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PCM5242: PCM5242 Differential Output buffer issue

Kris Panaro
Intellectual 315 points
Part Number: PCM5242

I am using a multiple feedback lowpass filter to buffer the output of the PCM5242 as an interface to my discrete single ended input amplifier.  I am finding that the buffer is actually adding noise to the output (reducing SNR), and modifying the buffer to single ended will reduce the noise by about 5 dB however now I have to add 6dB gain in order to get the signal where I need (thus adding 6dB of noise).  The filter I am using can be found on page 36 of the following datasheet: http://www.ti.com/lit/ds/symlink/pcm1690.pdf , I have modified the values slightly for 0dB gain using filter pro desktop, ensuring that the input resistors are above 5K so I don't load the output of the DAC.  

  • 0
    TI__Expert 5645 points
    Hi Kris,

    can you please clarify the question or problem you are trying to resolve w. the PCM5242? Is it regarding the noise at the output of the buffer? Thanks.



    Best regards,
    Ravi
  • 0 in reply to Ravi Patil
    Intellectual 315 points
    Ravi,
    1st question: is a multiple feedback filter appropriate for buffering the PCM5242 output? Do you have a recommendation for proper buffering of the PCM5242 output?
    2nd question: My implementation of this buffer with 0dB gain (example from datasheet link I gave has a gain of ~.7) leads to an SNR reduction of about 6dB. If I simply connect the recommended RC filter from the datasheet at the output of the PCM5242 and measure the SNR with nothing connected to the output, I get ~106dbu for each individual output referenced to ground. If I connect the MFB buffer circuit and measure the SNR at the output of the buffer, I get about 100dbu. Why am I getting a reduction in SNR when I connect the buffer? I did try to swap the polarity of the differential pair going into the buffer but the result was the same.

    Thanks,
    Kris
  • 0
    Intellectual 315 points

    Ravi,

          Please see my reply on the forum.


    Thanks,

    Kris

  • 0
    Intellectual 315 points

    Part Number: PCM5242

    Ravi,

    In response to your question, here is what I am looking for:


    1st question: is a multiple feedback filter appropriate for buffering the PCM5242 output? Do you have a recommendation for proper buffering of the PCM5242 output?
    2nd question: My implementation of this buffer with 0dB gain (example from datasheet link I gave has a gain of ~.7) leads to an SNR reduction of about 6dB. If I simply connect the recommended RC filter from the datasheet at the output of the PCM5242 and measure the SNR with nothing connected to the output, I get ~106dbu for each individual output referenced to ground. If I connect the MFB buffer circuit and measure the SNR at the output of the buffer, I get about 100dbu. Why am I getting a reduction in SNR when I connect the buffer? I did try to swap the polarity of the differential pair going into the buffer but the result was the same.

    Thanks,
    Kris