The speed of both serial and parallel interfaces shows a history of continual increase. The latest High-Definition Multimedia Interface (HDMI) and USB 3.1 specifications, for example, now allow a maximum speed of 6Gbps and 10Gbps respectively.
ESD Protection Standards
Higher data rates enable higher-resolution videos and faster data transfers, but also make it harder to protect interface devices against electrostatic discharge (ESD) strikes without affecting the quality of the signal.
ESD-induced damage can include gate oxide breakdown, excessive leakage, even the burnout of metal interconnects. As device geometries continue to shrink, so also do the voltage and current levels that can cause failures.
To make matters worse, many common high-speed interfaces are directly accessible to the end user, so the industry-standard ESD test, IEC 61000-4-2, aims to ensure that finished products can survive real-world conditions. The standard assumes that the user will not take any special ESD precautions, so the test pulses can reach up to 16A in 30 ns (IEC 61000-4-2 Level 4, 8kV contact discharge).
ESD Protection Circuit: TVS Diodes
To protect against damage, interface designers must add protection devices to clamp ESD pulses without compromising the signal integrity of the high-speed link.
Unfortunately, adding ESD protection also adds capacitance to the line, and it is important to keep this as low as possible. At gigabit speeds, maintaining impedance matching through the signal path is a priority to avoid reflections that increase jitter and degrade the signal.
Semiconductor diodes have many desirable characteristics, such as low clamping voltages, fast turn-on time, and better reliability. Until recently, diodes have had higher capacitance than other architectures, but newer devices offer capacitances below 1pF. As a result, transient voltage suppression (TVS) diodes are now the preferred choice for high-speed ESD protection.
There are both unidirectional and bidirectional TVS diodes, as shown in Figure 1. Both types are designed to be open-circuit during normal operation and provide a short to ground during an ESD event. 
Figure 1: The circuit representations of bidirectional and unidirectional TVS diodes (Image Source: Texas Instruments)
When a positive ESD strike occurs in a unidirectional TVS device, D1 turns on (becomes forward-biased) and current flows through zener diode Z1 to ground, since its breakdown is less than that of D2. A negative ESD strike turns on D2, shorting the energy to ground directly.
A bidirectional device can accommodate signals that swing both above and below ground as long as neither D1 nor D2 enters breakdown during normal operation. When an ESD strike occurs, one diode turns on and the other enters breakdown, dumping ESD energy to ground.
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How to Choose a TVS Diode: Key Specs
There are several key specifications to evaluate when selecting a TVS diode for high-speed interfaces:
I/O capacitance: Low capacitance is essential since the TVS diode appears as a parasitic capacitance to ground during normal operation. The data eye diagram is a methodology used to represent and analyze high-speed data signals. The eye is formed by superimposing many waveform samples on a single graph to give a statistical representation of the overall signal, as shown in Figure 2. 
Figure 2: A TVS diode should have a minimal effect on high-speed jitter, as shown here by the ESD8004 USB3.1 eye diagram (Image Source: ON Semiconductor)
The eye “opening” corresponds to one bit period. The smaller the eye, the more jitter exists on the system, and the smaller the error margin. Any additional parasitic capacitance will act to degrade the signal, manifested by a “closing” of the eye; a well-designed protection device should have a minimal effect, as Figure 2 illustrates.
Clamping Voltage: As discussed earlier, the threshold for ESD-induced failures decreases with shrinking device geometries. The clamping voltage must be low enough to protect downstream devices while still being above the normal operating voltage.
Figure 3: Current vs. voltage for a TVS diode (Image Source: Texas Instruments)
Dynamic Resistance: Since an ESD strike lasts only a few nanoseconds, the diode cannot short the energy to ground instantaneously, so its resistance during the turn-on time, known as the dynamic resistance (RDYN), becomes important. Figure 3 shows RDYN, which is measured using a technique known as Transmission Line Pulse Measurement (TLP). TLP characterizes devices in the time domain under pulse conditions. An ideal TVS diode would have a dynamic resistance equal to zero; the state of the art is currently 1Ω or less.
ESD Layout Guidelines
The PCB layout has a large effect on ESD performance. To avoid ESD propagation on the PCB, the ESD protection device must be placed as close as possible to the ESD source. ESD stress can propagate on both sides of the interface cable, so there must be a protection device at each end of the cable.
The PCB layout must also minimize parasitic inductance and capacitance. For example, shortening the ground path return to the GND vias reduces parasitic inductance, as does a symmetrical flow-through layout with the connector on one side of the ESD device and the transceiver on the other side. TVS diodes for high-speed interfaces use pinouts optimized for this arrangement. Figure 4 illustrates the pinout and recommended layout for ST's HSP061, a four-line unidirectional device.
Figure 4: High-speed TVS diodes such as ST's HSP061 have packages optimized for flow-through PCB layout. (Image Source: STMicroelectronics)
Typical ESD Devices
Many manufacturers offer ESD protection devices for high-speed interfaces. Texas Instruments, for example, has their TPD4E02B04 diode array, which allows 4-channel ESD protection for speeds up to 10Gbps. The part features 0.25pF capacitance per channel in a USON-10 package with flow-through routing.
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ON Semiconductor is also active in this market; their ESD8106 provides a complete USB3.0 solution by integrating two high-speed pairs, and the D+/D- line into a single UDFN14 package. And Littelfuse's PGB2010201KR ESD suppressors use polymer technology, feature a typical capacitance of 0.07pF, and come in a surface-mount 0201 package.
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Conclusion
Mitigating damage from ESD strikes is assuming greater importance as interface speeds push further into the gigabit range and operating voltages continue to trend lower. Any design that might come into direct contact with the end user must include a robust system of ESD protection which does not degrade a high-speed signal.
Transient voltage suppression diodes provide the needed protection, but designers must be aware of several key characteristics and techniques in order to implement a successful solution.