DIFFERENTIAL SIGNALING - Advanced Signal Integrity for High-Speed Digital Design - page 303

between V and V . Ideally, the phase difference between the waveforms propa-
gating on V and V is 180 ◦ , which keeps the energy in the odd mode. However,
if the phase relationship between V and V deviates from 180 ◦ as the signals
propagate toward the receiver, some of the energy will be converted from odd
mode to even mode. This phenomenon has many names, including mode conver-
sion, differential-to-common mode conversion , and ac common-mode conversion
(ACCM conversion) . In this text we use the term ACCM conversion .
ACCM conversion is caused by asymmetry between V and V in the dif-
ferential pair. The asymmetry can be caused by length differences, coupling
differences, etching differences, proximity effects, termination differences, bends,
or anything else that would make one leg of the pair look electrically different
from the other. Some of these examples are shown in Figure 7-8. To demonstrate
how intrapair asymmetry affects voltage and timing noise, consider the simple
(but very common) example where lengths of lines 1 and 2 in the pair are not
equal, as shown in Figure 7-8a. Assuming that the signal is launched differen-
tially, the difference in propagation delay between lines 1 and 2 will change the
phase relationship at the receiver because the voltage propagating on one leg of
the pair will arrive early, converting part (or all) of the differential signal to com-
mon mode. Figure 7-9 shows how a perfect differential signal at the transmitter
(Tx) is distorted at the receiver (Rx) when asymmetry exists in the pair. The dis-
tortion is proportional to the amount of voltage that exists in the common mode.
The amount of signal that is converted to common mode depends on the
length of the pair, the difference in propagation delays, and the frequency. This

line 1 V V

Z 01

V Z 02 V

line 2

∆ l

(a) (b)

S 1

V l 2

l 1

V

S 2

V V

(c) (d)

Figure 7-8 Examples of asymmetry in a differential pair that can cause differential to
common-mode conversion: (a) routing-length differences; (b) impedance differences due
to etching variation; (c) crosstalk differences; (d) length differences due to bends.