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
  • 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: