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DDC EVM Channel Differences

Drake
Prodigy 90 points
Other Parts Discussed in Thread: DDC114, DDC112

Hi,

   I am working with the DDC114 EVM. We have found at times that Side A and Side B are very different.

We are using 5ms integration times and everything else is default. Sometimes the codes are different by 10k.

Any clues to this behavior?

Thanks,

Drake

  • 0
    TI__Genius 14825 points

    Hello Drake,

     

    A good test to verify it is not something to do with the input is to put the device into test-mode by setting the TEST pin high and collecting data.  If you still see radically different A side-B side matching data, then you can look to the reference or supply or something systemic to the device.  If not, then you can probably focus on the inputs for further debugging.

     

    Jason

  • 0 in reply to Jason Bridgmon
    Prodigy 90 points

    Thanks Jason. I see your name on these schematics thought it looked familiar.

    We looked at the test mode and everything is ok there. Codes are very close. 

    Root cause looks to be something to do with our BNC cable hook up.

    Do you have any estimation what the trace capacitance is on the PCB?

    Other than what's in the design notes can you provide any best practices for implementing these parts?

     

    Thanks,

    Drake

  • 0 in reply to Drake
    TI__Genius 14825 points

    Drake,

    We did some ballpark measurements on an EVM for a newer DDC product EVM and found trace capacitance to be from 9pF to 11.5pF.  I would expect the DDC114 EVM traces would be similar.

    There is a document on the DDC112 EVM kit from 2000 that goes into some detail about how to lay out traces and shielding for the traces on the DDC112 - http://www.ti.com/litv/pdf/sbau015 - I suggest looking over pages 3 and 4 for some general guidelines that are applicable to the DDC114.

    Jason

  • 0 in reply to Jason Bridgmon
    Prodigy 90 points

    Jason,

       Can I get a hold of you directly? I see that you are one of the point men on this product and I would like to have the chance to chat with you directly and talk more about some specifics.

    Please send an email to drake.miller@gmail.com

    Thanks,

    Drake

  • 0 in reply to Drake
    Prodigy 30 points

    Hi Drake,

     

    have you found a solution yet? I encounter the same problem wehn connecting a large capacitance photodiode (2000pC) to the  DDC114EVM. The A anf B channels differ by about 14k. This however vanishes when disconnecting an reconnecting the photodiode. The effect is independent of the exposure time but depends on the DDC capacity (most prevalent at 50pC)

     

    Best regards,

     

    Sven