Conformity Magazine - June 2008 - (Page 44)

While there are several general rules that can be applied, such as using devices with lower dynamic resistance and using semiconductor-based ESD devices rather than poorly performing varistors and polymer solutions3, it is critical that the system undergo system level ESD testing per the IEC61000-4-2 industry standard specification to truly determine whether the ESD protection device is sufficient. Matched Capacitance Many design engineers are familiar with basic principles of high speed layout, such as avoiding vias, and keeping trace lengths matched. Vias and stubs cause impedance mismatches and reflections, while unmatched trace lengths contribute to signal skew and EMI problems. Just as importantly, it is critical to have matched capacitance between ESD protection devices to avoid these problems. Most high speed I/O ports, such as HDMI, DisplayPort, and eSATA, use differential signaling which consist of pairs of signal lines that swing opposite to each other around a center voltage. To maintain good signal integrity, they require very tight tolerances on intrapair skew. For example, DisplayPort requires a maximum of 20 pS of skew at the transmit pins of the system. Even small capacitance mismatches can cause large skew variations. For example, a seemingly small difference of 0.2 pF between two ESD devices can cause skew variations greater than 40 ps at critical points, reducing the margin in eye diagram mask tests. With discrete protection devices, this can be difficult to avoid; as shown earlier, BAV99s can range from < 1.0 pF to 3.0 pF, or 10 times the mismatch depicted in the example above. The best way to control capacitance matching is to use an integrated diode array. By manufacturing all of the diodes on a single die, typical diode arrays from leading ESD vendors can provide capacitance matching at < 0.05 pF, minimizing signal skew and EMI problems. With a multi-channel diode array, the system engineer can ensure channel-to-channel matching by design, and avoid large field variation when a new discrete diode vendor or wafer lot is merged into production lines later on. Packaging and Layout Issues Selecting an ESD protection device with low capacitance and matched capacitance is not in itself sufficient to assure good signal integrity. Choice of package can be just as important. Packaging of the ESD device can have several effects on signal integrity. The most obvious one is how easy or difficult it is to route a signal to a given package. Routing is particularly important with differential signals for several reasons: • Difficult routing can force the designer to use vias, affecting impedance; • Poor package choice can force the designer to route signals using unequal length, adding signal skew; • Changes to the distance between differential pairs can make it difficult to achieve matched impedance. The best way to route high speed signals is to use routing that goes straight through or straight under the ESD device, minimizing any discontinuities caused by routing. Figures 4 and 5 illustrate the differences between using discrete single channel devices vs. an optimized straight-through routing. Figure 3: Small 0.2 pF capacitance difference between lines can introduce significant skew 44 Conformity JUnE 2008

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Conformity - June 2008 Contents Editor's Note 700 MHz Auction Raises a Record $20 Billion FCC Modifies PCS and AWS Power Limit Rules for Broadband Wireless Commission Designates Spectrum Test Bed FCC Releases Report on Wireless Competition Commission Proposes $5 Million Fine for Slamming Using EDX for Non-Destructive Detection of Lead in Consumer Products ESD Open Forum Challenges in Testing - Improving Election Security and Accuracy Part 1 Challenges in Testing - Human Body Model: The Hidden Challenges High-Speed Signal Integrity Considerations for ESD Components Focus On...EMC Components Buyer's Guide FCC Levies $2.6 Million Fine for Junk Faxes FDA Updates Guidance on its Product Review Process EU Issues Updated Energy Star Regulations EU Repeals Directive on Veterinary Electro-Medical Equipment New Product Announcements Updated Standards List for the EU's PPE Directive CPSC Announces New Effort to Keep Out Hazardous Products CPSC Actions in the News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity - June 2008

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