Datasheet
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Load Circuits
Recommended Resistor Values:
For 5 V Nom Supplies, R
S
= 200 Ω.
For 3.3 V Nom Supplies, R
S
= 75 Ω.
R
S
R
S
R
T
= 100 Ω
Transmission Line
OUTPUT
INPUT
Recommended Resistor Values:
For 5 V Nom Supplies, R
T
= 200 Ω, R
S
= 90 Ω
For 3.3 V Nom Supplies, R
T
= 100 Ω, R
S
= 30 Ω
R
T
/2
Transmission Line
OUTPUT
INPUT
R
S
R
T
/2
Recommended Resistor Values:
For 5 V and 3.3 V Nom Supplies, R
T
= 100 Ω,
V
T
= V
CC
− 2.55 V
R
T
/2
Transmission Line
OUTPUT
INPUT
V
T
R
T
/2
_
+
TB5T1
SLLS589C – NOVEMBER 2003 – REVISED OCTOBER 2007
The test load circuits shown in Figure 6 and Figure 7 are based on a recommended pi type of load circuit shown
in Figure 18 . The 100- Ω differential load resistor R
T
at the receiver provide proper termination for the
interconnecting transmission line, assuming it has a 100- Ω characteristic impedance. The two resistors R
S
to
ground at the driver end of the transmission line link provide dc current paths for the emitter follower output
transistors. The two resistors to ground normally should not be placed at the receiver end, as they shunt the
termination resistor, potentially creating an impedance mismatch with undesirable reflections.
Figure 18. A Recommended pi Load Circuit
Another common load circuit, a Y load, is shown in Figure 19 . The receiver-end line termination of R
T
is provided
by the series combination of the two R
T/2
resistors, while the dc current path to ground is provided by the single
resistor R
S
. Recommended values, as a function of the nominal supply voltage range, are indicated in the figure.
Figure 19. A Recommended Y Load Circuit
An additional load circuit, similar to one commonly used with ECL and PECL, is shown in Figure 20 .
Figure 20. A Recommended PECL-Style Load Circuit
An important feature of all of these recommended load circuits is that they ensure that both of the emitter follower
output transistors remain active (conducting current) at all times. When deviating from these recommended
values, it is important to make sure that the low-side output transistor does not turn off. Failure to do so
increases the t
skew2
and V
OC(PP)
values, increasing the potential for electromagnetic radiation.
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