Datasheet
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Load Circuits
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
_
+
INPUT
R
T
= 100 W
OUTPUT
R
S
R
S
Transmission Line
Recommended Resistor Values:
For 5-V Nominal Supplies, R
S
= 200 W
For 3.3-V Nominal Supplies, R
S
= 75 W
TB5D1M , TB5D2H
SLLS579C – SEPTEMBER 2003 – REVISED JANUARY 2008
The test load circuits shown in Figure 3 and Figure 4
are based on a recommended pi type of load circuit
shown in Figure 15 . 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
Figure 16. A Recommended Y Load Circuit
transmission line link provide dc current paths for the
emitter follower output transistors. The two resistors
An additional load circuit, similar to one commonly
to ground normally should not be placed at the
used with ECL and PECL, is shown in Figure 17 .
receiver end, as they shunt the termination resistor,
potentially creating an impedance mismatch with
undesirable reflections.
Figure 17. A Recommended PECL-Style Load
Circuit
Figure 15. A Recommended pi Load Circuit
An important feature of all of these recommended
load circuits is that they ensure that both of the
Another common load circuit, a Y load, is shown in
emitter follower output transistors remain active
Figure 16 . The receiver-end line termination of R
T
is
(conducting current) at all times. When deviating from
provided by the series combination of the two RT/2
these recommended values, it is important to make
resistors, while the dc current path to ground is
sure that the low-side output transistor does not turn
provided by the single resistor R
S
. Recommended
off. Failure to do so increases the t
skew2
and V
OC(PP)
values, as a function of the nominal supply voltage
values, increasing the potential for electromagnetic
range, are indicated in the figure.
radiation.
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