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PCM4220: digital level is unstable

nicholas rose
Prodigy 100 points
Part Number: PCM4220

A proportion of parts have an unstable output level on both channels, suggesting that the converter gain has a random step change. The reference voltages are steady, as are the supplies and clocks, but the digital output level can change every few seconds on both channels.I'm driving it with a high level sine wave.

It may be worse when the part is cold. If it is operating correctly, hitting the part with freezer spray causes a short temporary gain increase of a few dB.

  • 0
    TI__Guru 57800 points

    Hi, Nicholas,

    This seems like a weird behavior, there was a known issue in the past where at low temperatures the part could show gain issues, but this is now fixed when testing the ICs. Do you have any input/output waveforms we can analyze?.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Prodigy 100 points

    Hi Diego - thanks for your reply.

    I think I've fixed it - it's both my fault and yours (TI).

    FS0 and 1 are normally low for 256x. A race condition in my logic is producing a 200nS pulse on FS1 every 2-5 seconds. Usually this has no effect, but every now and then will lock the converter into one of several low gain modes.

    You can demonstrate this by briefly shorting the signal to +3V with a bit of wire (make sure FS1 is pulled down not grounded). With a suitable input signal, display the ADC digital output, and it is undistorted, just has a gain level that is fixed normally but whose level is changed by flashing FS1. Spooky .....

    The temperature issues affect my race condition, not the part, although the freezer spray does slightly affect the gain.

    So, I can fix my timing, but it does indicate an undocumented strangeness in the FS1 line. Presumably applications that frequently change this signal should be aware of this.