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About VCOMR, VCOML and IREF of PCM1792A

user4177838
Prodigy 10 points
Other Parts Discussed in Thread: PCM1792A

Hi guys, 

Recently, I'am using PCM1792A to output signal. And I read its datasheet many time, and I was confused by pin20/21/22. Datasheet has few description about them, except figure 35. I found the IREF resistance will affect the value of current output. I want to know the relationship between current value and  IREF resistance ?

Besides, the PIN21/22, suggested to connect with 47uF to +5V. May I use a resistance to replace the capacitor?

Or can you tell me some detail about IREF, VCOMR and VCOML?

Thanks very much! 

  • 0
    TI__Mastermind 27850 points
    Hi,

    The data sheet value of 10K ohms for the Iref resistor is the value we officially recommend and we do not have data on the relationship between the Iref bias resistance and the current output. If you want to experiment you can go lower in resistance to change the current output, but this is not tested/characterized and have no guarantee of performance.

    As for pin 21/22, these are vital to the parts operation and require that 47 uF caps to +5V are used otherwise the part will not function correctly.

    Justin
  • 0 in reply to Justin Bohr
    Prodigy 30 points

    Hello,


    Could you please tell me if would be wise to increase value from 47uF to 100uF (or paralleling 10uF tantalums) for caps connected to VcomL/VcomR (pin 21/22)? Would this improve ripple or THD, especially when using noisy +5V PSU?


    Thank you,

    Raul.

  • 0 in reply to Raul Trifan
    TI__Mastermind 27850 points

    Hi Raul,

    Moving to 100 uF would not be a problem, but this will increase the settling time on startup. If you are worried about performance increase, you might be able to add a small ceramic (2.2uf) capacitor in parallel to add to the quick response of the charge reservoir (the capacitance on VCOM) and could improve THD slightly, but this is not proven, and needs to be tested.

    Justin

  • 0 in reply to Justin Bohr
    Prodigy 30 points

    Thank you very much for your answer Justin.

    I realized that my DAC's PCB has VcomL and VcomR separated and each one has a 47uF capacitor to the +5V, so a total of 2 caps. I believe this is different than TI's PCB layout recommendations, right? I think PCM1795A datasheet says that VcomL and VcomR should be bound together, then connected to +5V by a 47uF capacitor (please correct me if I'm wrong).

    So, will this affect audio quality in any ways, please? Should I solder VcomL with VcomR together or just forget about it?

    Thank you!

    Regards, 

    Raul.

    P.S.: Right now playing DSD x64 (through CM6631 + PCM1795A) has a lot of noise, THD+noise is about 82dB. Playback of 24-bit/192KHz has more than 115dB THD+noise. Perhaps DSD x64 playback noise issue might be firmware related after all.