Digital Signal Processing Reference
In-Depth Information
V
tt
RI
RI
+
−
+
V
tt
V
noise
(a)
RI
−
V
cm
RI
+
DO
+
RI
+
+
−
DO
−
RI
−
(b)
Figure 13.1
(a) In single-ended signaling the receiver switch point (
V
tt
) is with respect to the local
ground. (b) With differential signaling the crossing of complementary signals determines the switch
point.
13.2
What Are the Electrical Characteristics of Differential Signaling?
A diff-amp receiver amplifies the difference in voltage appearing on its inputs (
RI
+
and
RI
) and rejects voltages that simultaneously appear on both inputs. This is
why noise that couples equally to both inputs is ignored by the receiver. In fact, to
get the most benefit from differential signaling, we will intentionally exploit this
characteristic by deliberately routing the two traces forming the diff-pair so that
noise is encouraged to couple equally to both traces.
Taking the receiver ground connection as a reference, the difference in the
ground voltages between the transmitter and receiver appears to the receiver as a
noise voltage that adds or subtracts from the incoming signal. However, because
they do not use ground as a signal reference, the switch point of differential receiv-
ers is not affected by this type of
common mode
noise.
However, there are limits in the common mode voltage over which the circuit
will properly operate. The receivers'
common mode range
specifies the maximum
voltage that the signal may reach (measured with respect to the ground local to the
receiver) before the transistors within the diff-amp become unbiased and it loses
the ability to differentiate valid differential signals. This is a critical parameter to
assess when performing signal integrity simulations and when making laboratory
measurements.
−
