[SI-LIST] : Clamp diodes in models (was Input switchingthreshold & CPCI)

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From: David Haedge (d-haedge@raytheon.com)
Date: Mon Jan 10 2000 - 09:09:57 PST


Fellow SIers,

I modeled a circuit using vendor provided SPICE models and noticed that the
undershoot in the SPICE *never* got below -.7 volts, but that my board
measurement was showing -2.5 to -3 volts for 1 or 2 nanoseconds, then leveled
out at -.7 volts. I then asked the vendor why I was getting anomalous
results when the diode was forward biased. His answer was " Why would I ever
want to forward bias the diode? They basically never expected the diodes to
be forward biased and therefore did not worry about model accuracy for that
condition. Further investigation revealed that it was next to impossible in
SPICE to accurately model both bias conditions in the same model. The problem
as I understand it, is that when the diode becomes forward biased, it does
not does not absorb infinite current instantaneously, but the SPICE model can
and does. There is a capacitance that needs to be charged before the diode
actually clamps. This is why I was seeing the -2.5 to -3.0 volt undershoot
for a couple of nanoseconds. Another SPICE device guy told me that you really
need two separate and distinct diode models run in two different SPICE runs
to get the right answer. The same thing happens on overshoot. I am always
skeptical when I see nice straight lines at VSS-0.7 or VDD+.7 on any
simulation, SPICE, IBIS, or whatever.

David Haedge
Raytheon Company

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