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AUGUST 2020 I DESIGN007 MAGAZINE 41 the body capacitance. To make it simple, we look at a single capacitor, as shown in Figure 9. We set the main capacitance as a parameter so that we can step it and set the ESR and ESL to fixed values—10 mOhm and 1 nH, respectively. To see what happens, we intentionally do not specify the parallel body capacitance; the entry is left blank. We step the capacitance from 1 pF to 1F in four large logarithmic steps and sweep the frequency from 1 mHz to 1 THz. The result shows that, in fact, a parallel body capaci- tance is added by the simulator, but its value is not fixed; it depends on the other param- eters. With the values used here, the body capacitance seems to be approximately one million times smaller than the main capaci- tance. While this looks like a huge ratio (and it is), we see that if we simulate our circuit over many decades of frequencies, this small default body capacitance value still can cause unexpected artifacts. The good news is that it is easy to deal with; we just have to remember to call out specifically zero body capacitance, unless, of course, when we know its correct value and want to simulate the effect of the body capacitance. And a final note: Remember that all numeri- cal tools have to set limits for the input num- bers they can accept and process, whether the tools will tell you and remind you. Next time, when you see unexpected things in circuit-sim- ulation results, first make sure that the input numbers, including potential defaults, are set correctly. DESIGN007 References 1. Parallel Impedance of Four Groups of Capacitors. 2. High-Frequency Structure Simulator. 3. Berkeley SPICE. 4. LTspice. 5. LTwiki. Istvan Novak is the principal signal and power integrity engineer at Samtec with over 30 years of experi- ence in high-speed digital, RF, and analog circuit and system design. He is a Life Fellow of the IEEE, author of two books on power integrity, and an instructor of signal and power integrity courses. He also provides a website that focuses on SI and PI techniques. To read past columns or contact Novak, click here. Figure 9: Checking the body capacitance default value for the capacitor model.

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