You might not even need the cap and diode.
It looks like your using the diode and cap to generate a filtered voltage, ie a voltage that doesnt change quickly, if so you dont need to, the pics own reference voltage is good enough.
As Ron mentioned your on the edge of the reolution from the atod within the pic, to improve things an op amp would make a difference, you could capacitively couple the input of the op amp and use reasoanbly high gain to give you a good measurement.
Edit: Just occured to me that you might be using the comparator rather than the atod, this might work 4mV on the other hand is pretty close tolerance, you'd have to make sure noise isnt an issue.