Changing between reference voltages must be avoided when using the continues sampling method. When there are mixed inputs: measuring external voltages and resistor bridges like the one in figure 4, it's preferable to supply the resistor bridge from the 1.1 reference voltage. Be aware that the reference voltage can only deliver small currents. A load of a 4.7 kΩ resistor let the reference voltage drop from 1.0880 to 1.0857. In this case it's the best to buffer the AREF voltage with an opamp and connect the resistor loads to the opamp output.