UltraCAD

Introduction It s no news that the band pass requirements for power sys-tems on PCB s are increasing and that power supply imped-ance requirements are getting tighter. Bypass capacitor fabri-cation and assembly techniques are improving and pushing higher the normal self-resonant frequencies we have to deal with. ESR s (equivalent series resistance) are decreasing, pushing further down the floor of the power supply impedance curve. All this has created increased debate as to how to take advan-tage of this higher self-resonant requency and lower ESR. One argument is that lower ESR is thoroughly beneficial. Another is that, while lower ESR lowers the impedance at the mini-mum points, it also increases it at the maximum ( anti-resonant ) points, and therefore lower ESR is not necessarily beneficial.

Figure 3 Impedance curve for two capacitors in parallel The curve of impedance as a function of frequency is shown in Figure 3. It is instructive to look at this curve,

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  Capacitors, Ultracad

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