When using the XADC with the internal reference, the Gain Calibration Coefficient always reads 0x007F for both ADCs.
Why does this happen?
The XADC can digitally calibrate out any offset and gain errors in the ADCs and power supply sensor.
By connecting known voltages (i.e., VREFN and VREFP) to the ADCs and the supply sensor, the offset and gain errors can be calculated and correction coefficients generated.
These calibration coefficients are stored in status registers 08h to 0Ah for ADC A and 30h to 32h for ADC B.
However, when the internal reference is used, this gain calibration logic is disabled and the status registers for the gain coefficients always read back 0x007F, which means that the coefficient is 3F(the max positive value).
The reason for this is that the internal reference is trimmed during the factory test using the XADC, so the gain error is accounted for in the reference trim.
This means that the XADC performance specifications are met when using the internal reference and the gain calibration is not required. The coefficients should be ignored in this operating mode.