Definition

Edge-coupled differential pairs place two signal traces side-by-side on the same layer with controlled gap spacing. The electromagnetic coupling between traces carries the differential signal while providing common-mode noise rejection. Impedance is characterized by odd-mode (Zodd) and even-mode (Zeven) values, with differential impedance approximately 2x Zodd. Standard targets include 90 ohms (USB, HDMI), 85 ohms (PCIe, SATA), and 100 ohms (Ethernet). Edge coupling is the most common differential configuration due to simple manufacturing and easy access for testing.