Definition

Differential impedance (Zdiff) is the impedance measured between the two conductors of a differential pair when driven with opposite-polarity signals. It equals approximately 2x the odd-mode impedance of each trace (exactly 2x for symmetric coupling). Common differential impedance targets are 90 ohms (USB 2.0/3.0, HDMI), 85 ohms (PCIe, SATA), and 100 ohms (Ethernet, LVDS). Differential impedance depends on trace geometry, spacing between traces, dielectric thickness, and material properties. Tighter coupling (smaller gap) reduces differential impedance.