Well, I STILL don't see why you are talking about a comparator if you're looking at this with the adc. I don't quite understand what you are doing, exactly; can you explain this a little better. It appears that you are charging and discharging the sensor(cap) through a 220 ohm resistor, is that right? This appears to be a little small, since it will only give you a nominal time constant of 33 nS. I THINK what you would want to do is start charging the cap through a resistor, and sample the voltage output. From your samples you should be able to determine the time constant, and, thus, the capacitance.