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piccolo ACQPS

Max Sonnaillon
Intellectual 600 points

Hello,

I have experienced a very strange behavior of the on-chip ADC when changing the ACQPS register:

1) Increasing ACQPS does not help at all to reduce the noise level of the measurements.

2) Changing ACQPS affects the DC level of the measurements (average value). A very clear example of this is that I run the routine that TI provides to calibrate the offset every time I turn the DSP on, and the resulting offset depends on the ACQPS value! (the calibration is performed with an internal connection to the negative reference, so my external circuit is not the issue).

I remember having similar issues with the 2812 ADC some years ago.

Does anybody know anything about this? Will this be solved in future revisions of the silicon? Does the 28069 have identical ADC?

Thanks in advance.

Max

  • 0
    Prodigy 10 points

     

    Hi, Max .I also meet the same problem. If you solve it,can you tell your way?

     Thank you .

     my email: kyzone@163.com
     
     for example:
       ACQPS                           the correct result             my  sample  result
            6                                                 100                                  72             
            7                                                 100                                 85              
            8                                                 100                                  94                  
            9                                                 100                                  112         
            16                                               100                                 76               

                 

     
     
     
     
                           
  • 0 in reply to yu kuang
    TI__Expert 8500 points

    Max/Yu,

    The Piccolo F2802x errata (http://focus.ti.com/lit/er/sprz292e/sprz292e.pdf) lists an ADC first sample issue. I'm not overly familiar with the calibration routine, but when you do your calibration you probably need to take two samples and throw out the first result.

    Regards,
    Dave Foley

     

  • 0 in reply to David Foley
    Intellectual 600 points

    Dave,

    I have seen that errata, just in case I always discard the first sample. However, I haven't seen problems with the first sample in my circuits, the issue may be solved in new DSP versions (28035). The issue with the effect of ACQPS is independent from this issue, affects all the ADC results.

    The calibration routine uses all the ADC results, not only one or two. The result of the calibration routine is correct, it gives the offset of the ADC. 

    The issue here is that the offset changes with ACQPS, and the ACQPS does not help to reduce noise.

    It's very easy to verify this problem (try the offset calibration routine with different ACQPS values). I think that somebody in TI should do it and analyze this problem because is a very strange behavior of the ADC.

    Thanks,

    Max