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PCM1798 always outputs full scale

Other Parts Discussed in Thread: PCM1798, NE5534

Hi, I have implemented exactly the measurement circuit in figure 23 of the PCM1798 data sheet (SLES102A, page 18). However, when I try to give an I2S input, the output pins (17,18, 25, 26) go immediately to their maximum current values:

-the IOUTp pins [17,25] go to –5.5mA which corresponds to 800000 (–FS)

-the IOUTn pins [18, 26] go to –5.5mA which corresponds to 7FFFFF (+FS)

Please recommend what steps I should take first in order to debug this problem.  I have already tried changing the IC twice, so I think I have eliminated the possibility that I had a broken IC.

Thank you, 

Michael

  • Hi, Michael,

    That's really odd. I can't think of a reason why this would happen. 

    Please double check all your connections, make sure your power supply is stable and within spec, all bypassing nodes are stable, and your i2s signal is valid. 

    has anyone in the Community seen a problem like this on the current output DACs?

    d2

  • As it turned out, some of the digital pins were not configured correctly - it is working now.  I can't believe it took 5 debugging attempts to recognize the incorrect pins.  

    I did some square-wave tests - according to my 'scope the measurement circuit appears to have a pretty low slew-rate, it can't seem to get a very steep slope on high-frequency (1KHz +) test signals.  Blame the NE5534 op amps for this?

  • Hi, Michael,

    I'm glad you got it figured out!

    Can you post a scope shot of your slew rate? What load are you driving?

    -d2

  • http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/64/2746.mStereo_5F00_test_5F00_waveforms_5F00_0.pdf

    The load is a 1/4-watt  1MOhm thin metal film resistor, across Vout+ and Vout-.
    The waveforms shown are taken in MONO mode, left channel.

  • Hi, Michael,

    Sorry for the delay in getting back to you.

    Did you get this issue sorted out?

    -d2

  • Yes, I think so.  It seems that resolving a square wave in high-frequencies depends a lot on the sample rate.  My i2s source (a PCM2707) only goes as high as 48 KHz, so I think that is the reason I initially thought I was observing poor frequency response from the DAC or the analog filter stage.  Some more recent (and better!) test waveforms are attached.

    http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/64/2577.mStereo_5F00_2_5F00_wavforms.pdf