8.4.11 User Calibration
ADC calibration can be performed by the user without using calibration commands. This procedure requires the user to apply the appropriate calibration voltage as with using calibration commands, but in this case the user computes the calibration values based on the conversion result and then writes the value to the calibration registers. The procedure for user calibration is as follows:
- Set the OFSCAL[2:0] register = 0h, and GANCAL[2:0] = 400000h. These values set the offset and gain factors to 0 and 1, respectively.
- Apply zero voltage or short the inputs (example: set the ADC mux to internal short). Wait for the input voltage and the ADC to settle for a minimum of 63 conversions and then begin averaging of a number of conversion results. Averaging conversions results in a more accurate calibration. Write the 24-bit averaged value to the OFC register.
- Apply a DC or AC calibration voltage at least 5% below full-scale. Be sure not to be near or exceed 100% FSR otherwise the conversion data clips, resulting in erroneous calibration. Wait for the calibration voltage and the ADC to settle for a minimum of 63 conversions. Use Equation 15 or Equation 16 to compute the scale factor value.
Equation 15 shows the DC calibration voltage. Use the average value of the ADC data.
Equation 15.
Equation 16 shows the AC calibration voltage. Use an RMS value of the ADC data.
Equation 16.