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!!!  LIMITATIONS OF SPICE  !!!

Although SPICE is a useful tool, there are many limitations! Real electronic components can be a complex creatures with many behaviors. Simulation of a circuit is only as accurate as the behaviors modeled in the SPICE devices created for it. Many simulations are based on simplified models. For more complex circuits or subtle behaviors, the simulation can be misleading or plain incorrect. This can spell disaster if you're relying entirely on SPICE when developing circuits for manufacture.

SPICE can also lead you down a wrong design path. How? SPICE simulations are free of noise, crosstalk, interference, etc. - unless you put these behaviors in the circuit! Measuring 10 pA maybe cakewalk in SPICE land, but noise, stray conductive paths and bias currents are circuit killers in the real world.

SPICE is not the best predictor of component failures. SPICE will not say "ouch" or post a smoke graphic if you've exceeded the voltage, current or power limits of a device. Other failure modes are less obvious. Although you've nicely modeled the ON resistance of a CMOS analog switch, SPICE does not mimic its destructive latch-up condition. You must be the watch-keeper. You must know what dangers to look for and which behaviors are not modeled in your SPICE circuit.

SPICE is not a prototype substitute - the performance of the actual breadboard provides the final answer! However, a combination of theory, experience and software tools like SPICE can assist you in your circuit endeavors.

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