User Guide

4
-
2 Understandin
g
Optimization Principles and Options
Goals versus
Constraints
Goals and constraints represent the ideal behavior of a design. In
practice, this behavior is often unattainable.
Example: A gate cannot achieve zero propagation delay, but the
goal of the optimization process might be to come as close as
possible to that target value (that is, to reduce the error as much
as possible).
When solving problems involving both goals and constraints,
the PSpice Optimizer trades off meeting the target values for the
goals against violation of the constraints. This means that the
error indicator does not always reduce in value for a given
iteration.
When setting up an optimization, you must decide which
specifications are goals and which are constraints. In many
cases, there are several legitimate ways to describe the design.
Example: Assume you want to design a resistive terminator that
produces an output voltage of 3.75 V (± 0.1 V) at the junction of
the two resistors, and the Thevenin equivalent resistance of the
resistor combination must equal 100 Ω (±1 Ω).
Your objective is to find the best resistor values to meet these
two specifications:
•
output voltage of 3.75 V (V
e
)
•
equivalent resistance of 100 Ω (R
e
)
You can manually solve this problem using the following
simultaneous equations:
and
These equations solve to R1 = 133.3 Ω and R2 = 400 Ω, an exact
solution.
If there is more than one
g
oal,
the PSpice Optimizer combines
the errors by summin
g
the
squares of the normalized
values.
Fi
g
ure 4-1
Resistive
Terminator Circuit
5
R
2
R
1
R
2
+
------------------ 3 . 7 5=
R
1
R
2
⋅
R
1
R
2
+
------------------ 1 0 0=