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TM4C129ENCPDT: ADC coupling issue between ADC and GPIO pins

Jerome Cuenin
Prodigy 10 points
Part Number: TM4C129ENCPDT

- ADC is set as single ended on port GPIO_E_2

- Port GPIO_E_3 is set as standard input GPIO

-> ADC conversion (first sequencer step, no oversampling) give totally different results depending if the GPIO_E_3 is high or low, which shows a strong coupling on those two pins (GPIO_E_2 and GPIO_E_3), knowing that those two pins could be used as differential ADC.

-> I had to HW oversample (x32) and take the 2nd sampling to have correct values not influenced by the state of GPIO_E_3.

  • 0
    TI__Guru*** 135355 points

    Hello Jerome,

    Is the GPIO_E_2 left floating or is a specific voltage being applied to it? If a specific voltage, what is the voltage you are applying?

    Also if applying a voltage, is the voltage changing over time, and if so at what frequency does it change versus what sample rate is the ADC running at?

    Best Regards,

    Ralph Jacobi

  • 0 in reply to Ralph Jacobi
    Prodigy 10 points

    A voltage is applied to GPIO_E_2 (between ~0.8V and 2.2V), it varies very slowly (measure of an external voltage), nothing compare to the 1 mesure every 10seconds.

    Also the Vrefa+ of the ADC is 3,3V.

  • 0 in reply to Jerome Cuenin
    TI__Guru*** 135355 points

    Hello Jerome,

    Thanks, I'm trying to isolate what might be the actual root cause for your system here as the pins wouldn't just have a coupling effect just due to their differential usage. When configured as a GPIO, I wouldn't expect that PE2 would have an influence on the reading on a properly laid out board.

    That said, I did notice on the Errata that PE3 specifically has a glitch issue errata documented as ADC#13: https://www.ti.com/lit/pdf/spmz850

    The errata doesn't mention any external signal influences regarding the glitch, but that such an errata has been issued has me wondering if when the glitch occurs (glitch is very vague here), if it could be influenced by the adjacent pin.

    Is there any possibility you could repeat this setup on another pair of ADC pins which are adjacent to one another and see if the issue can be replicated or if its isolated to PE3? Also, can you apply the workaround from the errata to PE3 and see if allows for consistent readings regardless of the state of PE2?

    Best Regards,

    Ralph Jacobi