Printed Circuit Design & Fab - December 2007 - (Page 35)

Microstrip Differential Impedance (for traces routed on an outer layer) Zo = 87/Sqrt (Er + 1.41) * (ln (5.98H/ (0.8W + T)) Zdiff = 2 * Zo (1 - 0.48 e -0.96D/H) Unbalanced Stripline Differential Impedance (for traces embedded between planes) Zo = 80/Sqrt Er* ln (1.9 (2H + T)/ (0.8W + T)) * (1 - (H / 4 (H + C + T))) Zdiff = 2 * Zo (1 - 0.347 e -2.9D/B) FIGURE 4. Ideal length matching done at 8 GB/s. where W = trace width T = trace thickness H = distance to nearest reference plane Er = dielectric constant D = trace edge to edge spacing C = signal layer separation B = reference plane separation FIGURE 5. Approximate calculations of differential impedances in microstrip and stripline environments. Courtesy: National Semiconductor. destructive coupling within the differential signals, which is inversely proportional to the distance between the traces. Moreover, the closer the traces are to each other, the lower will be the effective impedance, and maintaining the original impedance will require reducing trace widths or adjusting the termination schemes. Rule #3: The trace separation within a differential pair needs to be constant over its entire length. The main reason behind this rule is Rule #2. If the differential traces are routed close together, then they are impacting the overall impedance. Now if this separation is not maintained all the way from the driver to the receiver, there will be impedance mismatches along the way, resulting in reflections. trace ■ Differential trace width and thicknesses ■ Differential air gap ■ Height of each trace from the plane ■ Dielectric constants of the media. The differential impedance can be calculated by the formula shown in FIGURE 5. BIBLIOGRAPHY Brooks, Douglas. Integrity Issues and Printed Circuit Board Design. Lattice Semiconductor Corp. “Differential Signaling, AN6019. May 2001. ” National Semiconductor. “Transmission Line RAPIDESIGNER Operation and Applications Guide. AN905. May 1996. ” Ritchey, Lee W. Right the First Time, Volume One. Conclusion Differential signaling has been around for a long time, and there is very little chance of it disappearing from the PCB design scene. It is imperative for both design and layout engineers to fully understand the concepts behind the need for differential circuits and the proper design rules, which if not followed correctly, will diminish their advantages and may even create more problems for the rest of the circuit. PCD&F SYED W. ALI, C.I.D, is a PCB layout engineer with Nexlogic Technologies Inc., San Jose, CA; info@nexlogic.com, 408-436-8150. With 35 years of experience Differential Impedance Calculations The impedance of edge coupled differential signals depends on the following factors: ■ Zo: Impedance of the single-ended Did you know? The redesigned PCB UPdate e-mail newsletter is now distributed daily! Edited by Ronda Faries, a former editor-in-chief of Printed Circuit Design magazine, and powered by the editorial content and industry expertise of UP Media Group’s two monthly magazines – Printed Circuit Design & Fab and Circuits Assembly – PCB UPdate provides subscribers with the latest PCB industry news via the convenience of a daily e-mail. Our goal is for PCB UPdate to be your primary and most trusted source of daily news, views and product information across the PCB design, fabrication and assembly markets. We make our own laminates in the U.S. • Material types in FR-4 Difunctional, Tetrafunctional and Multifunctional • Temperature ratings from 130˚C to 177˚C • Thickness ranges from .0025” to 3.000” • Coppers from 1/2 oz. to 6 oz. in stock • Black, Blue, Tan, Green, Orange & Natural • True Cryogenic G10 • Sheet sizes in 36”x48” • RoHS compliant, UL recognized and IPC approved Same Day Pricing and Lead Times Current, Inc. 30 Tyler St. Ext • East Haven, CT 06512 1-877-436-6542 • Fax 203-467-8435 www.currentcomposites.com Louis S. Owen, Director of Outside Sales 860-350-9607 • Fax 860-210-1748 PRINTED CIRCUIT DESIGN & FAB 35 Subscribe at pcbupdate.com/subscribe DECEMBER 2007 http://www.currentcomposites.com http://www.currentcomposites.com http://pcbupdate.com/subscribe

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Printed Circuit Design & Fab - December 2007 Contents Our Line Market Watch Around the World Happenings ROI 2007 PCB Designer Salary Survey Interconnect Strategies Positive Plating RF Design Ad Index Noise Reduction Supply Chain DfM Off the Shelf Marketplace The Signal Doctor

Printed Circuit Design & Fab - December 2007

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