I'm curious of a couple things as well. Why is the output of the front end op amp going through a 4066 instead of straight to the A/D convertor. Also, what is the time for A/D conversion because that would be the shortest possible sample delay due to limitations of the speed of A/D conversion. I am not familiar with the ADC0831 so I don't know what the conversion time is but an outboard convertor is necessary since the 16F84 doesn't have a built in A/D. I am building one using the 16F877 which does have a built in A/D. The minimum time is something like 20 to 25uS so its a little slow but since I am looking to build a boat tow unit, that will be plenty fast, just not for a coin/ring detector. When I do decide to build a coin detector, I will probably do the sampling and integration the old fashioned way and then read the result with the A/D so I can be sure of getting the 10uS to 15uS sample delay desired.
See ya,
Fred