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TMS320F28377D: Incorrect measured value on ADCINB0

Clément Letonnelier
Genius 3016 points

Part Number: TMS320F28377D

Hi,

We are undergoing an issue on ADCINB0 pin of our TMS320F28377D.

The voltage value measured on the board is equal to 1.156V but the result of our acquisition outputs 1.204V (1644 ADC value).

In our design we are using an external reference voltage at 3V.

We have the following configuration:

  • CPU frequency: 200MHz
  • ADC Prescale: 0xF (Clock / 8.5)
  • Resolution: 12 bits
  • SOC: SW trigger / ADCINB0 / 200 as sampling windows

We monitored the acquired voltage value down to the via below the BGA so we are pretty sure of the voltage value.

Any idea ?

Clément

  • 0
    TI__Mastermind 42930 points
    Hi Clément,

    Nothing stands out based on the setup settings you have provided. A 200 sample window (S+H of 1uS) and a prescaler of 8.5 (ADCLK = 23.5Mhz) are well within ADC operating specifications. Some things you can also check if you have not done so:

    - Ensure that the 3V VREF is stable at all times.
    - Channel B0 is also VDAC for DAC module. Check that DAC is not doing anything while ADC is converting the channel B0.
    - Can you try converting on other ADC channels/modules to see if this is only an issue with channel B0?

    Best regards,
    Joseph
  • 0 in reply to Joseph Casuga
    TI__Mastermind 42930 points
    Hi Clément,

    Checking to see if converting on channel B0 is still an issue for your application. If you still need assistance with debugging the issue, please let me know.

    Best regards,
    Joseph
  • 0 in reply to Joseph Casuga
    Genius 3016 points

    Hi,

    Sorry for not getting back to you earlier.

    We had the DAC configured to be using VDAC but we weren't using the DAC while the ADC is running.

    I changed our code to ensure the DAC is configured to use the internal reference instead of VDAC and did a test but the result was still the same.

    I confirm channel B0 is the only channel that present such an error.

    The only thing we didn't check yet as it's not that easy is to ensure the 3V external VREF we provide is stable at all time. But based on our design and the few measurements we did, I don't think we have an issue there.

    Clément

  • 0 in reply to Clément Letonnelier
    TI__Mastermind 42930 points

    Hi Clément,

    No worries, just wanted to check how the debug is going.  Since channel B0 is mux'd with VDAC, the internal channel capacitance on this pin is higher than the rest of the ADC channels (around 100pF).  This might be causing the drop in the reading especially if the source impedance is high.  Maybe you could try increasing the sampling time (ACQPS) to get the capacitance charged when sampling.

    Best regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Mastermind 42930 points
    Hi Clément,

    Not sure if you are still having this issue with channel B0 even when using higher sampling times or you have resolved it already. If not, please post in this forum so we can assist with the debug.

    Regards,
    Joseph