Noise in FET reverse-polarity protection for ADC
Lets examine VDD sag, due to series FET switch and local bypass capacitor:







simulate this circuit &ndash; Schematic created using CircuitLab

The added resistance of FET switch will reduce the VDD to ADC, and to the VREF.
Can you accept that?

The 100Ohm to ADC Ref pin causes an I*R error, because Ref pin demands charge as the ADC performs the analog-digital conversion. The average of that charge, over time, is a DC current flow; that current times 100Ohm is the Ref error.