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Some question about the output waveform in PCM1794A Digital Filter Bypass mode.

Ocean Chen
Prodigy 70 points

Hi all:

I have two question about PCM1794A output waveform.

First, I have to use PCM1794A (Digital Filter Bypass Mode) to replace PCM1704 for my case.

Cause I have designed digital filter and that PCM1794A internal filter will crash my function, I have to use Digital Filter Bypass Mode.

I try to use MONO standard 24-b Mode, the performance must be grate!

So that I change the mode to Digital Filter Bypass Mode (MONO).

Start my controller, I find that only R-channel has differ voltage output.

Q1: Can I use L-channel in Digital Filter Bypass Mode?

Second. In the Digital Filter Bypass Mode, I saw a confusing waveform as below:

Vary serious noise can crash my performance.

There is a more interesting experiment.

I short the DATA(pin 5) ground. Saying in other word, put the DATA always 0x000000.

Finally, I got the waveform as blow figure.

I measure all OP section until I/V section. I found the problem should be generated by DA.

It has serious ripple, this incredible thing let me a headache.

Q2: Could you please tell me why and how to solve this problem?

Thank you vary much.

BR,

     Ocean

  • 0
    TI__Mastermind 27850 points
    Hi Ocean,

    This seems to be the inherent out of band noise from the Delta Sigma as it approximates the value for the output. I believe this could be fixed by using a low pass filter to attenuate the OOB noise. To confirm this can you send us some data with frequency/time information so we can identify at what frequency these signals are occurring?

    Justin
  • 0 in reply to Justin Bohr
    Prodigy 70 points

    Hi Justin:

     

    Thanks for your reply. Because I have to maintain signal integrity in my case, I can't use a L.P.F with small B.W.

    I set the WDCK pin 192KHz (duty 50%) and SCK is 32xWDCK (I also test 64x but it is the same) .

    The digitizer can measure a almost DC value at 1/4x WDCK sample rate and set to output 0V,.

    But use 1/2x, 1 , 2, 4, 8, 16x(EXP.), we will find this strange wave as top picture.

    I feel this noise occurs regularly.Is this IC designer like to do?

    Ocean

  • 0
    Prodigy 70 points
    Can I solve this question?


    Ocean
  • 0 in reply to Ocean Chen
    TI__Mastermind 27850 points
    Hi Ocean,

    What signal bandwidth are you looking at? When you say 1/4x WDCK sampling rate, do you mean WDCK is operating at 1/4 SCK, or SCK is operating at 4 x WDCK? How much is the noise reduced in this mode?

    Justin
  • 0 in reply to Justin Bohr
    TI__Mastermind 27850 points
    Hi Ocean,

    When in digital filter bypass mode you must be operation the WDCK at 4x or 8x the desired sampling rate. You stated about when operating at 4x or (1/4 WDCK) you were getting better noise performance correct? The data sheet talks more about this on page 39.

    Justin