Posted on May 17, 2011 at 14:30The way the converter works is by taking a sample from a pin onto an internal capacitor. During the conversion the charge on the capacitor is changed a bit in a way that looks random but is related to the bit values of the result. Even with that bumping around, the capacitor ends up with a similar voltage to what it started out with.
When the sample capacitor is then switched to a unconnected pin, the charge equalizes and again remains a noisy approximation of the same level.
It's important to keep this behavior in mind. If you are getting a very noisy conversion, check that you are actually using the value from the channel you expect.
