DIFFERENTIAL SIGNALING - Advanced Signal Integrity for High-Speed Digital Design - page 296

A strategy that reduces the effect of the system noise dramatically is to dedicate
a pair of transmission lines for each bit on the bus. The two transmission lines
are driven 180 ◦ out of phase (in the odd mode), and the differences between the
voltages are used to recover the signal at the receiver using a differential amplifier.
This technique is called differential signaling and is depicted in Figure 7-2.

7.1 REMOVAL OF COMMON-MODE NOISE

Differential signaling is very effective for removing common-mode noise , which
is defined as noise present on both legs of the differential pair. If the bus is
designed properly so that the legs of the differential pairs are in close proximity
to each other, the noise on D + .

Therefore, assuming that the receiver's differential amplifier has a reasonable
common-mode rejection ratio, the noise will be eliminated. For example, assume
that there is noise with a magnitude of v noise coupled equally onto both legs of
a differential pair. The output of a differential amplifier with unity gain is

will be approximately equal to the noise on D −

v diff = (v D + + v noise ) − (v D − + v noise ) = v D + − v D − (7-1)

which removes the common-mode noise.
Figure 7-3 shows an example of a differential interconnect with noise present
on the ground plane ( v noise ). In this case, the noise is common to both legs of
the driver. Figure 7-4a shows a simulated stream of data bits at the input to a
differential amplifier with significant common-mode noise. Note that the digital
state of the single-ended waveforms v D + and v D − is indeterminable because the
noise is so large. However, since the noise is common mode, the bit stream can be
recovered when the signals are subtracted by a differential amplifier ( v D + − v D − ),
as shown in Figure 7-4b.

i drive

v D +

D D Z diff

R

R

R R

v D −

+
−

v noise

Figure 7-3 Differential driver and interconnect with common-mode noise present on the
ground. Noise on both the power and ground planes is very common in digital designs.