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PCM2900: ADC fluctuating THD

user3949174
Prodigy 160 points

Other Parts Discussed in Thread: PCM2900

Hello,

On my board for PCM2900 I am seeing THD for the ADC fluctuate from around 0.03% up to 0.14%, generally over the course of a couple of seconds. DAC THD is steady. I can't measure anything that changes in correspondence to this rising and falling THD. Have verified THD is not present on ingoing signal (on analog input pins).

Has anyone seen this or has anyone any ideas?

Schematic, video of THD and clock signal posted:

USB IN.pdf

  • 0
    TI__Guru** 114635 points
    Hi,

    Your schematic seems to be fine. Are there any layout files that we can take a look?
    The typical THD+N value is 0.1%. If an external LDO is used to drive Vccci (which is not this case) then the typical THD+N is 0.01%. The variation is within the specifications.
    Is there any variation on Vccci pin?
    How often is this THD variation? I'm sorry but the video is not very clear and I'm not able to read the information below the graph.

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • 0 in reply to Ivan Salazar
    Prodigy 160 points

    Hi Ivan,


    Layout attached below. I've done some screen captures of the distortion which hopefully is more clear, measured at 1kHz again, 8kHz and 100Hz as they show different distortion characteristics. (100hz is a picture as it remains steady).

    I can see no variation on Vccci.

  • 0 in reply to user3949174
    Prodigy 160 points
    I've replaced the PCM2900 and tried various values of crystal comp caps but nothing changes. Surely this isn't operating as expected? 100hz THD in particular looks peculiar. Any ideas anyone? Thanks.