RE: [SI-LIST] : Probing PCI slew rate test

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Mon Nov 22 1999 - 08:13:25 PST


>I have been working on trying to probe PCI buffers against the PCI
>spec slew rate load (1k to vss, 1k to vdd, 10pf to vss)
>
>These are pretty fast edges (spec up to 4v/ns) and what I have
>been looking at so far is to use a 8pf scope probe as a replacement
>for the 10pf load in the spec.

>I know that at fast edge rates a passive resistive load is better,
>but with a 1k scope input being on the same order of magnitude as
>the spec load I am not sure it is a good idea.

As far as loading is concerned, you could use either the 8pF scope probe
(properly compensated) and ~2pF of stray capacitance and 1K
pull-up/pull-down resistors, or the 1K passive scope probe (which serves as
the 1K pull-down) in parallel with just a 1K pull-up and 10pF.

But find out how good the 8pF scope probe is. Does it have the bandwidth?
Including calibration (compensation) error? What about its resonance? How
short/small can you make the probe+return wire loop? What is its impedance
vs. frequency? Some "hi-Z" scope probes may have a big loss factor at
higher frequencies, which cannot be represented by a 10M resistor || XpF at
several MHz.

Regards,
Andy


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