I know a PWM output can be smoothed using a simple low pass filter and using the highest PWM frequency, but I'm having a little problem finding out what state the PWM pin is in during the low (or off) phase of the PWM. Is the pin open or draining? Basically what I mean is during the low phase is the pin grounded, grounded via a pull-down or just floating with no connection?
The reason I need to know this is because the smoothed output will be fed into an op-amp to decouple it (the output needs to be used to bias another circuit and the smoothing resistor would mess that up, thus it needs to be decoupled). Since an op-amp has a very high input resistance, the low pass capacitor will not be able to discharge quickly enough and the voltage will rise to 5V regardless of the PWM duty cycle if the PWM pin is open during the off phase. If it's syncing current to ground then all is fine.
Can't supply any diagrams or anything, all this is in my head only atm.