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  • TI Thinks Resolved

ADS7953: EMI throws off reference?

Part Number: ADS7953

Hi,

I have an ADS7953 in my design, and I had a failure during EMC testing in the EMI chamber during what appears to be BCI at 1MHz.  The chip fails in such that all of the channels work for say 50% of the range (almost like there is an offset).  So it seems the reference voltage is being messed up?  What is causing this?

Some things to note:  the REF5025 is putting out a solid 2.5V, the layout is done as is shown in the data sheet, I have 4 unused channels that have the RC filter network on them, however they are floating.

If I replace the chip post failure, everything is fine, so it does seem that something is getting messed up in the chip.

What can I do to improve the robustness of my design to survive BCI?

Thank you!

Josh

  • Hi Joshua,

    Between the output of the REF5025 and ref input of the ADS7953, what passive components (resistors and/or capacitors) do you have connected?

    Can you confirm that the REF5025 is providing a solid 2.5V during testing? Does the entire system work post failure (after being reset and while not be subjected to EMI)?

    How much is the offset? Is it possible that the inputs are shifting during testing?
  • In reply to Evan Sawyer:

    Hi Evan,

    I have a 10uF, 6.3V X7R ceramic and a 22uF 10V X7R ceramic to ground.  I could not check the 2.5V while it was in the chamber, however post test the 2.5V is solid.  However the entire system is permanently "offset" on all of its values.  It appears to be off by about 50%.  And when I replace the ADS7953, everything is back to normal.

    Also note that +Vbd = 5V.

    Here is the schematic:

    Here is the layout:

    Thank you!

    Josh

  • In reply to Joshua Schofield:

    Hi Joshua,

    It looks like your schematic and layout were not successfully uploaded. Can you please try to attached them again?
  • In reply to Evan Sawyer:

    Hi Evan,

    Copy and paste into this webform appeared to not work, so I tried using the attach button.  Let me know if they come through.

    Thanks!

    Josh

  • In reply to Joshua Schofield:

    Hi Josh,

    Thanks for your patience. I'm working with a couple of colleagues, and we have a couple of questions to ask to help us clarify the source of the issue:

    1) Is the failure permanent and only fixed with a part replacement (as opposed to power cycling the part)?
    2) Does the problem go away after rewriting to the range selection register bit?
  • In reply to Evan Sawyer:

    Hi Evan,

    The failure is permanent, as in only fixed with a part replacement.  So when I replace the part, everything comes up fine.  But I am worried about the susceptibility that the part appears to have.

    Thank you,

    Josh

  • In reply to Joshua Schofield:

    Hi Evan,

    Do you have any advice?  We go back into the chamber in 2 weeks, so the only thing I can think of is grounding the floating pins, because if they were not grounded, they could be susceptible to bulk current injection (acting like antennas)?

    Thank you!

    Josh

  • In reply to Joshua Schofield:

    Hi Josh,

    Thanks for your patience, I've been speaking with several colleagues about possible reasons for the chip to fail BCI. It is possible that something in the device is causing the range to be set to 2*Vref, which would look like a 50% offset.

    - When testing the device after BCI and when you noticed the 50% 'offset,' do you recall if the conversions were all in the range of 0-2.5V or in the range of 2.5-5V?
    - When testing the reference after BCI and you noticed that it still output 2.5V, was the ADC still connected while measuring the reference output?

    Regarding the floating pins, it is a good idea to ground these. I don't believe this will make a difference though.
  • In reply to Evan Sawyer:

    Hi Josh,

    I am going to close this thread as we have an ongoing email. If during your next test the results are different from the first test please let us know (here or via email). I look forward to hearing back.

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