Overview

In many electronic circuits, capacitors are used either in power distribution bypassing or in high-speed signal termination and dc-blocking applications. For modeling purposes, there is a distinct difference between the two applications. In high-speed signal applications, the capacitor is usually embedded in 50 ? single-ended environment or a 100 ? sometimes 150 ? differential environment; this makes the parasitic capacitance to ground from the capacitor body and pads an important element of the model. On the other hand, bypass capacitors are connected to power distribution networks which typically have low impedance compared to the usual 50 ? trace impedance. Also, one terminal of bypass capacitors is usually connected to the return (ground) network. This application difference is highlighted in Figure 8.1.

Figure 8.1: Equivalent circuit of a capacitor (a) in high-speed signal circuits and (b) in PDNs. The simplified equivalent circuit is shown in (c).

Figure 8.1(a) shows a high-speed signal application where the capacitor is in series to a signal line. The C _p capacitance at either end represents the capacitance of solder pads to ground. Each C _p capacitance also represents half of the lumped body capacitance to ground. The pad capacitance is a few tenths of a picoFarad, (possibly a few picoFarads for large parts). In a 50 ? or 100 ? impedance environment, it is important to model this capacitive reactance. The connection of a bypass capacitor, as shown in Figure 8.1(b), effectively eliminates one pad capacitance...