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ADS1261: GANCAL command for ADS1261

Adrian Unkeles1
Prodigy 210 points
Part Number: ADS1261
Other Parts Discussed in Thread: ADS1260

Seeking clarification on section 9.4.7.4 of the datasheet. It says:

"To calibrate, apply a positive full-scale calibration voltage to the ADC, wait for the signal to settle, and then send the calibration command".

Should this full-scale calibration voltage should be applied to whichever differential inputs are configured in the INPMUX register? I'd like to use the voltage reference external input (on pins AIN0 and AIN1) as the positive full-scale calibration voltage if possible. If I use the voltage reference is the settling time a non-factor?

  • 0
    TI__Guru 53516 points
    Hi Adrian, The FS signal should be applied to whichever inputs you are using. But I did not understand what you meant by "using the voltage reference external input as the +FS voltage". Can you please explain in more detail what you intend to do here? Also please note that you should perform an offset calibration before the gain calibration. -Bryan
  • 0 in reply to Bryan Lizon86
    Prodigy 210 points

    Hi Bryan,

    Yes, I am doing offset calibration before gain calibration.

    I intend to have the external voltage reference (on the AIN0/1 differential pair) be used by the ADC when setting the full-scale calibration. During operation we only expect to measure positive voltages on the ADC in the range of 0 to 5 volts, thus it made sense to use our external voltage reference (GND and 5[V] rails) to set the full-scale calibration. Am I misunderstanding some aspect of the full-scale calibration?

    In my calibration process as of now:

    1) send a START command

    2) send a SFOCAL command and wait for averaging of conversion results

    3) send a WREG command to the INPMUX register, setting it to AIN0/1 (where my external voltage reference is attached)

    4) send a START command

    5) send a GANCAL command and wait for averaging of conversion results

    Hopefully I'm thinking about this and reading the datasheet correctly. Thanks for your insight.

  • 0 in reply to Adrian Unkeles1
    TI__Guru 53516 points

    Hi Adrian,

    Thanks for clarifying, I wasn't sure if you were intending to use the ADC's REFOUT feature as the full-scale signal into pins AIN0/AIN1 for the calibration. But now it is clear what you are trying to do.

    If you intend to use an external VREF with the ADS1261, then use you should connect this to one of the differential reference input pairs on the ADC as you described. Note that if you also use the same VREF for the calibration signal, then the calibration will not remove gain errors from the VREF, only the ADC. If you used a second precision signal source for the calibration, then you could also remove the initial inaccuracy inherent in your VREF. But if your discrete VREF is already accurate enough, then this is not that critical.

    Also note that the PGA must be bypassed in order to measure ground-reference inputs on the ADS1260. This is described in section 7.3 in the datasheet.

    -Bryan