I’ve been working on a project where I needed a consistent DAC signal to be sent from a control device to a sensor. The sensor wanted a voltage input which was steady, but arduino boards don’t have true DAC output, rather they use PWM to approximate a voltage over a given time. For many devices (like a simple LED) this is fine, but for higher speed systems, more precision is required. A simple solution is to employ a low pass filter, similar to that described here.
The site mentioned above has an excellent link for calculating your R and C values, based on the input signal your dealing with, which can be found here.
-Austin