As I have already said, every thing depends on A/D converter... As an exemple, if you use TL549 crystal isn't used, clock is used only for data sinchronization. Normaly A/D converts A signal and stores value in register, and you get signal, that new value can be read. That is sinchronization. How you start new conversion and so on, depends on the aplication. If that was your question...

For testing HF filter is not necessary.