • 0
    TI__Guru 52663 points

    Hi Koyo-san,

    To clarify, the customer had noise in the audible range before adding the filter.  Adding the filter resolved the noise in the audible range, correct?

    It might depend on how they were measuring this noise.  If you are directly measuring on the DAC output without any filter, your instrument (AP2, for example) might have to over range the input to accommodate the higher frequency switching.  The will reduce is resolution, making it look like there is more noise in the audible range.

    If you are using a class-D amplifier or a class-AB amplifier with output switching (chopping, "high-efficiency" mode), the out-of-band noise generated by the delta-sigma modulator can be modulated back into the audible range.  Adding a LPF filter will attenuate this noise and then it might now be modulated.

    Thanks,

    Paul