RE: [SI-LIST] : Best type of models, edge rates & load

Peterson, James F ([email protected])
Wed, 21 Jul 1999 08:07:57 -0400

As mentioned in this thread, C-Lumped test loads are not generating
realistic numbers. BUT, this lumped test load should be in the IBIS model
(Cref, Rref). Some SI tools adjust the analysis based on an unloaded
waveform. They throw out the lumped stuff. It seems an effective work-around
(we get good correlation between sim's and measured). Now this doesn't help
the data sheet, but it does help the simulation.
OK - time for a dumb question : what is Tco?

Jim Peterson
[email protected]
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
727-539-2719


-----Original Message-----
From: D. C. Sessions [mailto:[email protected]]
Sent: Tuesday, July 20, 1999 5:33 PM
To: [email protected]
Subject: Re: [SI-LIST] : Best type of models, edge rates & load

Mark Nass wrote:
>
> There has been some discussion recently about parameters of parts
> specified into 40pf caps and accuracy of models. I generate this type
> of data for our devices for our own use and our customers. So I am
> curious as to what people think would be the optimal way to generate
> Tco, Tsu, jitter parameters and IBIS models so that they would be
confident
> they were getting exactly what they needed for signal integrity and timing
> analysis. Any feedback?

If you had to pick just one load, it would be 50 ohms. Notice I didn't
write "50 ohms and 30 pf" or anything like it. To get the best sims we
need the high-frequency content of output waveforms, and artificially
filtering it out by adding a lowpass is not exactly helpful.

Of course, we need both the turnon and turnoff responses for both
the pullup and pulldown paths, so that's four tables (rising into
50 ohm pullup, falling into 50 ohm pulldown, rising into 50 ohm pulldown,
and falling into 50-ohm pullup.

Output timing should be to the 10% (initial) point of the edge. That's
easy to plug in as Vmeas so that the timing numbers add up, and also
easy enough to measure on a tester so that you're actually testing to
the critical design parameter.

No lumped caps anywhere (was this a surprise?)

-- 
D. C. Sessions
[email protected]


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