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TMS320F280039C: Analog Pin A1/B7/DACB_OUT ADC sampling 2.0V w/o PWM, but this pin ADC sampling 2.01V w/PWM.

Shaoxing Ke
Expert 3590 points
Part Number: TMS320F280039C

Hi Experts,

I am asking for my customer.

Customer using Analog Pin A1/B7/DACB_OUT ADC sample Temp, they found that PWM disable this pin voltage is 2.0V, but when enabled PWM this pin voltage is 2.01V. We do not know why this difference?

We check others analog pin voltage both less than 3.3V. Do you have any others suggestion here? Thanks.

  • 0
    TI__Genius 11011 points

    Hello Shaoxing,

    I don't quite understand the comment about other pins being less than 3.3V. They're all supposed to be less than 3.3V?

    It sounds like there is some board-level DC coupling happening between the PWM output and the A1 pin. How much isolation/filtering exists between the PWM outputs/power stages and this ADC input?

    What VREFHI source is being used? Is it coupled to the 3.3V power supply? Do you see the same effect when using the internal voltage reference? EPWM activity does not couple internally to the ADC inputs, so my first suspicion would be that this is a board design or layout issue.

    Best regards,
    Ibukun

  • 0 in reply to Ibukun Olumuyiwa
    Prodigy 30 points

    Hello Ibukun,

    we disconnect the A1 pin from the sampling circuit, the actual output voltage was 2.01V, without a decrease of 10mV and we moved the sampling circuit to A6 pin, the actual voltage is 2.01V, without a decrease of 10mV. 

    Could you assist in verifying on the demo board by toggling the PWM and observing with oscilloscope to check for any fluctuations in the sampled voltage at pin A1？

      

  • 0 in reply to zhijie ai
    TI__Genius 11011 points

    So the problem is not specific to the A1 pin?

    Ibukun Olumuyiwa said:
    What VREFHI source is being used? Is it coupled to the 3.3V power supply? Do you see the same effect when using the internal voltage reference?

    Please confirm on this question. If the ADC voltage reference is not properly isolated from the PWM, you could see errors in conversion.