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PCM6240-Q1: Waveform distortion occurs when the input signal is large

Leo Syu
Prodigy 110 points
Part Number: PCM6240-Q1


Tool/software:

Hi expert:

I split the amplifier output to the ADC input channel for loopback purposes. However, when the amplifier volume is set too high, the ADC waveform gets distorted.

The input channel was set to high swing mode, with an analog differential and AC-coupled input. I also tried setting the Digital volume control to a negative dB value, but it didn't work.

The picture below shows the distorted waveform from the ADC. I played a 2kHz tone at a high volume setting and captured the data from the ADC output channel.

I prefer not to use an automatic gain controller. If you have any other suggestions, please advise me.

Thanks

Leo.

  • 0
    TI__Mastermind 27670 points

    Hi Leo,

    What does the ADC input circuit connection look like and what is the input signal level (in Vpp or Vrms)?

  • 0 in reply to Daveon Douglas
    Prodigy 110 points

    Hi Daveon:

    Here is the circuit block diagram. I have only measured the voltage at IN1P, which is approximately 8.3V in Vpp

    Thanks

    Leo

  • 0 in reply to Leo Syu
    TI__Mastermind 27670 points

    Hi Leo,

    Can you attach the schematic and adjust your post to be in the internal forum?

    A couple of troubleshooting measures:

    • The ADCs internally sample at a very high rate. But they are optimized to take inputs only up to 20KHz. Higher frequencies will dramatically reduce the amount of full scale signal that the ADC can support (even by up to 40dB!) and can induce distortion. So I wouldn’t recommend using our audio ADCs to sample such high frequency signals however, If the high frequency signals contains audio band content (PWM output of class D would do), then its better to filter the output (using a external RC/LC) before connecting to our ADC inputs.
    • Ensure the ASI clocks are correct.

    Regards,