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TMS320F28335: Issue with eZdsp F28335 ADC Reading Waveforms

Benedict Sim
Prodigy 10 points
Part Number: TMS320F28335
Other Parts Discussed in Thread: CONTROLSUITE

Hello,

I'm trying to get the ADC of an eZdsp F28335 to read waveforms from a signal generator but the CCS graph waveform looks nothing like what the signal generator is producing.

I connected the signal generator directly to both ADCINA0 and an oscilloscope. I also connected ADCLO to AGND via TP1 of the board.

The CCS graphs are flat as expected for DC signals but not for others.

I'm using the Example_2833xAdcSoc.c example code from controlSUITE to do this.

Any help in this issue is much appreciated.

Thank you.

Ben.

  • 0
    Guru 192875 points
    Hi Benedict,

    CCS graphs can be effective with frequencies less than 10Hz. Anything above that would be distorted as above. This is because the Graph update rate cannot be lesser than 100ms due to debugger limitations.

    Regards,
    Gautam
  • 0 in reply to Gautam Iyer
    TI__Guru 60865 points
    Hi Benedict,

    A couple things to check here:

    (1) What is the ADC sample rate vs what is the scope sample rate? The ADC sample rate will be determined by the SYSCLK, the SYSCLK to ePWM clock divider(s), the ePWM period value, the ePWM count mode, and the ePWM to ADC trigger event settings.

    If you need to further reduce the sampling rate you could only take every nth sample and store it in the RAM buffer. You might also want to increase the length of the buffer.

    (2) The ADC input is not high impedance. It needs to be (ideally) driven by a high-speed op-amp and with low resistive/inductive impedance and moderate capacitance on the pin. In addition to increasing the drive strength and reducing the impedance, you can also increase the S+H duration by increasing the ACQ_PS setting.