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PCM1794A: Output of one side channel is reduced

Ryu Yamashita
Genius 3885 points
Part Number: PCM1794A
Other Parts Discussed in Thread: PCM1794

Hi team,

When we designed the board with the attached schematic, the output current of one channel drops at a frequency of about 1 in several 10 units.
Is there a problem with this schematic?
Also, are there any possible causes for the drop in output current on only one side channel?

7462.schematic.pdf

Sincerely,
Ryu

  • 0
    TI__Mastermind 31201 points

    Hello Ryu,

    We need to gather some more information  so we can do debug efficiently:

    • Can you elaborate more  on the channel drop, does the  output reduces in amplitude or disappears completely ? 
    • I assume you are seeing this at the output of the PCM1794 not after the conversion or filtering, correct? This way we can concentrate on PCM side .
    • If this issue is  at the output of the DAC , do you see only one of the diff outs has the issue or both ? (i.e,  IOUTL–  and IOUTL+   or just one of them)
    • Is it consistently L or R   (IOUTL–  and IOUTL+    or   IOUTR–  and IOUTR+ ) or sometimes the problem is on R and sometimes on L channel?
    • Have you plotted the I2S ( all in the same window and see the detail of edges when the problem shows up.
    • Have you monitor the input to make sure there is no hick up in DIN that is affecting the output? 
    • Once everything is checked out we might need to unload the PCM and see if that gives us another datapoint.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Genius 3885 points

    Hi Arash,

    Thanks for the reply.
    I answer below.

    • Can you elaborate more  on the channel drop, does the  output reduces in amplitude or disappears completely ? 
      Output will be reduced.
    • I assume you are seeing this at the output of the PCM1794 not after the conversion or filtering, correct? This way we can concentrate on PCM side .
      I am checking with the output of the op amp. It is at test pins 123 and 129.
    • If this issue is  at the output of the DAC , do you see only one of the diff outs has the issue or both ? (i.e,  IOUTL–  and IOUTL+   or just one of them)
      Both of them are lowered.
    • Is it consistently L or R   (IOUTL–  and IOUTL+    or   IOUTR–  and IOUTR+ ) or sometimes the problem is on R and sometimes on L channel?
      I have had it happen on both channels but more often on L.
    • Have you plotted the I2S ( all in the same window and see the detail of edges when the problem shows up.
      I did not check it.
    • Have you monitor the input to make sure there is no hick up in DIN that is affecting the output? 
      We did not check, but the voltage on the VCOM pin increased when the problem occurred.
    • Once everything is checked out we might need to unload the PCM and see if that gives us another datapoint.

    Sincerely,
    Ryu

  • 0 in reply to Ryu Yamashita
    TI__Mastermind 31201 points

    Hello Ryu.

    The lower current at the output along with increased VCOM voltage  that you see can be resolved by replacing the internal bias decoupling caps at the VCOM pin, you can use a   ceramic capacitors such as  GRM32ER71A476KE15L  47 µF ±10% 10V Ceramic Capacitor X7R 1210 for VCOM pin.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Genius 3885 points

    Hi Arash,

    Thanks for the reply.
    Does this mean that the problem is more likely to occur when using capacitors other than the recommended ones?
    And why does this problem occur?
    I am currently using OS-CON.

    Sincerely,
    Ryu

  • 0 in reply to Ryu Yamashita
    TI__Mastermind 31201 points

    Hi Ryu,

    No, I just happened  that I had  a similar behavior (The lower current at the output along with increased VCOM voltage  ) that was due to the VCOM cap.

    I don't have any supporting proof that  the type is very critical but at least we know it is the cap that can cause this. I would go with the suggested type to be same. I don't have the internal schematic to review and tell you what can be the reason for it.

    Regards,

    Arash