Capacitance Requirements for High Speed Signals
Abstract
This application note addresses the capacitive loads presented by electrostatic discharge (ESD) protection diodes to high-speed signals. With the industry trend towards smaller chipset sizes with higher data rates, the chipset’s tolerance to transient voltages has decreased, increasing the need for ESD protection diodes. Higher data rates are very sensitive to parasitic capacitance, as a large capacitance can cause impedance mismatches. A large capacitance can distort the signal and cause the data to become corrupted or unreadable. For high speed signals, the ESD protection diodes need to have an ultra-low capacitance to not disrupt the signal transmission. With this dynamic, it becomes important to understand how much capacitance a high-speed signal can tolerate and still maintain proper signal integrity.
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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.
Figure 1-1 Segment of Two-Wire Transmission Line
Figure 1-2 Equivalent Circuit for a Two-Wire Transmission Line 