Digital Signal Processing Reference
In-Depth Information
From this we see that, although the traces were routed with a 50
Ω
odd-mode
impedance, the differential impedance is actually twice that (100
Ω
).
13.4
How Are Differential Transmission Lines Terminated?
From Figure 13.8 we see that a proper termination is formed by connecting a resis-
tor equal to the transmission line impedance between each traced and a common-
mode voltage. For instance, for a diff-pair having a 50
Ω
odd-mode impedance,
attaching to each of the traces 50
resistors connected to a supply voltage (such as
the 0.5V common mode from the previous section) would properly terminate the
line.
A second approach appears in Figure 13.9, which shows that a valid termi-
nation can be formed by placing resistance directly across the traces forming the
diff-pair. It is evident the value for a single resistor would be twice the value of
RT
,
and we have just seen that for proper termination
RT
should be made equal to the
odd-mode impedance. For instance, attaching a 100
Ω
resistor across the diff-pairs
will properly terminate them when they have an odd-mode impedance of 50
Ω
. This
makes intuitive sense: Because the differential impedance is equal to twice the odd-
mode impedance, a single resistor equal to the differential impedance should be
a proper differential termination. These two ideas are illustrated in Figure 13.10.
Figure 13.10(a) shows the common-mode termination, while Figure 13.10(b)
shows the differential termination technique. We can see in Figure 13.10(a) that if
the common-mode voltage is made zero, the two resistors are effectively connected
to ground. In fact, diff-pairs are sometimes terminated with a resistor equal to the
odd-mode impedance (
Z
oo
) connected between each of the traces and ground. This
scheme is attractive for those drivers that require a connection to ground to estab-
lish a DC bias for proper operation, but such a termination is not truly differential.
Any difference in the voltage between the ground at the transmitter and the local
ground at the receiver where the termination resistors are connected will appear as
a voltage offset to the receiver, which can erode receiver noise margins.
Ω
Z
=50
Ω
oo
50
Ω
DO+
RI
+
0.75V
VDO+
+
5mA
RT
0.5V
0.5V
+
V
cm
0.5V
RT
DO
−
0.25V
RI
−
50
Ω
Z
=50
Ω
oo
Figure 13.9
Output voltages measured with respect to ground.
