There is a unique class of calls that reach my desk centered on the subject of Spice; that is, the Simulation Program with Integrated Circuit Emphasis. This program, originated at the University of California - Berkley in the early 1970s, has become a standard simulation tool especially in the analysis of analog circuits. Most, if not all of the manufacturers of analog ICs will provide, at no charge, a library of Spice models of many of their linear ICs.
Spice can be a very powerful addition to the engineer's tool box. It allows the user to examine the action of circuits without touching any test equipment or even the devices. You can examine many aspects of the circuit such as bias points, gain, and even preliminary stability. My colleague, Neal Albaugh, has developed many useful circuits and gained valuable insight through creative use of Spice.
This is an ideal situation, except the Spice model is not the real world. First, it is a model, a mathematical expression that attempts to react in the world of the simulator as the real part reacts in the real world. The key word here is attempts. At some level of detail the model fails to react as the real part reacts.
The danger with Spice is that the results are presented with such definition and authority that they become difficult to challenge. If you begin to suspect the results where can you turn? With a real world breadboard the questionable device can be replaced. In the Spice world replacing the device is not usually an option. An analysis that may give good results with a model from one supplier may not give acceptable results with a model from a second supplier or even a different generation model from the same supplier.
It is not uncommon for the dedicated Spice user to begin to believe the computer results over the real world. One caller was quite upset with his results on the computer, however, the silicon worked fine. As I attempted to isolate the problem, he stated that since the Spice results were not consistent with the production units there must be something wrong with the silicon. This user had reached the point depicted by the sentiment that some engineers treat Spice as the town drunk treats the street lamp, more for support than illumination.
At Texas Instruments the Spice model library contains 326 zipped files covering almost 1000 part numbers and situations. Each of these files may contain up to three files. Some of these models are over twenty years old. We are continuing to refine the techniques used to develop new models and include more features, but seldom revisit old models.
Like any tool, Spice has value. However, like any tool, it can be misused and thereby give erroneous results. Spice is a good tool to use to decide which circuits are worth realizing on a breadboard or prototype. My attitude remains to work with the Spice simulations and then do the final simulation in silicon.
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