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ADS5400: Sampled values rounded up to the nearest multiple of 256

Nissim Ofek
Prodigy 20 points
Part Number: ADS5400
We have a problem with these ADCs. There are amplitudes that the ADC will round (up) to the nearest multiple of 256.
The following figure demonstrate the problem. We feed the ADC with a DC voltage, take a trace and step it up. When the sampled
value is far from a multiple of 256, we see only the input noise. When it is within a certain range below a multiple of 256, it is
rounded up.
We tested many of these devices. Some behaves perfectly, some worse (the data below was taken from the worse part). Some
parts show this behaviour only below 768, some only below 1024. Also, how wide is the "dangerous" range differs from part to part.
It never happens blow 256. It never happens for values slightly above a multiple of 256. We didn't see it on parts from a previous batch.
Any ideas?
  • 0
    TI__Mastermind 20080 points

    Hi Nissim,

     

    On the results that are collected, is the board and part separate from each other?

    Are you measuring different parts on different boards or different parts on the same board always?

    We need to disconnect the board on which the ADC is on and the ADC itself.

     

    I would suggest swapping out a known good ADC that passes this testing and put it in place of an ADC that is not performing well on the same board.

     

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 20 points

    Hi Rob,

    Sorry for the delay. It took us some time to do the replacement.

    To your questions:

    1. The ADC is part of the board under test.

    2. I measured all the ADCs on about 15 boards (2 ADCs on each). All showed similar behavior.

    3. We had an older board with ADCs from a different batch. The old board behaved perfectly. After swapping,

        the new board behaves perfectly as well.

    Any suggestions?

    Thanks,

    Nissim

  • 0 in reply to Nissim Ofek
    TI__Mastermind 20080 points

    This discussion has been taken offline to resolve.

    Regards,

    Rob