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PCM2906C audio problem

Val
Prodigy 145 points
Other Parts Discussed in Thread: OPA353, PCM2906C, HID2

Hi,

I just finished mounting a PCM2906C and OPA353 based ADC, I'm using the USB Bus 5V power,

complete schematic:

USB Option photo usb.gif

The Codec is regularly 'seen' by my Windows 7 laptop, I can record audio,
but I get a terrific audio quality: there's an annoying digital repetitive noise, I tried to record a sweep,
here's the recorded WAV file (stereo 16-44.1, but I recorded only Left Channel).

LINK

The less noise part is without the source connected, more noise when source is connected.

In a word, I get an intermittent noise, which modulates my sound wave,
here's a shot of the recorded WAV file:
photo test_usb_audio102.png

This way, the codec is not useable at all.
I've also tried to remove the uA78M33 to see if it was the cause of noise, but the result is the same.

Note: the HID0, HID1, HID2 pins are left disconnected (floating) in my design, while in the evaluation board, they're pulled high through 1.5k resistors. I'm not sure if this affects the correct operation mode.

Any idea on what's happening?

Thanks in advance.

  • 0
    TI__Guru**** 189399 points

    Hi, Val,

    Please see this post.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 145 points

    Hi Don,

    Thank you for the reply.

    I made a totally new p.c.b., much more similar to the EVM board, and that solved the interference problem.
    My mind still puzzling about the HID0,1,2 lines that were left floating on my first board.

    I still had a bad signal/noise figure, around -27dBFS, whenever connecting my audio source.
    Way too high.
    Adding a 150K-ohms resistor in series to my function generator output (which has a 50-ohm output),
    that made the whole circuit useable and set the noise to acceptable levels.

    Is this normal?
    Does the circuit needs to see a high driving impedance?
    i.e. a current source instead of a voltage source?

    Noise floor is -90dBFS without the source and around -60 with the 150k res and source connected.

    What do you think? Anyone had the same experience?

    Thank you in advance. Have a great day!

    Respect,

    Val.

  • 0 in reply to Val
    TI__Guru**** 189399 points

    Hi, Val,

    That's strange. I wonder if you don't have a ground loop?

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 145 points

    Hi Don,

    Thank you for the quick reply.

    Yes, that's strange.
    As for the ground loop, I've isolated the whole pcb, by removing the source and replacing it through a 470 ohm resistor. The recordings are quiet.

    I thought that if the ground loop is within the pcb, it should show up anyway, right?

    The problem arises whenever I connect my (low Z) generator.
    I also tried with different cables with different lenghts.
    No luck.
    The cable is a RG 223 U with double shield.
    The only practical cure is adding a resistance in series.

    :(

  • 0 in reply to Val
    Prodigy 145 points

    I have a doubt:
    Do I still have to install the USB Filter driver for the PCM2906C revision?

    I'm asking because I found the chip still being rather noisy.

    TIA

     

    Val

  • 0 in reply to Val
    Expert 1720 points

    Do you have your signal generator galvanically isolated from a ground? I am asking, because it looks like you have ground loop via USB cable, computer, power socket to the generator and back to the USB codec.

  • 0 in reply to Pavel Ruzicka
    TI__Guru**** 189399 points

    Hi, Val,

    The PCM2906C uses the USB Audio Class drivers in Windows, so no additional driver(s) should be needed.

    I think Pavel is correct in thinking about a group loop. Thanks for jumping in and commenting on that, Pavel! I look forward to seeing more posts from you!

    -d2