利用 Spice 使生活更便捷(英)

　　A computer-based simulation of your analog and digital circuit is important. Such a simulation is important because using your preferred computer Spice program for analog simulation or IBIS (I/O Buffer Information Specification) program for digital simulation can reduce initial errors and development time. If you correctly use your simulator, you can identify circuit errors and nuances before you go to your breadboard. The first step to correctly using these tools is to determine the expected performance of your circuit before you start simulations. Then, after you run a simulation of your circuit, you verify your design before you spend time soldering the circuit. These tools also help you to troubleshoot your bench circuit. Using simulations is a great way to try out ideas by creating what-if scenarios.

You can view the results from these software tools on a PC with user-friendly graphical-user-interface suites. These tools fundamentally provide dc operating points, small-signal gain, time-domain behavior, and dc sweeps. At a more sophisticated level, they help you analyze harmonic distortion, noise power, and gain sensitivity and perform pole-zero searches. This list is incomplete, but generally, Spice and IBIS software have many of these fundamental features. By finessing the Monte Carlo and worst-case-analysis tools in Spice, you can predict the yields of your final product. At the onset of a project, using a breadboard circuit may be expensive and time-consuming. A simulation may speed up your application-circuit time to market.

But beware. You can effectively evaluate analog and digital products if your models or macromodels are accurate enough for your application. The key words here are "accurate enough." These models should mimic the actual performance of the component without carrying the burden of too many circuit details. Too many details can lead to convergence problems and extremely long simulation. Too few details can hide some of the intricacies of your circuit’s performance. Worse yet, your simulation, whether you use a complete circuit model or just the macromodels, may misrepresent what your circuit will do. Remember that a Spice or an IBIS simulation is simply a pile of mathematical equations that, with any luck, represents what your circuit will do. In essence, a simulation produces imaginary results.

The nay sayers in the industry will tell you that your computer-based-simulation tools do not work and that using them is a waste of time. These people are a bit misguided and have a limited view of what these tools can do. Sure, if you misapply a Spice tool, it can lead you astray. These simulation tools do not replace good engineering judgment. But, like any other tools, they are only as good as their users. Before you run a simulation, it is critical to have a good idea of how the final outcome will look. Any insight that you gain from your simulations is a bonus. Better yet, Spice simulations point out problems that you would never anticipate. In most cases, these simulation tools use double-precision calculations. Such calculations ease the detection of low-level problems that are impossible to find on the bench.

The questions are: Does my model accurately mimic the device over temperature? Does the simulation produce distortion results that are representative of the device? Do I receive a good portrayal of the device’s ac response? Do I expect the model to simulate these parameters and to what degree of accuracy? What information do my models provide? Did I accurately include the parasitic characteristics of my board and components?

The only way to answer these questions is to have a feel for what your circuit will do in real life before you start your simulations. Then, ask challenging questions about your simulation results. There is no replacement for good engineering judgment.