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PCM1792A: Noise on output

Jordy Odekerken
Prodigy 20 points
Part Number: PCM1792A

I am using the PCM1792A and noticed some unwanted behaviour.

1) The DAC is running (48kHz, 3,072MHz, 12,288MHz or 24,576MHz). The output has HF Noise on it, even when sending nulldata over the PCM interface. When I stop the clocks, the output is clean. I've measured the clocks and their phase but they look good. What could cause this HF noise on the output?

2) I've also seen dips every Fs/8 in the output if B4 in Register 20 (DFTH) is set. Is this normal?

The DAC is configured as follows:

B6 in Register 20 (SRST), 300us delay, B4 and B5 in Register 18 (FMT0 and FMT1, 24 bit Left Justified).

My schematic:

Schematic

  • 0
    TI__Guru 52663 points

    Hi Jordy,

    This DAC uses a delta-sigma modulator, meaning that the output switches very quickly between different DC values to produce an averaged signal that is the desired output.  This means that there will be higher frequency noise that we recommend you filter using multiple active filter stages.  In your schematic, you have a single first-order filter.  We usually follow this with an active 2nd-order filter. 

    If you look at the output of a PCM DAC with a spectrum analyzer, you see that the noise (called out-of-band noise, or OoBN) starts to increase around 50%×fS.  This means that most of the noise energy is outside the audible range if you are using a 44.1kHz or 48kHz sampling rate or faster.  

    https://www.ti.com/lit/an/slea048/slea048.pdf?ts=1590593270996

    https://www.ti.com/lit/an/sbaa322/sbaa322.pdf?ts=1590593380792

    Check out these two articles.

    The PCM1792A uses an 8x over sample rate, so you will see the delta-sigma modulator switch at about that rate.

    Thanks,

    Paul