Digital Signal Processing Reference
In-Depth Information
Differential
d
r
i
v
e
r
Z
=50
Ω
oo
50
Ω
DO
+
+
RI
+
1V
+
−
R
V
out
term
RI
−
DO
−
50
Ω
0V
Z
=50
Ω
oo
RI
−
0.75V
V
= 0.5V
cm
0.25V
RI
+
V
out
t
Figure 13.6
A 50
Ω
differential driver connected to a differential receiver. The signal appearing
across
R
term
swings 0.25V above and below a 0.5-V common-mode voltage. The receiver output
shown at the bottom of the fi gure swings rail to rail.
Differential
d
ri
v
e
r
Z
=50
Ω
oo
50
Ω
DO
−
DO
+
RI+
VDO
+
+
0.5V
RT
+
0.25V
V
cm
+
−
0.25V
RT
0.5V
+
0.5V
VDO
−
RI
−
DO
−
50
Ω
Z
=50
Ω
oo
Figure 13.7
Differential driver connected to a differential pair. The outputs switch about the com-
mon-mode voltage
Vcm
. Voltages on
DO
+ and
DO
−
are measured with respect to
Vcm
.
The circuit consists of a 0.5-V supply voltage around which the outputs switch
(the common-mode voltage,
V
cm
), and two 0.5-V supplies and 50
Ω
resistances
connected to the
DO
+ and
DO
outputs. The termination resistor
R
term
shown in
Figure 13.6 has been separated into two equal valued resistors called
RT
. They are
connected together to the common-mode voltage supply.
When driving a logic high, the common-mode supply adds to the supply con-
nected to the
DO
+ pin. When measured with respect to ground,
DO
+ is a 50
−
Ω
re-
sistance in series with a 1-V source. Similarly, the supply connected to the
DO
−
pin
