Common Mode vs Differential Mode Signals
- 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.
- Common mode impedance (Zcom): it is what the common mode voltage driver sees across the differential pair.
- 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.
- 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
- A differential driver A (e.g. H-Bridge) transmits complementary signals on a communication interface usually postfixed as _TX_P and _TX_N
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