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28335 ADC results jumping around

Di Pan
Prodigy 130 points

Hi all, I am using a ezDSP kit for my project and I found my adc results are sort of unstable and jump around a lot.  I disconnected the board from other circuit and directly tied the ADC pin to the ground.  Surprisingly, I found even the fifth bit (after trunk out the lower four bits) changes from time to time in debug mode.  I am using the cascaded sequential mode.  I've tried changing the adc clock prescale, but could not improve the results.  Any idea what I did wrong?

Thank you very much!

  • 0
    Genius 5885 points

    Hi Di,

    did you tie VREFLO to GND?

    Regards, Mitja

  • 0 in reply to Mitja Nemec
    Prodigy 130 points

    Mitja,

    Thanks for replying.  I didn't....  I tried to tie it to GND.  The result becomes much better for some of the pins.  I found in the manual that VREFLO should be tied to AGND.  Is there any issue if I directly tie it to GND?

    Di

  • 0 in reply to Di Pan
    Genius 5885 points

    Hi Di,

    The results should be good for all of the pins. Check the contacts if otherwise.

    When it comes to grounding, my recommendation to newbies is: have a common ground, only when you know what you are doing migrate to split ground system (analog ground, digital ground and maybe even power ground) as you can do more harm than good with split ground system. You really really have to know what you are doing and you need to be very meticulous when connecting different systems and different supplies.

    You should check TI HW design recommendations for C2000 family (there is a PDF somewhere)

    Regards, Mitja

  • 0 in reply to Mitja Nemec
    Prodigy 130 points

    Thank you very much Mitja!  I think I am a bit confused.  In simutaneous sampling mode, all ADCB channels results look good.  ADCA channels will read, the reading will vary as I change the voltage applied to the pins,  but results are not accurate.  Is it because there is not enough time gap as the converter swithes between two S/H? 

    Also I am not clear about the input channel select register.   I am looking at the document below.  The description is quite short.  Is there a better reference?

    TMS320x2833x Analog-to-Digital Converter
    (ADC) Module
    Reference Guide

    Basically, I need to acquire 14 channels of analog signals at 8 kHz frequency.  I am trying to configure the ADC to operate in cascaded sequencer mode with either sequencial or simotaneous sampling mode.  Any suggestion for that?

    Thanks again!

  • 0 in reply to Joe Prushing
    Prodigy 130 points

    Thank you, Joe.  Any suggestion for the issue I am having?

  • 0 in reply to Di Pan
    TI__Genius 15201 points

    Di-

      I think you may be experiencing the simultaneous sampling latency issue described in the errata ( http://www.ti.com/lit/er/sprz272f/sprz272f.pdf ).  Since you only need 14 samples, you could set the sequencer to sample 16 times and throw away the first two, or you should be able to increase the sample window to greater than 160ns and still meet your sampling requirements.

     

    best regards,

    Joe

  • 0 in reply to Joe Prushing
    Prodigy 130 points

    Hi Joe,

    It seems like what I am expericing.  The first two channels are the ones that are jumping around.  I would try your suggestions. 

    Thank you very much!

    BR,

    Di