User Guide

Cautions and recommendations for simulation and analysis
189
must be applied to both past and future samples of the
input in order to properly represent the inverse of the
Laplace expression.
For example, the expression {S} corresponds to
differentiation in the time domain. The impulse response
for {S} is an impulse pair separated by an infinitesimal
distance in time. The impulses have opposite signs, and
are situated one in the infinitesimal past, the other in the
infinitesimal future. In other words, convolution with this
corresponds to applying a finite-divided difference in the
time domain.
The problem with this for PSpice is that the simulator only
has the present and past values of the simulated input, so
it can only apply half of the impulse pair during
convolution. This will obviously not result in
time-domain differentiation. PSpice can detect, but not fix
this condition, and issues a non-causality warning
message when it occurs. The message tells what
percentage of the impulse response is non-causal, and
how much delay would need to be added to slide the
non-causal part into a causal region. {S} is theoretically
50% non-causal. Non-causality on the order of 1% or less
is usually not critical to the simulation results.
You can delay {S} to keep it causal, but the separation
between the impulses is infinitesimal. This means that a
very small time step is needed. For this reason, it is usually
better to use a macromodel to implement differentiation.
Here are some guidelines:
•
In the case of a Laplace device (ELAPLACE), multiply
the Laplace expression by e to the
(-s
∗
<the suggested delay>).
•
In the case of a frequency table (EFREQ or GFREQ), do
either of the following:
•
Specify the table with
DELAY=<the suggested delay>.
•
Compute the delay by adding a phase shift.
Pspug.book Page 189 Wednesday, November 11, 1998 1:14 PM