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UCD3138A: EADCVALUE register ERROR_VALUE is showing unreasonable value

Julie Zhu
Prodigy 470 points
Part Number: UCD3138A

Hi,

 

We found in prebias mode, the EADCVALUE register ERROR_VALUE is showing some abnormal number. We keep the voltage level unchanged, enable prebias, from memory debugger, I saw the DACSTAT.DAC_VALUE is constant, EADCVALUE.ABS_VALUE is not changing, but the error_value can change from 0 to -254. Why would this happen?  seems not right.

 

Thanks,

 

Julie

  • 0
    TI__Genius 17090 points
    That's probably caused by the way the memory debugger works. It reads everything a byte at a time. So if the error is going back and forth between 0x1ff (-1) and 0x002 (2), sometimes it will read the low byte when it is an 1, and then read the high byte when it is a1. So that would mean you would get a 0x102, which is a -254. I'd also expect to see maybe a -255 and -256, as well as maybe a +255. If you're getting anything very much between those, then I'd worry.
  • 0 in reply to Ian Bower
    TI__Prodigy 470 points

    Hi, Ian

    Thanks a lot for your quick response.  Your explanation makes sense. Now I don't worry about the very big number like -255, but I still get some relatively big number like 30, 40. I thought in prebias, if the voltage is stable, the error should be 0. however, I kept reading, and I got some random number from 0, 6, 30, 40......  can't figure out what is wrong with the configuration. Do you have any idea about this?

    Thanks,

    Julie

  • 0 in reply to Julie Zhu
    TI__Genius 17090 points
    OK, I'll need to know more about your setup - what board are you using, how are you connecting the hardware, what are all the bits you have modified with the memory debugger, and what are their states?

    Also, what are you trying to accomplish? What are you using the prebias mode for?
  • 0 in reply to Ian Bower
    TI__Prodigy 470 points

    Hi, Ian,

    we are using UCD3138A 40 pin open loop EVM. We connect vin to front end EAP0, enable prebias. The behavior we expected is: if the Vin is stable, then we should be able to see the DAC value is fixed, and the DAC Error value should be a small positive/negative offset because of the noise.  But we observed is :  the DAC value keeps changing, then the error value can be a relatively big number. Absolute value is constant, which seems normal.  

    We are doing experiment to see if we can do the vin feedforward if the prebias works as we imagined. 

    Thanks,

    Julie

  • 0 in reply to Julie Zhu
    TI__Genius 17090 points

    That's interesting.  If the absolute value is staying constant, then it sound like it's working to me. Have you tried experimenting with reducing the Pre Bias range to see what effect it has?

    I'm not sure how you are planning to use it as a voltage feed forward.  The EADC doesn't make a very good ADC for a quickly changing signal, either in prebias or SAR mode.   It doesn't have anti-aliasing or a sample and hold circuit.  So it keeps trying to approximate the dynamic signal, and sometimes it can't catch up.  It sounds like the prebias keeps running a successive approximation.  That wouldn't surprise me, though I've never looked at prebias mode before. 

  • 0 in reply to Ian Bower
    TI__Prodigy 470 points

    Hi, Ian,

    I did a test program and found out we can't use this method to do vin feedforward. 

    Thanks for answering my questions and helping us debugging this issue.

    Regards,

    Julie