User`s guide

B
-
10 Conver
g
ence and “Time Step Too Small Errors”
Example: A first approximation to an opamp that has an open
loop gain of 100,000 is:
VOPAMP 3, 5 VALUE = {V(in+,in-)*1e5}
This has the undesirable property that there is no limit on the
output. A better expression is:
VOPAMP 3, 5 VALUE =
+ {LIMIT(V(in+,in-)*1e5,15v,-15v}
where the output is limited to +/- 15 volts.
Transient Anal
y
sis
The transient analysis starts using a known solution - the bias
point. It then uses the most recent solution as the first guess for
each new time point. If necessary, the time step is cut back to
keep the new time point close enough that the first guess allows
the Newton-Raphson repeating series to converge. The time step
is also adjusted to keep the integration of charges and fluxes
accurate enough.
In theory the same considerations which were noted for the bias
point calculation apply to the transient analysis. However, in
practice they show up during the bias point calculation first and,
hence, are corrected before a transient analysis is run.
The transient analysis can fail to complete if the time step gets
too small. This can have two different effects:
1
The Newton-Raphson iterations would not converge even
for the smallest time step size, or
2
Something in the circuit is moving faster than can be
accommodated by the minimum step size.
The message PSpice puts into the output file specifies which
condition occurred.