User Guide

5
-
2 Tutorial: Optimizin
g
a Desi
g
n (Passive Terminator)
The Passive Terminator
Desi
g
n
Figure 5-1 shows a simple terminator that you could use, for
example, for one line of a backplane. The top end of R1 connects
to a +5 v DC supply; the lower end of R2 connects to ground.
The center point of the two resistors provides the line
termination.
This design must meet two specifications:
• Voltage at the junction of the two resistors (V
center
) must lie
within a specified range.
• Thevenin equivalent resistance of the combination (R
equiv
)
must equal a specified value.
Assume that the DC supply has negligible internal resistance,
and that:
• V
center
must be 3.75±0.1 V.
• R
equiv
must be 100±1 Ω.
Your objective is to find the best values for R1 and R2 that meet
the stated specifications. 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. As you complete this tutorial, compare this solution to
that produced by the PSpice Optimizer.
Fi
g
ure 5-1
Resistive
Terminator Circuit
5
R
2
R
1
R
2
+
------------------ 3 . 7 5=
R
1
R
2
⋅
R
1
R
2
+
------------------ 1 0 0=