# Common Mode vs Differential Mode Signals

Created: 5/19/2020last Updated: 5/19/2020

**Definition: **

**Even mode propagation**refers to the way signal propagates on differential pair (P & N traces), where signals have**equal in magnitude and polarity**.**Odd mode propagation**refers to way signal propagates on differential pair (P & N traces), where signals have**equal in magnitude but inverted polarity**.**Even mode impedance (Zeven)**: it is defined as the impedance on**a single trace**of the differential pair when driven by a**differential mode voltage.****Odd mode (Zodd):**it is defined as the impedance on**a single trace**of the differential pair when driven by a**common mode voltage****Differential impedance (Zdiff):**it is what the differential**driver**sees across the differential pair.- Zdiff=2*Zodd

**Common mode impedance (Zcom):**it is what the common mode voltage**driver**sees across the differential pair.- Zcom=0.5*Zeven

**Characteristic impedance (Zo):**it is the impedance of a single trace on seen by a single ended driver, which is also commonly know as Single Ended (SE) trace.- It's a function of transmission line inductance and capacitance.

**Application**

- Differential impedance, odd mode propagation, and odd mode impedance are key concepts in differential high speed signals design, where differential pairs are used in board layout.
- In real design, we are given a differential impedance of 100 ohms which is measured by Time Domain Reflectometry (TDR). TDR will given both a odd and even impedance.
- Due to the fact differential traces are tightly coupled to reference ground planes in PCB stripe line design,
**Zdiff=2xZodd=2xZo**

- Due to the fact differential traces are tightly coupled to reference ground planes in PCB stripe line design,
- A differential driver A (e.g. H-Bridge) transmits complementary signals on a communication interface usually postfixed as _TX_P and _TX_N

**Q&A**

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

- Zeven= Zo (P/N)+Mutual Impedance (P&N)

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

- Zodd= Zo (P/N)-Mutual Impedance (P&N)

*How do differential impedance Zdiff relates to odd mode impedance in an ideal differential pair? *

- Zdiff = 2 x Zodd

*When does Zodd equal to Zo?*

- The odd-mode impedance of the loosely coupled pair equals the characteristic impedance of the SE trace, which is what generally seen in the PCB design for coupled trace . For tightly coupled trace (i.e differential P and N are routed closer to each other), mutual impedance is higher, so the Zodd will be lower.

**Reference and Further Reading:**

- "The basics of Time Domain reflectometry", https://hvtechnologies.com/blog/basics-time-domain-reflectometry-tdr
- "What is Differential Impedance and Why do We Care?", https://www.signalintegrityjournal.com/blogs/12-fundamentals/post/1665-what-is-differential-impedance-and-why-do-we-care