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Noise in PCM1804

Wes Brenner
Prodigy 20 points
Other Parts Discussed in Thread: PCM1804, OPA4134

Hi:

I'm trying to evaluate the use of a PCM1804 for use as an input for a bare metal app on a Raspberry Pi.

I constructed the circuit for the single ended to balanced converter as described in the datasheet except I used three of the opamps in the quad OPA4134's for each stereo channel.  I then put them through the PCM1804 circuit suggested in the datasheet.  The problem is that on my scope I see the least most significant 16 or so bits are filled with noise at 192k and the least most significant 8 or so bits at 32k.  In looking for noise problems I shorted the inputs to the first circuit and powered the circuits with batteries +9v and -9volt. Still had the noise.  When I run a audio generator into it at 1khz I can see each bit turn on as I advance the input level once I get to the noise level and continue smoothly until all bits are filled and the overflow is indicated.  

   Circuits were constructed with surface mount to dip converters and point to point wiring.  Used the tantalum, ceramics, and film caps suggested in the datasheet and on-board  lm7805,  lm7905, and ld1117 3.3v regulators and fxo-hc53 series 3.3v oscillator running at 24.576mhz.  My frequency counter confirmed the lrck line was running at 191.998khz and bck was running at 12,287,835hz.

I would appreciate any suggestions on what and where to look to find the noise source and any suggested solutions. 

Wes Brenner

  • 0
    TI__Mastermind 27990 points
    Hi Wes,

    It sounds like you are testing the part in a way where you are holding a DC value is this correct? Have you measured the THD of the output?

    Justin
  • 0
    Prodigy 20 points
    Hi Justin:

    Thanks for looking at my problem.

    Initially,  I was testing with a sine wave from an audio generator.  I saw no change in the bottom 15 or 16 bits until I brought the signal level up to that threshold and then the most significant 8 or 9 bits acted as expected (turning on in turn as I increased the input.  I then tested with the input to the opamps shorted to ground.  The bottom 15 or 16 bits were still active. I measured the voltage on the pcm1804 and the Vin-. Vin+,and Vref for both channels all seemed identical on the scope at about 2.4 volts flat lines.  Is the layout critical and if so what should I look for to avoid generating the noise.  I'm sorry but I don;t have a thd measurement capability. It may help you to see the scope traces of the l/r clock on top trace and the data out on the bottom trace.  Note that the noise starts at about the same bit in left and right channels.
    Any suggestions of what to look for would be greatly appreciated.

    Wes Brenner