Conformity Magazine - June 2008 - (Page 42)

One common problem in identifying device capacitance is that it is often inadequately specified or in some cases specified in misleading ways. The engineer should look for two key parameters in a datasheet: consist of two steering diodes on two independent dice, which are typically connected between power and ground, with the signal line connected in the center via high inductance bonding wires. BAV99s all specify their capacitance as a maximum of 1.5 pF; however, this is the capacitance of each individual diode, so the actual junction capacitance can be twice that, or 3.0 pF. In practice, the capacitance on these parts can vary up to 200% from vendor to vendor and lotto-lot, creating unforeseen impedance mismatches, signal skew, and EMI problems. This type of issue can often go unnoticed in the test lab, only to be discovered when systems are shipping and quality issues are being Figure 2: Single Channel reported by end customers. BAV99 diode structure Finally, as mentioned, lower capacitance often results in higher clamping voltages and residual current reaching the protected circuits. With high speed circuits migrating to smaller geometries that are more sensitive to ESD damage, solutions that previously worked to protect a system may no longer work in the next generation design. This makes it critical for engineers to carefully balance the tradeoff of capacitance vs. ESD protection capability. • Typical junction capacitance (I/O to Ground) • Maximum junction capacitance (I/O to Ground) Most vendors list their typical capacitance, but many do not list their maximum capacitance. If this is not specified, it is possible that lot-to-lot production variations from the ESD vendor may cause signal integrity problems in the end systems. Unfortunately, this is not the only datasheet comparison problem. Some ESD component vendors highlight their ESD devices based on I/O to I/O capacitance rather than I/O to Ground capacitance. It is important to identify the correct, consistent parameter, as well as the related effects. The actual junction capacitance (I/O to Ground) can be more than 100% higher than the capacitance between two I/O pins, causing impedance mismatches and requiring layout changes to compensate for the higher capacitance. I/O to I/O capacitance can be related to cutoff frequency, or crosstalk, depending on which pins are referenced. Discrete diodes are another source of problems in this regard. Single channel diode solutions such as BAV99s have historically been used by engineers to protect moderate speed ports such as VGA and full speed (12Mbit/s) USB. These signal diodes are not as reliable for ESD protection as ESD-rated diode arrays, but are popular because they are inexpensive and available from numerous vendors. They Figure 1: Decreasing capacitance improves signal attenuation at higher frequencies 42 Conformity JUnE 2008

Table of Contents Feed for the Digital Edition of Conformity - June 2008

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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