ADC2y provides the ability to compute offset and gain corrected conversion data using user-programmable offset and gain correction registers. The same calibration values are used for all sequence steps, however, the offset and gain correction factors are not applied when the internal temperature sensor is selected for measurement by MUX2y. As shown in Figure 8-8, the 16-bit offset correction value (OCAL2y[15:0]) is subtracted from the conversion data before being multiplied by the 16-bit gain correction factor (GCAL2y15:0]). Output data are rounded to the final resolution and clipped to +FS and –FS code values after the scaling operation. The offset and gain calibration coefficients must be stored in external nonvolatile memory and programmed into the offset and gain calibration registers each time the device powers up or resets because the ADS131B23 registers are volatile.

Figure 8-8 Calibration Logic Block Diagram

The 16-bit offset calibration value is provided in two's-complement format and programmed into the OCAL2y[15:0] bit field. Table 8-9 shows example offset calibration values. The LSB size of the offset calibration value is calculated using Equation 10 and Equation 11 depending on the selected gain setting.

The 16-bit gain calibration value is provided in two's-complement format and programmed into the GCAL2y[15:0] bit field. One LSB of the gain calibration value equals a gain correction factor of 1/2¹⁶ = 0.000015. Table 8-10 shows example gain calibration values.

The recommended calibration procedure is as follows:

1. Preset the offset and gain calibration registers to OCAL2y[15:0] = 0000h and GCAL2y[15:0] = 0000h, respectively.
2. Perform an offset calibration by shorting the ADC2y input in one or multiple sequence steps internally to AGNDy using the respective input multiplexer setting (SEQ2y_STEPn_CH_P[3:0] = 1001b), or short one of the ADC2y inputs externally at the system level to include the offset error of the external filter stages. Set the gain for the respective sequence step to 1 or 2. Acquire multiple conversion data and write the average value of the data into the offset calibration registers. Averaging the data reduces conversion noise to improve calibration accuracy.
3. Perform a gain calibration by applying a precision calibration signal to one of the ADC2y inputs or at the system level to include the gain error of the external filter stages. Choose the calibration voltage to be less than the full-scale input range to avoid clipping the output code. Clipped output codes result in inaccurate calibration. For example, use a 1.2-V calibration signal when using gain = 1. Acquire multiple conversion data and average the results. Use Equation 12 to calculate the gain calibration value.
   Equation 12. Gain Calibration Value = (expected output code / actual output code)
   The expected output code for a 1.2-V calibration voltage using gain = 1 is: (1.2 V / LSB size) = 7AE1h, where LSB size = (1.25 V / 2¹⁵). If the actual measured output code is 6FB6h for example, then the gain calibration factor calculates to 1.1. The resulting gain calibration value to write into the GCAL2y[15:0] bit field is: (1.1 – 1) / (1 / 2¹⁶) = 199Ah.