There is an internal 8.1K resistor between P2 and the internal AGND. This 8.1K along with the external bypass capacitor forms a Low Pass Filter that filters any noise on AGND. Because of this high value of internal resistance, it is not adviceable to use P2 as a reference AGND for any analog system. The best way to bring out AGND is by using a RefMux user module or by writing to testmux bits in the ACBxxCR2 register and through an analog output buffer. The analog output buffer is a low impedance source and will provide a stable AGND. When using P2 as AGND reference, an external OpAmp buffer needs to be placed at P2 to get low impedance AGND reference source.