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ADS7816 possible latch up state

Jan Zomerdijk
Prodigy 20 points
Other Parts Discussed in Thread: ADS7816, REF02

Hi All,

My question is : (see above circuit)

Is it possible for the ADS7816 to become in a latch-up state when the Vref is powered up approx. 500us before VCC ?

Vref and Vcc are derived from the same supply but Vref is measured to rise quicker than Vcc. This allows Vref to become approx. 1V-1.2V higher than Vcc during start-up.

Vref comes from a REF02 and I've seen that sometimes the output of the REF02 collapses. This happens very randomly and with about 1% of the produced boards.

Thanks in advance,

Jan 

  • 0
    TI__Guru**** 180741 points

    Hi Jan,

    Sorry about the slow reply on your question regarding the ADS7816.  As noted in the absolute max ratings table on page 3 of the data sheet, the analog inputs (which includes Vref) should not exceed the analog supply by more than 300mV.  Does the collapse of the REF02 coincide with a latched up ADS7816?  Have you tried increasing the value of R45 or decreasing C65 to get Vcc to ramp up faster than the reference?

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Hello Tom,

    Thanks you for the answer.

    The collapse of the REF02 output does indeed coincide with the latch up of the ADS7816, that triggered me to start measuring Vref and Vcc.

    I've looked a bit further for the problem and I think the problem may (also) be ESD related. There were more complaints about this machine that point into that direction. Right now I'm setting up an ESD test stand to investigate. Something that was already on my wish list....It will take a while because I'm not that familiair with performing those type of tests and I want to use this machine and it's problem to gain experience.

    Changing the filtering components is a good tip and may be a solution for powering up the device in the right order, I'll work that out.

    Do you know if anything changed with the ADS7816 since 2001 that caused the device to become more sensitive to ESD ? Maybe a different chip process, scaling down the structures etc. ? Funny thing is that it seems that the older machines are little or less succeptible to this problem.

    best regards,

    Jan

  • 0 in reply to Jan Zomerdijk
    TI__Guru**** 180741 points

    Hi Jan,

    Thank you for confirming the conditions of the REF02 match up with the erroneous behavior of the ADS7816.  I've been over the QC records for the ADS7816 and don't see any process change notice regarding scaling of structures or the movement of the fabrication process from one plant to another.  Limiting the current from the REF02 by increasing R45 to something like 150-200 ohms should help.