Digital Signal Processing Reference
In-Depth Information
11.8 DIFFERENTIAL CURRENT-MODE TRANSMITTERS
11.8.1 Operation
Differential current-mode transmitters are often used for high-speed data trans-
mission. In this case, the transmitter operates by injecting a current onto the
transmission line. Figure 11-31a depicts a simple differential transmitter design.
The transmitter uses complementary input signals,
v
in
and
v
in
, which ensure that
only one side of the circuit is in the conducting state at any given time. Thus,
the transmitter uses the differential input signals to steer the current from the
constant current source,
i
S
, to the desired side of the circuit. The flow of cur-
rent creates a voltage drop across the source termination resistor,
R
TT
,onone
side of the circuit, while the side that has no current flow is pulled up to
V
DD
,
thereby creating the output signal levels
v
out
and
v
out
. The relationship between
input and output signals is summarized in Table 11-4. The table demonstrates
that the differential signal swing has a magnitude (2
i
T
x
R
TT
) that is twice that of
the singled-ended swing.
V
DD
V
DD
R
TT
R
TT
R
TT
R
TT
v
o
ut
v
out
v
out
v
out
v
in
v
in
i
s
i
s
i
s
(a)
(b)
Figure 11-31
(a) Differential current mode simple transmitter design; (b) linear model.
TABLE 11-4. Relationship Between Differential
Transmitter Input and Output Signals
v
in
/v
in
Low/High
High/Low
V
DD
−
i
T
x
R
TT
v
out
V
DD
V
DD
−
i
T
x
R
TT
v
out
V
DD
=
v
out
−
v
out
−
i
T
x
R
TT
v
diff
i
T
x
R
TT
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