In message <3.0.2.32.19980118014641.00930100@postoffice2.mail.cornell.ed
u>, Sean Breheny <shb7@CORNELL.EDU> writes
>Hello all,
>
>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? I can remember when I asked a few other questions about this
>device on the list before, at least one person said something like "if you
>do [blah blah blah, etc.] it will then have a nice 50 ohm input impedance"
>While 50 ohms is good for many devices where I want the load matched to the
>generator, lets say I want my scope to see the output of an oscillator with
>Zout = 1K, I will see little or nothing if my Zin(scope) = 50 ohms!
>Besides, what happens when my 50 ohm scope gets attached to a signal which
>is riding on a 12 V DC level? My scope will have to dissipate 2.88 Watts!
>It is true that the cable going to the probe will probably have a
>characteristic impedance of around 50 ohms, but if I compensate the
>probe(or the internal input nework), I should be able to compensate for any
>added reactance due to the cable, right?

The impedance of a scope probe cable isn't 50 ohms - it's far higher
(although I've no idea what the actual value is!). If I can remember
back that far, I seem to recall that the impedance is calculated from
the diameter of the outer screen and the diameter of the inner core,
where the smaller the inner, the higher the impedance If you look in a
scope cable the inner is a single, very thin, wire - often corrugated, I
presume to keep it in the middle of the air-spacing.

>I expect the scope's front end to have abt a 50 MHz BW. My input network
>must connect the BNC input connector to a MAX4107 300MHz BW op amp which
>has an input which looks like a 1Meg resistor in parallel with 2pF of cap.
>This input network will have to provide variable attenuation.
>
>I would appreciate any suggestions that anyone has regarding what input
>impedance is best. I would think the higher, the better.

The standard input impedance for scopes is 1Meg and about 30pF
capacitance, I can't thing of any real reason for wanting to change from
that!. If you keep to the standard specs, that means you can use all the
usual scope probes etc.

It sounds a very interesting project, I would be intersted to hear how
you progress.

--

Nigel.

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