Differential signaling is widely used in high-speed interfaces such as USB, HDMI, Ethernet, PCIe, and LVDS. Proper differential impedance design is critical for signal integrity, noise immunity, and electromagnetic compatibility. This article explains the principles and practical implementation of differential impedance control.
Differential impedance (Zdiff) is the impedance measured between two differential signal traces when driven with opposite polarity signals. It is determined by the trace geometry, dielectric material, and layer stack-up. Standard differential impedance values: 90Ω (USB), 100Ω (Ethernet, LVDS, HDMI), 85Ω (PCIe).
Edge-coupled differential pairs are the most common implementation. Tight coupling (small spacing) improves noise immunity but increases susceptibility to manufacturing variations. Loose coupling eases fabrication but increases common-mode noise sensitivity.
Intra-pair skew (length mismatch) causes phase shift and reduces differential signal quality. Industry standards specify maximum skew: 5 mils (0.127mm) for USB 2.0, 1 mil (0.025mm) for USB 3.0, 2 mils (0.05mm) for HDMI. Match lengths by adding serpentine traces in the shorter path.
Minimize vias on differential pairs. When vias are unavoidable, place them symmetrically for both traces. Use return vias adjacent to signal vias to maintain impedance and reduce EMI.
Field solvers (such as Polar SI9000, Ansys Q2D) are used to calculate trace dimensions for target impedance. Manufacturers typically provide controlled impedance stack-ups with recommended trace width and spacing for specific dielectric materials.
Traces on outer layers with reference plane on adjacent inner layer. Susceptible to external noise but easier to route. Impedance affected by soldermask.
Traces sandwiched between two reference planes. Better EMI performance and more stable impedance. Preferred for critical high-speed signals.
Time Domain Reflectometry (TDR) is used to measure characteristic impedance of manufactured boards. TDR testing should be performed on test coupons or dedicated impedance structures on the production panel.
Differential impedance control requires careful stack-up design, precise trace dimensions, and collaboration with PCB manufacturers. FANYE Technology provides comprehensive high-speed PCB design services with guaranteed impedance control for your critical interfaces.