差分阻抗计算Differential Impedance What’s the Difference? Douglas Brooks Just when you thought you had mastered Zo, the characteristic impedance of a PCB trace, along comes a data sheet that tells you to design for a specific differential impedance. And to make things tougher, it says things like: “… since the coupling of two traces can lower the effective impedance, use 50 Ohm design rules to achieve a differential impedance of approximately 80 Ohms!” Is that confusing or what!! This article shows you what differential impedance is. But more than that, it discusses why it is, and shows you how to make the correct calculations. Zo (a) Z11 i1 i 1 2 ki2 Z22 (b) i2 ki1 1 Z11 i1 + ki2 Rdiff 2 Z22 i2 + ki1 (c) Single Trace: Figure 1(a) illustrates a typical, individual trace. It has a characte……