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PCM5122: Output waveform sanity check

Joshua Sanders
Prodigy 40 points
Part Number: PCM5122

Hello,

I'm writing firmware to control a PCM5122 DAC mounted on a custom development board.

When I play a 15kHz sine wave sampled at any sampling rate, the DAC output waveform looks surprisingly jagged (photo below).

Here are some details about my setup:

-The DAC circuit schematic is the one from Figure 75 in the PCM5122 datasheet, including the RC filters on the output lines. Beyond the essential components, a 24.576MHz clock IC is available to generate SCK in master mode configuration. 

-The I2S and I2C signals are generated by a microcontroller dev board (Teensy 4.1)

-The jagged waveform appears in both I2S slave mode with PLL configured, and in I2S master mode with the external clock source.

Is this the expected output of the PCM5122 without an additional output stage to filter the signal? Or is something misconfigured?

Thank you!

-Josh

  • 0
    TI__Guru 52663 points

    Hi Joshua,

    If you are operating at 15kHz with some sample rate (lets say 48ksps), then you would have approximately 3 points per period, which would look pretty distorted.  In this case, you see more than that.  That is because PCM5122 features a digital filter and interpolator.  

    The interpolator will try to calculate additional points between the input points to smooth the output and reduce distortion.  The PCM5122 will interpolate to 384ksps in most cases.  So now you would see about 26 points per period.  This is what you are seeing.  While all quantization errors will increase the harmonics, this is a vast improvement to the 48ksps input.  

    So you are seeing these 26 steps, with an RC filter smoothing the rising edge.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 40 points

    Hi Paul,

    This is what I was looking for, thanks!

    Out of curiosity, was the signal passed through additional filter(s) to get the published DAC specs (112dB SNR, -93dB THD+N @ -1dBFS)?

    If I'm reading correctly, the tests were done using "20-Hz HPF, 20-kHz AES17 LPF". Do I need to add these filters to the outputs to achieve the published spec, and if so is there a recommended set of components and/or an application note that would be helpful?

    Thanks,

    -Josh