User Guide

1
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8 Thin
g
s You Need to Know
Constraint
A constraint defines the performance level that
the design must fulfill in which the target value exceeds, falls
below, or equals a specified value (e.g., an output voltage that
must be greater than a specific level). The constraint
specification includes:
• The name of the constraint.
• A target value and an acceptable range.
• A circuit file to simulate. (See note on previous page.)
• An evaluation for measuring performance.
• An analysis type used for simulation-based evaluations.
• An allowed relationship between measured values and the
target value, which can be one of the following:
The constraint specification can also include the name of the file
containing Probe goal function definitions (.prb file).
Constraints are often nonlinear functions of the parameters in
the design.
Example: Bandwidth can vary as the square root of a bias
current and as the reciprocal of a transistor dimension.
Performance
The performance of a design is a measure of
how closely its specifications’ calculated values approach their
target values for a given set of parameter values. When there are
multiple specifications (at least one of which is a goal), the
PSpice Optimizer uses the sum of the squares of their deviations
from target to measure closeness. For a single specification
(goal), the PSpice Optimizer uses either the goal’s value, or the
square of its deviation from target.
<= measured value must be less than or equal
to the target value
= measured value must equal the target value
>= measured value must be greater than or
equal to the target value
See Optimization on page 1-5
for more on least-squares and
minimization al
g
orithms.