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ads1263: Self Calibration vs. User Calibration

Tony Gedwillo
Prodigy 20 points
Other Parts Discussed in Thread: ADS1263

I have an ads1263 ADC, and I am trying to make sense of some of the calibration procedures in the documentation.  Section 9.4.9.6 says to apply a "positive full-scale dc signal to the ADC".  Does this need to be the exact full-scale reading?  Or can this just be greater than or equal to full scale?

I am especially confused because the user calibration section (9.4.9.9) requires a voltage that is less than the positive full-scale value.  This makes sense, because you use the actual and the expected value in the calculation that this section cites (equation 23).

How does the auto-calibration in section 9.4.9.6 do it?  The only sense I can make of this is that you have to try to get as close to the positive full-scale reading as you can without going over and saturating the ADC.  The ADC must just use the full scale value in the "expected" value in equation 23.  Is this correct?  Can anyone help me correctly calibrate my ADC?

  • 0
    TI__Mastermind 44680 points

    Hi Tony,

    Yes, the ADC expects a perfect full-scale voltage (Vref/PGA) to perform gain calibration and will use 7FFFFF as the "expected code value" in equation 23.

    For a PGA gain of 1 V/V, you could simply connect the reference source to your inputs; however, for other gains it is not as easy to generate a perfect full-scale voltage. My recommendation would be to apply the "User Calibration Procedure" using the SYSGCAL1 command like so...

    Perform the system gain calibration routine with an input signal less than 100% full-scale. The reason why, is that the system gain calibration routine only works if you apply a perfect 100% full-scale input voltage, which is very difficult to guarantee and very easy to accidentally over-range the ADC. There are multiple ways of doing this, but the easiest way may be to simply apply a 95% full-scale input signal (for example) and run the SYSGCAL command as usual. Once the ADC calculates the gain calibration coefficient for the FSCAL[2:0] registers, you can correct for the 95% full-scale input by multiplying the FSCAL[2:0] registers value by 0.95 and writing this result back to the FSCAL[2:0] registers....The gain calibration is still only as good as the accuracy of the full-scale "reference" voltage, but at least this way you can avoid over-ranging the ADC and account for signals less than full-scale.

    Best Regards,
    Chris