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ADS1261: Full Scale Calibration

ttd
Mastermind 8580 points
Part Number: ADS1261

Dear Technical Support Team,

①Is it ok to execute only GANCAL for Full-Scale Calibration automatically?

　Output code is over 7FFFFFh while applying full-scale Vin(for example, fullscale positive Vin is ＋2.5V when Gain=1 ).

　Does fullscale calibration algorithm try 95% of expected fullscale automatically like describing  User Calibration Procedure?

② If ① doesn't work , should user do " User Calibration Procedure" manually?

Best Regards,

ttd

  • 0
    TI__Mastermind 44680 points

    Hi ttd,

    1. I would always recommend performing offset calibration BEFORE gain calibration. Performing calibration out of order, or skipping offset calibration, means that the offset error will be present in the gain error measurement, and this will skew the gain error correction factor.

      The ADS1261 does not automatically scale the gain correction to 95%, so this is something you may need to do before issuing the GANCAL command.

      Alternatively, you could change the input voltage to be 95% of the ADC's positive full-scale voltage and run GANCAL. Then after the ADC has calculated the gain error correction factor (and written it to the FSCAL[2:0] register) you would need to multiply this value by 95% and write it back to the FSCAL[2:0] registers. The reason for this is the ADC assumes that the input signal is equal to 100% positive full-scale.
       
    2. Correct. The ADC clips the output codes to 7FFFFF whenever a signal larger than 100% full-scale is measured (this measurement includes the gain error, hence for the potential for measurements greater than 100% full-scale). Reducing the gain correction factor will allow you to measure the signal and gain error without any clipping. Comparing the measured value to 799998h is similar to multiplying the measured correction factor by 95%, that I mentioned above.
       

    I hope that helps!

  • 0 in reply to Christopher Hall
    Mastermind 8580 points

    Hi Chris Hall,

    Thank you for your reply.

    Additional question,

    When ①G=1.1(just example) over 7FFFFF and issuing the GANCAL command, then ADS1261 compensate from ①G=1.1 to G=1.0 automatically? 

    Based on your mention before, user need to calibrate manually from ①G=1.1 to ③=0.95.

    After issuing the GANCAL command, ADS1261 calibrate from ③G=0.95 to ②G=1.0 automatically.

    I hope the both calibration automatically(from ①G=1.1 to ②G=1.0 and from ③G=0.95 to ②G=1.0) when issuing GANCAL .

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Mastermind 44680 points
    Hi ttd,

    The ADS1261's GANCAL calibration does not automatically adjust for the previous value programmed into the FSCAL[2:0] registers.
    ... the User Calibration Procedure described in section 9.4.7.6 of the datasheet does not use the GANCAL command at all. If you follow this procedure, you'll have to manually perform these calculations to determine the final values to write to the FSCAL[2:0] registers.

    To determine if this process is even necessary for you at all...
    First measure your 100% full-scale signal. If your output code(s) are less than 7FFFFF (i.e. not clipping) then you do not need to reduce the gain correction factor to avoid clipping - in this case you can simply perform the GANCAL operation with the default FSCAL[2:0] register values.

    It is only when the output codes are clipping that you need to either reduce the input signal, or reduce the gain scaling factor to be able to accurately calibrate for full-scale.

    In many cases, unless you have external components in front of the ADC that are contributing to a large gain error, the factory trimmed gain error of the ADC will be very good and not require gain calibration.