Sean said:

>I am continuing work on my PIC-based digital storage osc. and I am actually
>in the building phase right now, although the front end is not completely
>finished. This brings me to my question, what should I make the input
>impedance?

(stuff skipped)
>I would appreciate any suggestions that anyone has regarding what input
>impedance is best. I would think the higher, the better.
>

Lower you can always go, by adding external circuitry. Higher you can't.
Sounds like you've got a good start on the trade-offs involved. An input
that appears to the external world like 10 Megohms paralleled by ten to a
few dozen pf is standard for a 10X probe.

One thing you didn't mention is the kind of protection you intend to
provide for the internal element that is being fed this signal. Protective
circuitry, if any, may affect the input impedance.

For general-purpose use, make it as idiot-proof as possible. Someone,
somewhere, will shuffle across the carpet and touch the input. You'd like
it to survive. Fortunately, the PIC part is pretty durable, but if there
are other devices attached to the input, they may not be as forgiving.

Good luck, I hope this helps,

Reg Neale