The impedance of this pin changes over time.
Moreover, the sampling capacitor's capacitance is in the order of a few pF.
During sampling, a current flows in order to charge the capacitor, but normally it is just leakage current.
Therefore, the impedance fluctuates according to the status.
This A/D pin must be driven using a low impedance in the order of just a few kΩ.
This is because, during sampling, the sampling capacitor for conversion must be charged within the A/D accuracy.
A high accuracy cannot be obtained when the drive impedance is high.