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PCM1772 + TPA4861

Matt Garrison
Prodigy 20 points
Other Parts Discussed in Thread: TPA4861, PCM1772

I'm working on a design using the PCM1772 DAC and TPA4861 amplifier.  My question is how to interface between them.  Do I need an LPF as shown in the PCM1772 datasheet?  Or can I connect directly between them?

If I need an LPF, how do I design one?  I found the filters in the DEM-DAI1772 User's Guide -- would these be appropriate?

My first shot at this design did not have an LPF.  For the gain components, I used Ci = 1uF, Ri = 15K, Rf = 11.8K (gain of 1.57 V/V).  It is at least mostly working -- can't tell at the moment if the distortion on it is a hardware or software problem.

Other comments?  Thanks...

  • 0
    TI__Guru**** 189399 points

    Hi, Matt,

    The LPF is probably a good idea - since the PCM1772 is a sigma-delta converter, it has a noise shaper which pushes the in-band noise out-of-band. Sometimes, this out of band noise can wreak havoc on the input stages of amplifiers, so it's always a good idea to put an LPF between them.

    The filter design in the EVM users guide is probably a good choice.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    hai

    we are facing the problem in PCM1772 with more noise in playback of audio file.

    we tried to make LPF with input series resistor to PCM1772, its solves the problem in non-working boards, but actually

    working boards are not working with this change. Assuming input IC capacitcance within PCM1772.

    Its an interface between DM355 TI Processor to PCM1772PWG4, pls help in this case to solve the issue.

     

    Thanks & Regards

  • 0 in reply to SURESH KUMAR74621
    TI__Guru**** 189399 points

    Suresh,

    A series resistor may not get you a LPF. I suggest you use the series resistor, and then a cap to ground to get a true LPF.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    Hai,

     

    Data sheet recommend to use series resistor input to the PCM1772.

    The series resistor combines with the stray PCB and device input capacitance to form a low-pass filter that reduces

    high-frequency noise emissions and helps to dampen glitches and ringing present on the clock and data lines.

    Hardware is already ready, we are facing issue in some of the units.

    Thanks & Regards

    Suresh kumar

  • 0 in reply to SURESH KUMAR74621
    TI__Guru**** 189399 points

    Suresh,

    Sorry, I misread your post. We were talking about the OUTPUTS before... I thought you were talking about that still. Do you have an LPF located there?

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    hai,

    we don't have LPF on board.

    only R on series between Processor and PCM1772.

    As per datasheet, he assumed this series resistor with internal chip capacitance forms LPF.

    As we increased series resistor value, we found that noise reduced.

     

    But my basic doubt is: how this works in CMOS digital signals, are there no effects of reflection due to impedance mismatch.