A third option for ADC reference is the internal analog power supply. However, an increase of linearity error results with this connection, and therefore, use this option only for less-critical applications, such as ADC self-diagnostics.
For critical applications, do not use the power-supply reference option. For applications that use the power-supply voltage as the reference voltage, connect the power-supply voltage to the external reference inputs, and select the appropriate external reference bits in the REFMUX register. For example, to measure a 6-wire load-cell, connect the bridge excitation voltages to the external reference inputs, and select the appropriate REFMUX bits.