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28355 Total ADC Error

Isaac Pilli1
Intellectual 310 points
Other Parts Discussed in Thread: TMS320F28335

Hi,

Working with TMS320F28335 ADC, running ADC_cal() and using 25MHz  ADCCLK :

From the data sheet ( SPRS439M.pdf p. 169 ) :

Offset error ...................................................................................   +/–  15 LSB
Overall gain error with internal reference ..............................   +/–   30 LSB

Looking at http://www.maximintegrated.com/app-notes/index.mvp/id/748 figure 5 definition,

my interpretation is that the worst case ADC error is actually 15+30 = 45 counts, which are > 1% of the total of 4096 counts !

Is this a correct assumption?

Any recommendations to reduce the error?

Isaac

  • 0
    TI__Guru 60865 points

    Hi Isaac,

    Yes, worst case error will be the 45 LSB you have, plus worst case linearity error of 2 (INL) + 1 (DNL) LSBs = 48LSBs.

    Keep in mind that this will only occur at worst case temperature, voltage (VDDA), and process variation.  Also, gain error will only be as much as 30LSBs at full scale.  

    You can attempt to periodically calibrate out offset error by sampling ground, then adjusting conversion results as necessary.  You can do something similar for gain by adding a precise voltage reference near full scale (say 2.5V), sampling periodically, and digitally adjusting other conversion results as necessary.  

  • 0 in reply to Devin Cottier
    TI__Expert 6525 points
    Hi Devin,

    Could you please tell the overall gain error value in datasheet means before running ADC_cal() or after run ADC_cal() ? My customer wants to know does ADC SPEC on datasheet measure before run ADC_cal() ? Thank you for your reply in advance.
  • 0 in reply to Lisa Ding
    TI__Genius 14051 points
    Hi Lisa, DS performance is guaranteed only after running ADC_cal().
  • 0 in reply to Joe Prushing
    TI__Expert 6525 points

    Hi Joe,

    I see. thanks for reply.