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3. Differential Pair: Two complementary traces (positive and negative) routed close together, used for differential signaling. Differential Pairs with incorrect (left) and correct (right) tuning The above images show an important point about differential pairs that sometimes goes overlooked - Differential pairs must be matched in length and symmetry along every millimeter of routing to avoid creating common mode voltage signals, which generate noise and unwanted electromagnetic fields. Delaying both traces by the same amount prevents signal interference. So it is not sufficient to match differential pairs in length only. They must match in the signal phase as well, hence strict adherence to trace symmetry or at least equivalent delay, which is commonly achieved through routing symmetry. f Application: Common in high-speed applications like USB, HDMI, and DDR, as differential pairs are immune to common-mode noise as long as they are routed with proper phase matching. f Impedance Control: Controlled by trace width, spacing between the pairs, and proximity to the ground plane. Key Factors Affecting Impedance Control Several factors influence impedance in transmission lines, and controlling them ensures that signals remain within defined impedance thresholds: f Trace Width and Thickness: Wider or thicker traces lower impedance, while narrower traces increase it. f Distance to Reference Plane: The farther a trace is from its reference ground plane, the higher the impedance. f Dielectric Constant (Er): The material between the trace and ground plane affects impedance; materials with lower Er tend to increase impedance. f Trace Spacing (for differential pairs): The distance between differential pair traces affects both their characteristic impedance and coupling strength. Basics of Routing Differential Pairs and Length Matching 1. Differential Pair Routing: f Route differential pairs in close proximity to each other with consistent spacing. This maximizes coupling and improves noise immunity. f Maintain a consistent gap between the traces across the entire path to control differential impedance. 13 www.cadence.com OrCAD X High-Speed Digital Design Guide