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PCM1794: Issue with THD+N from board to board

Leo A.
Intellectual 515 points
Part Number: PCM1794
Other Parts Discussed in Thread: OPA1612

I have a design that is currently being manufactured using two PCM1794A chips in monaural mode. The THD+N varies drastically between boards and even between left and right channels on the same board.

The datasheet lists a THD+N of -108dB as the nominal but there is no min or max listed.

My question is how much can it be different from chip to chip. I am seeing between -101 and -111dB between different chips, is this normal or is this some kind of weird manufacturing error that is going on?

Here is what the output circuit looks like, it uses 0.1% resistors and 1% COG capacitors.

  • 0
    TI__Guru 52683 points
    Hi Leonid,

    While we do not specify a max for the mono mode, we do specify one for the 2VRMS stereo mode, which we have a max of ~101dB. So for that parameter we have a typical value of ~108dB and a max 101dB. Now that max value can't be directly applied to the other specification, but it should give you some idea of the expected range. It is very difficult to specify a max for THD as it can be impacted by so many other factors, such as the discrete Rs and Cs in the I-V stage, power supply noise, and board layout. You could see if supply noise is different between the boards.

    This is not to say the THD cannot be improved, but overall I would say that you are seeing number that are really unexpected.

    Thanks,
    Paul
  • 0 in reply to Paul_Frost
    Intellectual 515 points

    I really don't think it's a power supply issue because I have seen a 6-7db difference in THD+N on the same board between the two channels and the same power supply is used for both channels. So PS noise is not the issue.

    Also the SNR is always at around 130dB regardless of the THD+N.

    I do not see what you are referring to in the datasheet the only spec that I see for mono mode is 0.0004% at 44.1kHz which would equate to -108dB

    In any case are you saying that the performance chip to chip can vary all the way to -101dB from -108dB, that is very large and would equate to more than 2 times the distortion?

  • 0 in reply to Leo A.
    TI__Guru 52683 points

    Hi Leonid,

    The spec I was referring to is the THD+N spec for the 2VRMS stereo mode.  It is the only THD spec where we provide a maximum, and only at the specified conditions.  It would not be valid to apply this maximum to the other conditions, but it does help us get an idea for a reasonable range.  

    Can you confirm that you you are using bandwidth limited output filter on your instrument? This specification is conditional based on the 20-Hz to 20kHz bandpath filter.

  • 0 in reply to Paul_Frost
    Intellectual 515 points

    All of my measurements are on 20Hz to 20kHz bandwidth. I use an APx525 with AO52 option.

  • 0 in reply to Leo A.
    Intellectual 515 points

    Please respond.

  • 0 in reply to Leo A.
    TI__Guru 52683 points
    Hi Leonid,

    The point I was making was that THD of 0.0004% to 0.0008% is not unreasonable for this device. Variation from board to board and channel to channel may be more to do layout, supply, and the output amplifiers as well.

    Thanks,
    Paul
  • 0 in reply to Paul_Frost
    Intellectual 515 points

    Ok thank you. That is very disappointing to know that I can have a unit with swing of 6dB+ in distortion between channels.

  • 0 in reply to Leo A.
    TI__Guru 52683 points
    Hi Leonid,

    I understand. Though it is still possible that there are other factors that are impacting this. For clarification, are these mass-produced devices or hand built? If hand built you could try seeing if there is more consistency by thoroughly cleaning the board. Excess resin or flux from assembly can result in unexpected leakage paths around the amplifiers that are sometimes dependent on the voltage potential of the component. That can impact distortion.

    You mentioned that you have seen the same SNR on the boards, so I do not think system noise from the design is really the issue, but is possible that you have some random interference during the measurement- is the THD repeatable? For example, if you test board A, then B, then A again, do you see the same result?

    Finally - a bit of long shot - try to very thoroughly inspect the board, even if it was mass produced for good solder joints. This will be especially true around the DRG package amplifiers. I have personally seen reflowed boards that had cold joints at the pins. Electrically they seemed connected, but they caused many problems. The thermal pad on those devices can result in an area of the board that did not reach as high of a temperature.

    Goodluck,
    Paul
  • 0 in reply to Paul_Frost
    Intellectual 515 points

    Paul,

    Do you have any documentation on how to properly wash the PCM1794A and OPA1612 parts.

    I am having a few more boards made and want to share your feedback with manufacturer, along with the wash recommendations.

    Thanks;

    Leo

  • 0 in reply to Leo A.
    TI__Guru 52683 points
    Hi Leo,

    I do not have any specific recommendation for those devices, but TI has a few documents that describe recommended cleaning procedures for SON and QFN devices.

    www.ti.com/.../snoa550e.pdf
    www.ti.com/.../snoa040b.pdf
    www.ti.com/.../slua271b.pdf

    Thanks,
    Paul