User`s guide

Worst-Case Analysis 13
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Worst-Case Anal
y
sis
This section discusses the analog worst-case analysis feature of
PSpice A/D. The information provided in this section will help
you to apply it properly and with realistic expectations.
Overview of Worst-Case
Anal
y
sis
Worst-case analysis is used to find the worst probable output of
a circuit or system given the restricted variance of its
parameters. For instance, if the values of R1, R2, and R3 can
vary by +
10%, then the worst-case analysis attempts to find the
combination of possible resistor values which result in the worst
simulated output. As with any other analysis, there are three
important parts: inputs, procedure, and outputs.
Inputs
Besides the circuit description, two forms of information are
required from the user:
• parameter tolerances
• a definition of what worst means
PSpice A/D allows tolerances to be set on any number of the
parameters that characterize a model. Models can be defined for
nearly all primitive analog circuit components, such as resistors,
capacitors, inductors, and semiconductor devices. PSpice A/D
reads the standard model parameter tolerance syntax specified in
the .MODEL statement. For each model parameter, PSpice A/D
uses the nominal, minimum, and maximum probable values, and
the DEV and/or LOT specifiers; the probability distribution type
(such as UNIFORM or GAUSS) is ignored.
The criterion for determining the worst values for the relevant
model parameters is defined in the .WC statement as a function
of any standard output variable in a specified range of the sweep.
In a given range, the measurement must be reduced to a single
value by one of these five collating functions:
not
included
in: