Protecting Your High Speed Interfaces and Minimizing Signal Impact

Gigabit Ethernet, 10 Gigabit Ethernet, HDMI, DisplayPort, USB 3.x, Thunderbolt, and other interfaces are becoming commonplace on many types of devices, and can operate at bitrates of 10 Gbps or higher. As the speeds of these interfaces have continued to increase, the design parameters that ensure proper operation have become more and more strict. Even minor deviations from recommended design practices can have significant signal integrity impacts and cause performance problems. 

Ver Productos relacionados

All of these interfaces operate at frequencies ranging from hundreds of Megahertz to several Gigahertz.  In order to ensure signal integrity and minimize EMI and immunity challenges at these speeds, most high speed interfaces use differential signaling and reduced switching voltages. Differential signaling provides inherent noise cancelling and immunity benefits, while the use of reduced switching voltages help to minimize noise at the interface and on the system power bus. These features aren’t silver bullets, though—layout and component choices can significantly impact their effectiveness. If not designed properly, the reduced switching voltages also create an inherent problem with noise immunity, as coupled RF energy can impinge on the interface’s noise margins more easily. 

For these reasons, careful attention to circuit implementation is crucial to achieving good signal integrity, proper interface operation, and proper support of longer interface cables. This is particularly true when selecting and implementing transient protection on the interface. The most important considerations are that the differential pairs must be symmetrical and well matched. Realizing this goal requires attention to many details, however. It may seem easy to achieve proper trace length matching and impedance control, but the realities of connectors, protection devices, EMI control devices, and other inline components create areas where the signal path must deviate from optimal routing and where components directly impact the signals on the transmission line. Stubs and changes in the impedance of the transmission lines will cause reflections which can result in ringing and other degradations of the signal waveform in multi-GHz systems. In multi-GHz systems, attention to stubs is very crucial as ¼ wavelength stubs are relatively easy to create, and these act as an effective short to ground. Unmatched trace lengths can also result in jitter and invalid data due to timing errors. Sources of series impedance like common mode chokes can reduce the signal edge rates and amplitude of the signal, and capacitive loading from transient voltage suppressors (TVSs) can cause further deterioration of signal edge rates. Any or all of these conditions can cause enough deterioration to the waveform to inhibit proper operation of the interface. 

Ver Productos relacionados

In addition to the above concerns, when adding transient protection for high speed differential signaling, it becomes critical to pay attention to the unpredictable nature of transient events.  Because they can manifest as common mode or differential mode spikes on transmission line pairs, symmetry and matching become a critical design practices. Symmetry is best described as ensuring the differential pair has identical (or as close to identical as possible) routing with respect to trace length, adjacent ground structures, and component placements. To minimize differential coupling of transient events, transmission line pairs should be length matched, closely coupled, and have ground islands or traces on either side to ensure a constant impedance along the length of the trace. Components must also be mounted in equivalent locations along the length of the traces to ensure any signal degradation due to loading or impedance changes is matched and there are not significant lengths where the signals are not differentially matched. If individual transient suppressors are used, they should be positioned as shown on the left side of Figure 1 instead of having stubs routed out to a row of TVS devices to the side of the interface, mirroring the circuit’s schematic on the right side of the diagram. Stubs introduce multiple problems including reflections and additional trace impedance that reduces the effectiveness of TVS devices. When the stubs are not equal lengths, these effects are magnified because they manifest differently on the separate lines in the differential pair and could result in a potentially damaging differential spike remaining on the lines, even after proper TVS operation.

Figure 1&2: Implementation of Individual TVS devices

Newer multi-line TVS devices have been designed specifically for differential pairs in high speed interfaces. These lay on top of the traces and provide exceptionally well matched performance and unparalleled symmetry on high speed signal pairs. Use of these devices is highly recommended.

Figure 3: Inline TVS Protection Device

Matching refers to design practices that ensure that the protection activates simultaneously and equivalently, thus providing equal protection to all lines in the interface and preventing differential spikes. Fundamental matching practices include ensuring that components are of the same value and inserted in equivalent locations on all lines of signal pairs, and that tolerances of individual components are evaluated to ensure proper protection across normal performance variance. 

Final considerations include proper TVS selection, ensuring TVS devices are located between the physical connector and any isolation components, and keeping the overall distances between these areas of the circuit as short as possible. All TVS devices present some capacitive load on the signals they protect, and it is important to select a good match to your interface to prevent signal degradation. Today’s highest speed 10 Gbps interfaces such as SuperSpeed USB 3.1, PCIe, and Thunderbolt 2 require capacitances under 0.3 - 0.4 pF to ensure minimal impact to the signal, while slower legacy interfaces like USB 2.0 (480 Mbps) can operate with capacitances as high as 1.5 pF.

With proper component selection and attention to symmetry and matching during the circuit design process, it is possible to create robust devices that support today’s highest transfer speeds while also exhibiting high immunity to ESD and other transients that pose the greatest threats to these interfaces.