User Guide

Chapter 6 Analog behavioral modeling
182
The output of the device depends on the type of analysis
being done. For DC and bias point, the output is simply
the zero frequency gain times the value of EXPR. The zero
frequency gain is the value of XFORM with s = 0. For AC
analysis, EXPR is linearized around the bias point (similar
to the VALUE parts). The output is then the input times
the gain of EXPR times the value of XFORM. The value of
XFORM at a frequency is calculated by substituting j·w for
s, where w is 2p·frequency. For transient analysis, the
value of EXPR is evaluated at each time point. The output
is then the convolution of the past values of EXPR with the
impulse response of XFORM. These rules follow the
standard method of using Laplace transforms. We
recommend looking at one or more of the references cited
in Frequency-domain device models
on page 6-181 for more
information.
Example
The input to the Laplace transform is the voltage across
the input pins, or V(%IN+, %IN-). The EXPR property
may be edited to include constants or functions, as with
other parts. The transform, 1/(1+.001·s), describes a
simple, lossy integrator with a time constant of 1
millisecond. This can be implemented with an RC pair
that has a time constant of 1 millisecond.
Using the part editor, you would define the XFORM and
EXPR properties as follows:
XFORM = 1/(1+.001*s)
EXPR = V(%IN+, %IN-)
The default template remains (appears on one line):
TEMPLATE= E^@REFDES %OUT+ %OUT- LAPLACE
{@EXPR}= (@XFORM)
After netlist substitution of the template, the resulting
transfer function would become:
V(%OUT+, %OUT-) = LAPLACE {V(%IN+, %IN-)}= (1/1+.001*s))
The output is a voltage and is applied between pins
%OUT+ and %OUT-. For DC, the output is simply equal
to the input, since the gain at s = 0 is 1.
Pspug.book Page 182 Wednesday, November 11, 1998 1:14 PM