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PCM5242: PCM5242 Suggested Diff to SE buffer

Kris Panaro
Intellectual 315 points
Part Number: PCM5242
Other Parts Discussed in Thread: PCM5142, OPA1688, OPA1622, PCM4104, PCM1792A

Hello,

      Is there a recommended DIFF to SE buffer circuit for the PCM5242?  I realize there is a recommended passive low pass filter on the datasheet, however I need to convert the differential output to single ended to interface to my amplifier without effecting the noise/distortion performance of the PCM5242.

Thanks!

  • 0
    TI__Guru* 84915 points
    Hi, Kris,

    My colleague will take a look at this. He will answer as soon as possible.

    Thank you!

    Best regards,
    Luis Fernando Rodríguez S.
  • 0
    TI__Expert 5645 points
    Dear Kris,
    Apologies for the delayed response.
    Since you want the SE output, I would recommend using PCM5142 which may help you avoid needing DIFF to SE buffer conversion altogether. Would this be possible in your application?

    Best regards,
    Ravi
  • 0
    Intellectual 315 points

    Ravi,

          I wanted to take advantage of the 4.2V output of the 5242.  If I go with single ended, I will be forced to add 6dB of gain thus increasing the noise floor by the same amount.  My application requires ~8V to drive the headphone output (600 Ohm), so using the 5242 reduces my need for excessive gain.  Can you offer a solution?  I have tried to implement a diff amp using a 33078 op amp, but for some reason I am getting a 6dB increase in noise floor (from -103dbu to -97dbu) with a gain of -1 (5.1K input and 5.1k feedback).  Should I use a manufactured diff amp to do this, or do I just need proper impedance matching on the output?


    Thanks,

    Kris

  • 0 in reply to Kris Panaro
    TI__Expert 5645 points

    The datasheets for the OPA1688 and OPA1622 both show differential to single-ended conversion on a DAC output in the applications section. If you are only planning for 600 ohm headphones, and none of the lower impedance types which are a much more difficult load for the amplifier, then I would suggest the OPA1688.

     http://www.ti.com/lit/ds/symlink/opa1688.pdf

    Differential to single-ended conversion using a difference amplifier with equal resistance values results in a gain of 6 dB. The DAC noise is a differential input to the circuit, so the DAC noise will be amplified by 6 dB as you found. Only DAC noise which is a common-mode (equal in magnitude and phase at the DAC outputs) would be removed by the difference amplifier. 

    Best regards,

    Ravi

  • 0
    Intellectual 315 points

    Thanks for the feedback Ravi.  Is there a part similar to the 5242 with higher SNR or is the 5242 best in its class?

  • 0 in reply to Kris Panaro
    TI__Expert 5645 points

    Hi Kris,

    As far as the Delta Sigma DACs go, PCM4104 does have higher SNR (-118dB) and does have differential voltage output.

    But absolute best in class DAC is PCM1792A which is SW controlled device w. -132dB SNR. It's a differential current output. It really depends on the application and the constraints that apply to the overall solution. Please share the overall system details and we can definitely look into any other DAC that may match the requirements better.

    Thanks and Best regards,
    Ravi
     

  • 0
    Intellectual 315 points

    Ravi,

          Unfortunately I need the mini dsp included with the DAC.  I do have another project coming up that will need a higher performance DAC without DSP so this is good to know.  Aside from the DSP, here are the other important specs:


    THD+N: 0.002% or better

    SNR: -114dB or better 

    Output: 4.2V DIFF or 2.1V SE

    I2S interface, 24 bit, 48K - 192K fs

    I2C control


    Thanks!