UNIVERSITY OF CALIFORNIA
College of Engineering
Department of Electrical Engineering
and Computer Sciences
B. E. Boser |
Software |
EE 40 |
SPICE
Cellular phones and computers are just two examples of some of today's extremely complex electronic systems. Such devices contain millions of circuit components, and simple trial and error is not an effective way of ensuring that the final product will work properly. As a result, designers rely on circuit simulators to verify the performance of a circuit before fabrication.
The most popular component level circuit simulator available today is called SPICE (Simulation Program with Integrated Circuit Emphasis), which was developed at the University of California, Berkeley, in the 1970s under the guidance of Prof. Pederson. Today vendors offer many different versions of SPICE, many with graphical interfaces and new features added.
Common Problems
Regardless of the version of SPICE that you are using, you may occasionally run into problems. Here are a few common causes and solutions:
- No path to ground: you need at least one "ground" symbol in your schematic.
- Less than two connections: every terminal of every component needs to be connected. E.g. you cannot have a resistor that has not both of its terminal connected to other components.
- Inaccurate results: By default, SPICE calculates node voltages to 0.1% accuracy. If you suspect that this may not be sufficient (e.g. when calculating currents through small valued resistors), decrease the value of the simulation parameter "reltol" and resimulate.
- No convergence: this is the most dreaded and timeconsuming problem to fix. It occurs typically for large circuits with feedback, or for circuits that contain too many ideal devices. Lower the gain of amplifiers, lengthen the rise and fall times of input waveforms, and add series resistance to voltage sources. If this does not help reduce circuit complexity until you get convergence. Save the operating point (check the manual on how to do this), and then add components slowly, resimulating each time with the previous operating point and saving the new one. Hopefully you won't ever need to do this in EE 40, but you may want to remember these suggestions in case you ever run into problems later.
LTSpice
LTSpice is based on Berkeley SPICE 3. Unlike most other free versions of SPICE it has no restrictions on circuit size but has a somewhat limited choice of built-in models and favors parts from a particular vendor (Linear Technology). It is possible, however, to add new parts.
- Download or run on instructional machines
- Tutorial
- Documentation
- Reltol: got to simulate - control panel - SPICE
- Components
- 555 Timer: Misc/NE555
- LMC6482: subsitute Opamps/LT1884
- IRF510: nmos, right click, pick new mosfet, scroll down to IRF510 (scroll down, the list is not in an obvious order)
- Plotting
To make plots from LTSpice more visible, do the following:
1) Go to tools -> control panel, under Waveforms check the box "Plot data with thick lines".
2) Go to tools -> color preference, under WaveForm, chose "background" in "Selected Item", and push the bar for Red, Green and Blue to the right end. Click ok.
PSpice
Very popular version of SPICE. A free demo version with limited circuit size (ok for EE40) is available on the web; an unrestricted version is available on instructional computers.
Multisim
Commercial SPICE designed specifically for learning. No free versions, but the tool is available on our servers.
Matlab
Matlab is a general purpose computation tool with applications in many engineering disciplines. For example, it has very powerful tools for graphing (e.g., Bode plots), designing filters, etc. It is availble on the instructional servers. You can also purchase a student version from the vendor. A free version, Octave, is also available. It works well for many tasks (e.g. frequency response) but lacks many more specialized features.
Tutorials and documentation are available on the web.