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PCM1860 not seeing 24-bit resolution.

Warren S
Prodigy 140 points
Other Parts Discussed in Thread: PCM1860EVM, PCM1860, PCM1861

I have designed a board with the PCM1860 to operate in Master Mode.  For testing I have been looking at the I2S signals DOUT, BCK, and LRCK.  A problem that I am having is that the 7 LSB never vary, leading me to question what is happening.  I also have a PCM1860EVM evaluation kit, and I am seeing the same results.  The upper 17 bits vary with signal, but the 7 least signifcant bits are fixed at 0.

I have attached an image showing this (ignore the pink trace). Any thoughts on why the LSB would be fixed at 0, and not at least varying wildly at LSBs tend to do?

  • 0
    TI__Mastermind 27920 points
    Hi Warren,

    This is because the PCM1860 is capable of operating to -103 dB SNR, which in ENOB is about 17 bits. If you use the PCM1861, which is capable of -110 db SNR, you will get more LSBs toggling past 17.

    Justin
  • 0 in reply to Justin Bohr
    Prodigy 140 points

    Thanks Justin.   That makes sense; though I was expecting to see those bits vary even pseudo-randomly.  So it is safe to say that those lower 7 bits will always remain low regardless of input signal?

    The PCM1861 appears to be a direct drop-in replacement, is this right?   Just wondering why it doesn't share the same datasheet as the PCM1860.

  • 0 in reply to Warren S
    TI__Mastermind 27920 points
    Yes, the entire PCM186X family are pin to pin compatible so you can scale from 2 channels with hardware control, to 4 channels ad software control, with both performance grades.

    Justin