DIFFERENTIAL SIGNALING - Advanced Signal Integrity for High-Speed Digital Design - page 301

Therefore, the common-mode noise propagates in the even mode .
The voltage present on each leg of the differential pair can be calculated from
the modal voltages by solving (7-2):

v D + = 0 . 707 v odd + 0 . 707 v even = 1 + 0 . 707 ( 1 . 41443 v cm ) = 1 + v cm

v D − =−

+ =− +

0 . 707 ( 1 . 41443 v cm ) =−

+ v cm

0 . 707 v odd 0 . 707 v even 1 1

In a differential bus, the line voltages are subtracted with a differential amplifier
at the receiver which eliminates the noise.

v D + − v D − = ( 1 + v cm ) − ( − 1 + v cm ) = 2

To summarize, a differential signaling scheme eliminates energy in the even mode .

7.5 COMMON TERMINOLOGY

For systems with more than two signal conductors, the terms even mode and
odd mode are no longer applicable . When analyzing differential buses, it is
common to refer to a pair being driven with two signals 180 ◦ out of phase
as the differential mode and two signals driven in phase as the common mode .
The differential- and common-mode terms are simply naming conventions and
are not technically modes at all. Analysis of the modal voltages propagating on
multiconductor systems as described in Section 4.4 will show that the digital
states do not correspond directly to modal voltages for systems with more than
two signal conductors.
The differential-mode impedance is defined as twice the odd mode, and the
common-mode impedance is one-half the even mode. The odd- and even-mode
impedance values are described in Section 4.3.1.

Z differential = 2 Z odd (7-5a)

Z even
2

Z common = (7-5b)

It should be noted that equations (7-5a) and (7-5b) are typically used for the
purpose of specifying design guidelines. The actual impedance of a differential
pair may not correspond directly to Z differential if there is significant coupling to
adjacent pairs. Equations (7-5a) and (7-5b) are representative of the true pair
impedance values only if the interpair coupling is weak. Remember: In a mul-
ticonductor system with

N signal conductors, there will be N unique modal

impedance values.
It is also true that for a system with N signal conductors, there will be N modal
propagation velocities. If the transmission lines are routed in a homogeneous
dielectric (such as a stripline), all the modal velocities will be identical. However,
for a nonhomogeneous dielectric (such as a microstrip), the differential- and