RE: [SI-LIST] : What speed scope should I consider?

Ingraham, Andrew ([email protected])
Fri, 21 May 1999 08:06:11 -0400

>Also note that the loading of the probe on the cirucit isn't just the DC
>impedance of the probe. A 10 MOhm, 10 pF probe represents an input
>impedance of
>26.5 Ohms at 600 MHz. (unless I've done the math wrong)

Your math is right, but it may be even worse than this.

I don't have a reference handy, but I have read or been told that many
10Mohm, x10 probes also have a significant resistive loss or shunt factor
(perhaps due to dielectric losses?) at high frequencies. Thus the input
impedance is something much less than 10Mohm in parallel with 10pF.

I haven't used a x10 probe in a very long time.

Get good probes, then pay utmost attention to grounding and probe leads. 1"
is too much.


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