Why is AREF connected to a capacitor in the Arduino Pro Mini Schematic?

Question

I'm currently looking at the Arduino Pro Mini schematic and have noticed that the pin AREF (analogue reference) is connected to a 0.1uF capacitor as shown below:

I have done some research and read this great explanation into AREF and now understand its purpose as a voltage reference. What I do not understand however is the purpose of the capacitor. Is it as a means of stabilising the pin when an external voltage is applied, or something else?

A full schematic of the Arduino Pro Mini can be sourced from here.

Answer 1

On the datasheet for the Atmega328P, in the Analog-to-Digital Converter section, is the sentence:

The voltage reference may be externally decoupled at the AREF pin by a capacitor for better noise performance.

Also:

The internal voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve noise immunity.

Thus the capacitor is there to reduce noise on the AREF pin.

Answer 2

An ideal capacitor presents an impedance of wC (radian frequency * capacitance -- ohms). Of course ideal capacitors do not exist in real component selections or circuits and all include parasitic losses and inductance. The parasitic components depend on the type of the capacitor, lead length, connecting circuits, etc. Oscilloscopes, network analyzers and spectrum analyzers serve to identify fundamental performance of voltage reference points.

The decoupling capacitor should be connected using short leads to a ground plane layer and the Aref pin.