User`s guide

6
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36 Analo
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Behavioral Modelin
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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 6-35 for more
information.
Example
The input to the Laplace transform is the voltage across the input
pins, or V(%IN+, %IN-). The EXPR attribute 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 symbol editor, you would define the XFORM and
EXPR attributes 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.
For AC analysis, the gain is found by substituting j·ω for s. This
gives a flat response out to a corner frequency of 1000/(2π) =
159 Hz and a roll-off of 6 dB per octave after 159 Hz. There is
also a phase shift centered around 159 Hz. In other words, the