It performs nonlinear dc and transient analyses, fourier analysis,
and ac analysis linearized at an operating point. At this point
the analog is stronger than the digital. (In fact, the digital
part is rather weak.) It is fully interactive and command driven.
It can also be run in batch mode or as a server. The output is
produced as it simulates. Spice compatible models for the MOSFET
(level 1 and 2) and diode are included in this release.
This version includes several improvements including real Fourier
analysis and better time step control based on truncation error.
There are other minor improvements.
Since it is fully interactive, it is possible to make changes and
re-simulate quickly. The interactive design makes it well suited
to the typical iterative design process used it optimizing a circuit
design. It is also well suited to undergraduate teaching where
Spice in batch mode can be quite intimidating. This version, while
still officially in beta test, should be stable enough for basic
undergraduate teaching and courses in MOS design, but not for
In batch mode it is mostly Spice compatible, so it is often possible
to use the same file for both ACS and Spice.
The analog simulation is based on traditional nodal analysis with
iteration by Newton's method and LU decomposition. An event queue
and incremental matrix update speed up the solution for large
It also has digital devices for true mixed mode simulation. The
digital devices may be implemented as either analog subcircuits or
as true digital models. The simulator will automatically determine
which to use. Networks of digital devices are simulated as digital,
with no conversions to analog between gates. This results in
digital circuits being simulated faster than on a typical analog
simulator, even with behavioral models. The digital mode is
experimental and needs work. There will be substantial improvements
in future releases.
ACS also has a simple behavioral modeling language that allows
simple behavioral descriptions of most components including capacitors
and inductors. Unfortunately, it is not well documented.
ACS uses an object oriented approach to modeling. Complex models
like MOSFETS are made of simpler ones like resistors, capacitors,
diodes, and any other models that may already exist. The model
designer does not need to worry about details like convergence
checking, bypass checking, integration, or how the new device plugs
into the solution matrix because these are already taken care of
by the basic models. This results in a dramatic improvement in
the time it takes a researcher or model designer to install a new
model, compared to Spice.
The source and documentation can be obtained by anonymous ftp from
ee.rochester.edu or cs.rit.edu in /pub/acs. It can also be obtained
by dial-up (USA) 716-272-1645 in /pub/acs. It may be distributed
under the terms of the GNU general public license. The dial-up
also has some test circuits, pre-compiled executables for Next,
Sun4, MSDOS and possibly others, and documentation in dvi and
If you are tired of Spice and want a second opinion, you want to
play with the circuit and want a simulator that is interactive, or
you want to study the source code and want something easier to
follow than Spice, try ACS.
ACS is an ongoing research project. It is being released in a
preliminary phase in hopes that it will be useful and that others
will use it as a thrust or base for their research. I also hope
for some comments that may help me direct my research.
Albert Davis, 136 Doncaster Rd., Rochester, NY 14623.
email: [email protected] or [email protected]