Signal Integrity and Radiated Emission of High-Speed Digital Systems

4

Capacitance

Capacitance is another very important line, filtering, and IC device parameter for Signal Integrity (SI) and Radiated Emission (RE). In this chapter, self and mutual capacitances are theoretically introduced, considering the coupling between two generic conductors. The capacitance C matrix concept for two coupled wires having a reference return conductor is introduced as background to build a C matrix for n-multiconductor transmission lines. Calculation of the inductance L matrix for multiconductor lines, starting with the C matrix, is considered. Finally, the definitions of Differential Mode (DM) and Common Mode (CM) capacitances are given.

4.1 Capacitance Between Conductors

The capacitance is a very important parameter in modeling interconnects. Starting with the basic definition of capacitance, the concept is then extended to a general multiconductor formulation which is essential, as many interconnects consist of several coupled lines. Particular emphasis is given to the capacitance C matrix and its negative off-diagonal terms, as this concept is fundamental to understanding the output of a static field solver for building transmission-line models.

4.1.1 Definition of Capacitance

Capacitance is a property of a geometric configuration defined by two conducting objects surrounded by a homogeneous dielectric. It describes the ability of the given configuration to store electrostatic energy [1–3]. Consider, for example, the geometry shown in Figure 4.1, where two ...