@nctnico
I certainly appreciate your reply; but again, I am not an expert in filter design, and since you did not mention specifics in your recommendation, I don't clearly see how my circuit will really benefit from the RC addition you propose. I must repeat what I said in my opening post about the reason I used large resistors and small capacitors in combination with an op-amp, as opposed to just using a cheap R-C LPF instead of an op-amp -- "capacitor discharge time" affects my ADC acquisition time. As you can see from my schematic, my existing design uses small 1nF capacitors in the input side of the op-amp. Discharge time is short, which is what I want. Using an op-amp circuit and then adding and RC filter on the ADC side would, at least in my mind, defeat the purpose of my having used the op-amp filter in the first place.
Again, I've tested the circuit on a breadboard, and I am not see any ill effects of "capacitive sampling" from the ADC to the output of the op-amp. Indeed, I've not read anyone online about the negatives of using an op amp on an ADC input. If anything, most forums (including the Microchip forum) are filled with talk recommending an op-amp to protect ADC inputs.
My circuit is working on a breadboard, but I've not mass produced a PCB yet, hence my opening post. I am just curious if my resistor and capacitor values chosen are reasonable, and hearing specifics from someone with extensive experience in op-amp LPF design is desired. Also, I am wanting to know if I can reasonably get away with using 5% tolerance resistors (again, see my opening post for the schematic) rather than more expensive 1% resistors. I'm unsure if I even need more expensive C0G capacitors, or if X7R will suffice. And lastly, within the tolerance range of the components chosen, I am wanting to know if there is worry of oscillation or similar such instability. Thanks.