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TMS320F28335 adc gives more error in adc result

Johnson John
Intellectual 630 points
Other Parts Discussed in Thread: TMS320F28335

Hello

We are using TMS320F28335 in our boards and seeing more error (~350 counts offset at 2.7Vdc input condition and ~60 counts offset in 0.3V condition ) in ADC conversion .  All other boards are working and Only this board we are facing this issue.

We checked adc reference (using internal reference) across (ADCREFP and ADCREFM) and measured 0.666Vdc instead of 0.75 (datasheet typical). Can anyone tell the issue, Why are we getting 0.666Vdc? Is it causing this error issue? We checked power supply input processor and adc input voltages and NO issues.

regards

  • 0
    TI__Genius 13586 points
    I do think the 0.66V could be a clue to the problem.
    Can you can post the value which get loaded into the ADCREFSEL register? I might be able to determine if something looks fishy with the factory trim.

    Is this a new design or well established? How many working boards?

    The best thing to do at this point is an A/B swap. In other words remove the device from the failing system and put it on a known good board and take a device from a known good system and put it on the failing board. This will narrow down whether it is the board, device, solder short, etc. If you cannot do this, I would at least double check the cap and resister values on REFP/REFM/RESEXT looking for incorrect component values, solder shorts, open contacts, and so forth. I should also ask whether the failing board ever worked?
  • 0 in reply to Joe Prushing
    Intellectual 630 points
    Thank you for the prompt response.

    Yes, ADCREFSEL register value is 0x18AB

    In CCS software, I changed the ADCREFSEL value to 0x0000 for testing purpose and the adc reference (across (ADCREFP and ADCREFM) was changed to 0.700Vdc (Not reaching 0.75V). If I use working board it is changing 0.790Vdc max.

    No, this failing board ever worked.
  • 0 in reply to Johnson John
    TI__Genius 13586 points

    The factory trim for internal reference looks very reasonable.

    I'll go back to my previous advice.  I'm not sure how automated your PCB build flow is, but I once saw an assembly house put 22ohm rather than 22kohm on ADCRESEXT which had a significant performance impact.  If the REFP/M/RESEXT components are within standard tolerance (even up to 20%) you should be OK, but an order of magnitude off will cause problems.

    If in the end you cannot find anything on the board, and in particular an A/B swap shows the problem following the device, you may just need to send the device back through the QTS (customer return) flow.