User Guide

Chapter 12 Monte Carlo and sensitivity/worst-case analyses
308
direction (better or worse) in which the collating function
changes with a small increase in each model parameter is
recorded.
Finally, for the worst-case run, each parameter value is
taken as far from its nominal as allowed by its tolerance,
in the direction which should cause the collating function
to be its worst (given by the HI or LO specification).
Outputs
A summary of the sensitivity analysis is printed in the
PSpice output file (.OUT). This summary shows the
percent change in the collating function corresponding to
a small change in each model parameter. If a .PROBE
statement is included in the circuit file, then the results of
the nominal and worst-case runs are saved for viewing in
the Probe window.
Caution: An important condition for
correct worst-case analysis
Worst-case analysis is not an optimization process; it does
not search for the set of parameter values that result in the
worst result.
It assumes that the worst case occurs when each
parameter has been either pushed to one of its limits or left
at its nominal value as indicated by the sensitivity
analysis. It shows the true worst-case results when the
collating function is monotonic within all tolerance
combinations.
Otherwise, there is no guarantee. Usually you cannot be
certain whether this condition is true, but insight into the
operation of the circuit may alert you to possible
anomalies.
Pspug.book Page 308 Wednesday, November 11, 1998 1:14 PM