I should also add that - when converting voltages, you gain a lot of accuracy from oversampling.

For example, if you need to multiply an A/D reading by 14, rather than take one sample and multiplying it by 14, take 14 samples and add them up. At 40Mhz, a single conversion takes only about 20uSec, so the oversampling doesn't add much overhead.

http://en.wikipedia.org/wiki/Oversampling