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Why do we specifically use 80 for ADC zero offset calibration in f28027 (c2000 piccolo launchpad)?

DHRUMEEL DUTIA
Prodigy 30 points
Other Parts Discussed in Thread: TMS320F28027, CONTROLSUITE

I am using a C2000 Piccolo launchpad (tms320F28027). I am a complete beginner.

I was going through the zero offset calibration function provided in the ControlSuite and the datasheet. We initially add 80 (50h) as offset and then subtract the board noise after repeated conversions. 

My question is - why 80? 

  • 0
    TI__Guru 61235 points
    The value 80 is a compromise. You want the value large enough to offset any negative offset error, but you don't want it so large that conversions start to incur gain error. Remember that the OFFTRIM is applied in analog before the conversion. Suppose the unadjusted offset error in the ADC is positive, say 60. You are now converting 80+60 = 140 LSB, and gain error can start to affect the calibration. So, you don't want to make the 80 any larger.

    We used to suggest an iterative process where 40 decimal was used and iteratively added to the OFFTRIM until any negative offset error disappeared during calibration. I suspect this was thought to be overly complicated, and now TI just suggests using 80 as the static OFFTRIM value during calibration. 80 is larger than the worst-case uncalibrated negative offset error for the ADC.

    Regards,
    David