[SI-LIST] Re: How does the loading affects the rise time of a PAD?
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 21 Aug 2001 14:18:17 -0400
The round-trip delay of a 3 inch trace is about 1.1ns. For the 15 inch
trace, it's 5.5ns.
For risetimes (much) longer than these amounts, the trace could be treated
like a lumped load.
For faster risetimes, and assuming low-loss etch, you could treat the trace
like a 50 ohm load (to the line's previous voltage level) for the duration
of the risetime and up to the round-trip delay; and you should NOT treat the
trace like its equivalent lumped load.
The 3 inch trace with a 1ns risetime is a borderline case. The reflection
may appear before the end of the risetime, so it could affect it.
Lengthening it to 5 inches may increase the risetime, but above 5 inches, it
should have little effect.
Of course if you measured your risetime between (say) 10% and 90% of the
FINAL value, and if the driver is intended to be reflected-wave or is not
very strong, then your results will differ. In that case, longer 50 ohm
traces will indeed "slow down" the measured "risetimes" proportionately.
Andy
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