Definition

Odd-mode impedance (Zodd) is the impedance seen by each individual trace in a coupled pair when driven with equal but opposite signals (differential mode). The coupling between traces lowers the effective impedance compared to an isolated single-ended trace. Differential impedance equals approximately 2x Zodd (exactly 2x for symmetric coupling). For a 100-ohm differential pair, each trace sees about 50 ohms odd-mode impedance. Odd-mode impedance decreases as trace spacing decreases (tighter coupling), which is why differential pairs are routed with controlled, tight spacing.