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PCM1798 Output Noise

Dan Cartman
Prodigy 60 points
Other Parts Discussed in Thread: PCM1798

I have a board which uses 4 x PCM1798. They all have identical layout, component values and output circuitry, the only difference is in the routing of the MCLK/BCLK/LRCLK (all of which are schmitt trigger buffered) and they all have a different source data I2S line from the DSP.

Three of them are behaving perfectly nicely and with zeroes on the data input from the DSP I get a scope trace on the output of the I to V stage that looks like this :

However, the fourth PCM1798 has a some nasty spikes on the output of it's I to V stage :

Now, I'm sure that I'm sending zeroes to the data line so this is something that the PCM1798 is generating itself. When I send audio data it's reproduced correctly, but with this noise on top which obviously wrecks the performance.

I've got 3 boards doing exactly the same thing so I'm pretty sure it's not a manufacturing fault. The frequency of the spikes seems to be sample rate related (they're longer at 48k than at 96k) but they're otherwise pretty random.

I'm guessing that the problem is something to do with the layout - it may not be a coincidence that the problem DAC is the one furthest from the DSP. Before I start hacking this PCB about to work out what's going on, has anyone got any suggestions of what could cause these spikes?

  • 0
    TI__Guru**** 189401 points

    Hi, Redeye,

    That's an interesting problem.

    Is the power supply clean on the fourth device?

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    Hi Don,


    Yes, both analog and digital supplies seem to be clean on the fourth device.

  • 0 in reply to Dan Cartman
    TI__Guru**** 189401 points

    Hi, Redeye,

    Very interesting... There must be something on the PCB causing this to happen...

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    Yes. I was hoping for a bit more of a clue than that. I'll phrase the question another way :

    What input conditions would cause those output spikes on a PCM1798?

    From my investigations so far there is no doubt that it's the PCM1798 generating the noise. The question is why. And why the other three PCM1798 on the same PCB don't have that output noise.

  • 0 in reply to Dan Cartman
    TI__Guru**** 189401 points

    Hi, Redeye,

    I'm thinking it either has to do with the power supply, or data going to the IC. Maybe there are some reflections on the data lines going to the IC?

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    Hi Don,


    After a bit of investigation I've got a few more clues, though I still can't work out what the problem is.

    There were some reflections on the data lines causing some overshoot, but 50R resistors on the outputs of the data line buffers seems to have sorted that out but the noise problem is still there.


    I've modified the code in my DSP to output a synthesized 200Hz sine wave to see if that gives me any more information and this is what I've come up with. This is the output of the IoutL- pin after the I to V stage on a good channel (yellow) and a noisy channel (blue) :

    When you zoom right in, this is what I get :

    To me that looks pretty conclusively like it's the PCM1798 which is generating the noise. The question is why?

    I'm not sure I know enough about the internals of the DAC to be sure, but instinct tells me that this is more likely to be on the analog than the digital side. Have you any ideas what would make these two DACs with the same digital input give such vastly different outputs?

  • 0 in reply to Dan Cartman
    TI__Guru**** 189401 points

    Hi, Redeye,

    Can you try swapping the data (left to right) to see if the noise remains on the left channel?

    Also, since the output is differential, you may want to look at the other output to see if it has a complementary signal on it which will cause the resulting signal to be cleaner.

    There is no noise on the VCOML terminal which might be contributing to this noise, is there?

    -d2

  • 0 in reply to Dan Cartman
    Prodigy 80 points

    Dear Redeye,


    Did you solve your issue ? We are experiencing a similar issue with PCM1798.


    Best regards,

    Olivier

  • 0 in reply to Olivier SCHMITT
    Prodigy 60 points

    Hi Olivier,


    Yes, we did solve the issue and I forgot to post back here.

    The problem turned out to be a PCB assembly error which meant that there were different value capacitors in the I to V stage on the +ve and -ve outputs of the problem channels, so obviously when they were debalanced there was still a lot of noise on them.

    So, if you have a similar problem I'd check your RC values are all matched in your I to V stage and debalancing stage.

    After spending ages trying to figure this out I felt a bit of an idiot when we realized the problem - when we'd machine built the boards we'd assumed the values were correct but somehow a mistake had crept in. Lesson learned.

  • 0 in reply to Dan Cartman
    Prodigy 80 points

    Hi Redye,

    Thank you for this follow-up ; we'll check according your advice. It is easy to get mixed up with the capacitors at assembly time.


    Best regards,

    Olivier