Simon,

If you are trying to perform PWM DAC, feed PWM into a low pass filter. Ensure that PWM frequency differs from cutoff frequency by at least a couple of orders of magnitude. That should provide you with good resolution and signal quality. The more these two frequencies differ and the higher the order of the filter the better the signal quality.

http://www.microchip.com/download/...00538c.pdf

Cory Spackman