ADC1y provides the ability to compute offset and gain corrected conversion data using user-programmable offset and gain correction registers. As shown in Figure 9-8, the 24-bit offset correction value (OCAL1y[23:0]) is subtracted from the conversion data before being multiplied by the 16-bit gain correction factor (GCAL1y[15:0]). 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 ADS131B26-Q1 registers are volatile.

Figure 9-5 Calibration Logic Block Diagram

The 24-bit offset calibration value is provided in two's-complement format and programmed into the OCAL1y[23:0] bit field that spans across the ADC1y_OCAL_MSB and ADC1y_OCAL_LSB registers. Table 9-10 shows example offset calibration values. The LSB size of the offset calibration value is calculated using Equation 6.

The 16-bit gain calibration value is provided in two's-complement format and programmed into the GCAL1y[15:0] bit field. One LSB of the gain calibration value equals a gain correction factor of 1/2¹⁶ = 0.000015. Table 9-11 shows example gain calibration values.

The recommended calibration procedure is as follows:

1. Preset the offset and gain calibration registers to OCAL1y[23:0] = 000000h and GCAL1y[15:0] = 0000h, respectively.
2. Perform an offset calibration by shorting the ADC1y inputs internally using the respective input multiplexer setting (MUX1y[1:0] = 10b), or short the inputs externally at the system level to include the offset error of the external filter stages. 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 the ADC1y 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 150-mV calibration signal when using gain = 8. Acquire multiple conversion data and average the results. Use Equation 7 to calculate the gain calibration value.
   Equation 7. Gain Calibration Value = (expected output code / actual output code)
   The expected output code for a 150-mV calibration voltage using gain = 8 is: (150 mV / LSB size) = 7AE148h, where LSB size = (2 × 1.25 V) / (8 × 2²⁴). If the actual measured output code is 6FB587h for example, then the gain calibration factor calculates to 1.1. The resulting gain calibration value to write into the GCAL1y[15:0] bit field is: (1.1 – 1) / (1 / 2¹⁶) = 199Ah.