User`s guide

Cautions and Recommendations for Simulation and Analysis 6
-43
A good example of this is the expression {S}, which
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 A/D 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 A/D
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.
One more point about {S}. You can delay it to keep it causal, but
keep in mind that 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 useful 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.
Cheb
y
shev filters
All of the considerations given above for Laplace parts also
apply to Chebyshev filter parts. However, PSpice A/D also
attempts to deal directly with inaccuracies due to sampling by
applying Nyquist criteria based on the highest filter cutoff
frequency. This is done by checking the value of TMAX. If