Common Mode vs Differential Mode Signals

Created: 5/19/2020last Updated: 7/06/2023

Introduction

In the realm of signal integrity and high-speed circuit design, understanding the behavior of differential pairs is crucial. A fundamental aspect of differential pairs is the concept of even mode and odd mode propagation, which governs how signals travel along the pair. Additionally, various impedance parameters play a significant role in characterizing the differential pair's behavior. This includes even mode impedance (Zeven), odd mode impedance (Zodd), differential impedance (Zdiff), common mode impedance (Zcom), and characteristic impedance (Zo). By comprehending these concepts and equations, engineers can design differential pairs with optimized performance and ensure reliable signal transmission in their electronic systems.

Definition:

Application

Q&A

Q: How do even mode impedance relate to characteristic impedance of each differential pair line?

A: Zeven = Zo (P/N) + Mutual Impedance (P&N). The even mode impedance is determined by the characteristic impedance (Zo) of each individual differential pair line and the mutual impedance between the P and N traces.

Q: How do odd mode impedance relate to characteristic impedance of each differential pair line?

A: Zodd = Zo (P/N) - Mutual Impedance (P&N). The odd mode impedance is determined by the characteristic impedance (Zo) of each individual differential pair line and the mutual impedance between the P and N traces.

Q: How does differential impedance (Zdiff) relate to odd mode impedance in an ideal differential pair?

A: Zdiff = 2 x Zodd. In an ideal differential pair, the differential impedance (Zdiff) is equal to twice the value of the odd mode impedance (Zodd).

Q: When does Zodd equal to Zo?

A: The odd-mode impedance of the loosely coupled pair equals the characteristic impedance (Zo) of the single-ended (SE) trace, which is typically observed in PCB designs for coupled traces. In tightly coupled traces where the differential P and N traces are routed closer to each other, the mutual impedance is higher, resulting in a lower Zodd value.

Summary