eldo_user

Speed and Accuracy
Global Tuning of the Accuracy—EPS
where V(i) is the voltage value at the current iteration i and V(i-1) is the value at the
previous iteration.
• For branch currents:
|I(i) − I(i − 1)| < RELTOL × |max(|I(i)|, |I(i − 1)|)| + ABSTOL
where I(i) is the current value at the current iteration i and I(i-1) is the value at the
previous iteration.
• For charges:
|Q(i) - Q(i-1)| < RELTOL*|max(|Q(i)|, |Q(i-1)| + CHGTOL
where Q(i) is the value at current iteration i and Q(i-1) the value of the previous
iteration.
The same RELTOL parameter controls the relative tolerance for voltages, currents, and charges.
However, when voltages, currents, or charges become ‘small’ in absolute value, a relative
tolerance becomes useless, thus absolute tolerances are required. Of course the typical orders of
magnitude of voltages, currents, and charges are quite different, thus the necessity to have
specific absolute tolerances (VNTOL, ABSTOL, CHGTOL).
As a rule, if using the default settings, a simulation that goes well should use at most three or
four Newton iterations per time step. This can be less if the circuit is almost linear, or if the
settings such as the RELTOL value are relaxed. If many more iterations are needed, it means
that either the time step control options have been relaxed a lot (and potentially the truncation
error is large), or there are strongly non-linear characteristics in the circuit, which are difficult to
solve. The number of Newton iteration will also increase if the RELTOL value is reduced to
obtain more accuracy.
Related Topics
Speed and Accuracy in Eldo
Tips for Improving Simulation Performance
Global Tuning of the Accuracy—EPS
A global parameter, EPS, is used as a general controller to set the required accuracy of a
simulation. When this parameter is changed, a collection of lower-level parameters are adjusted
accordingly, affecting both the Newton convergence tolerances and the time step control
tolerances.
Is is always possible to set these low-level parameters individually. However using EPS
guarantees that the adjustments to the low-level parameters are consistent, which is not always
easy to achieve.
Eldo® User's Manual, 15.3 1253