1 Introduction

High-speed signals are typically routed by impedance matched transmission lines. This can be in the form of traces on a printed circuit board (PCB) or a cable connecting a source to a sink. ESD protection diodes are usually placed on a PCB very near to the connector where ESD is anticipated. The transmission line architecture of most high speed signals on a PCB is in the form of differential signal pairs. A simplified model of this is shown in Figure 1-1. Each unit length of the transmission line is comprised of an inductor, resistor, and capacitor. In an equivalent circuit with small enough unit lengths, the losses become negligible enough to consider only the inductance and capacitance as shown in Figure 1-2. The equation for the characteristic impedance, Z₀, then simply becomes:

The inductance in an ESD protection diode which is in series with the differential line is usually nonexistent (this is especially true for the device that has one protection pin for each protected line) while the parallel capacitance is somewhere on the order of 0.1 pF to 1 pF. This arrangement presents mostly capacitance to the node of the transmission line, so that the characteristic impedance becomes much lower at that point.