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PCM1795: How to debug the PCM1795

Matthias R.
Prodigy 40 points
Part Number: PCM1795

Hello,

I am using the PCM1795 and I am supplying clocks and data but don't get an analog output. All analog outputs are at a current of -1.2mA no matter the digital input. Some more parameters of my setup:

* 32bit, right-justified format

* SYCLK = 5MHz

* BCK = 1.25MHz

* LRCK = 19.53125kHz (Note: _not_ an audio application so non-standard sample rate.

* All clocks verified using logic analyzer

* All registers in their default state except register 18 where I set FMT to 001

** I can read back the register contents periodically and the values are correct.

** If I send 0x9200 to the control port I get back 0x0010.

* Reset conditions have been checked. Delay between reset and configurationis long enough. Configuration is kept so assumed OK.

* All power supplies within +-1% of nominal

How can I proceed debugging this issue? Is there any way to read out the status of the baudrate auto-detect?

Best regards,

Matthias

  • 0
    TI__Guru 52683 points
    Hi Matthias,

    If you are seeing 1.2mA with no input signal, then I suspect that there is something electrically wrong with your configuration. At bipolar zero input (or mute) the output should be 3.5mA. Can you post your schematic?

    Thanks!
    Paul
  • 0 in reply to Paul_Frost
    Prodigy 40 points
    Hi Paul,

    thanks for your input. I was indeed to focused on the digital part because I couldn't see anything wrong with the vlaues of the currents. Of course 0V should be ~-3.5mA. I checked my design and found out that while the TIA I am using does have a R2R output the inputs are far from R2R. My simulation was hiding that fact from me by sinking current into the TIA (-) input pin which of course won't happen.

    My working hypothesis is now that replacing the OP amp will do the trick. I will order new parts and report back once I know more.

    Thanks for your help already.

    Matthias
  • 0 in reply to Matthias R.
    TI__Guru 52683 points
    Ok, let me know if you need help selecting a device.

    www.ti.com/.../sboa237.pdf

    This tech note may be useful as well. Thanks!
  • 0 in reply to Paul_Frost
    Prodigy 40 points

    Hello,

    I continued debugging the analog circuit with another OP amp today. I am using the circuit described here:

    https://e2e.ti.com/support/data_converters/audio_converters/f/64/t/235364 

    The TIA op amp has rail-to-rail inputs and outputs. I tried different values for the biasing the TIA's + pin but I never get the correct 0 scale current. Could you provide an equivalent circuit of the DAC's current outputs clearly indicating the direction of the specified output current please? I want to be 100% sure that I understand the output.

    Best Regards,

    Matthias

  • 0 in reply to Matthias R.
    TI__Guru 52683 points

    Hi Matthias,

    The current will ways be flowing out of the device.  The voltage potential on the left side of Rf should be higher than the right side.  Can you note on the schematic the values you are using for Rf, the divider resistors, and the resistors in the differential stage?

    Thanks!

  • 0 in reply to Paul_Frost
    Prodigy 40 points
    Hi Paul,

    the TIA gain resistor was too large so it was actually clipping the filter. The issue was that I misinterpreted the negative currents from the datasheet as going into the device (the DAC sinking current to 0V).

    Thanks for your help,

    Matthias