Chapter 6 - PCB Design for Signal Integrity

Abstract

This chapter addresses the electrical aspect of Printed Circuit Board (PCB) design. Desirable electrical characteristics of a circuit and its PCB include low noise, low distortion, low cross talk, and low radiated emissions, and so on. The purpose of this chapter is to introduce the issues that cause PCB performance problems and how to route the PCB to minimize them and maximize signal integrity. An overview of signal integrity, electromagnetic interference, and electromagnetic compatibility is presented and applied directly to PCB design. There are three goals in designing PCBs for electrical performance and signal integrity: the PCB should be immune from interference from other systems; it should not produce emissions that cause problems for other systems; and it should demonstrate the desired signal quality. A common factor relating these three issues is electromagnetic waves. Topics such as loop inductance, ground bounce, ground planes, characteristic impedance, reflections, and ringing are discussed. The idea of “the unseen schematic” (the PCB layout) and its role in circuit operation on the PCB is introduced. Look-up tables and equations are provided to determine required trace widths for current handling and impedance as well as required trace spacing for high-voltage designs and high-frequency designs. Various layer stack-up topographies for analog, digital, and mixed-signal applications are also described. A demonstration on how to use PSpice to simulate transmission lines to aid in circuit design and PCB layout is also provided.