Texas Instruments SLLU040A, Руководство пользователя

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Typical Test Results
2-6
LVPECL drivers need a 50-
Ω
termination to V
T
. A modification of Figure 2-2
and the above instructions are used when evaluating an SN65LVDS101 with
a direct connection to a 50-
Ω
oscilloscope. With power supply 1 in Figure 2-2
set to 3.3 V, power supply 2 should be set to 1.3 V (2 V below V
CC
) to provide
the correct termination voltage.
CML drivers need a 50-
Ω
termination to V
TT
(V
TT
is either V
CC
, 2.5 V, or
1.8 V). A modification of Figure 2-2 and the instructions for the SN65LVDS100
are used when evaluating a SN65CML100 with direct connection to a 50-
Ω
oscilloscope. With power supply 1 in Figure 2-2 set to 3.3 V, power supply 2
should be set to either V
CC
(3.3 V), 2.5 V, or 1.8 V to provide the correct
termination voltage.
Dual termination of the output can be achieved by placing 49.9-
Ω
resistors at
R2 and R3 and connecting to an oscilloscope as described above.
If the EVM outputs are to be evaluated with a high-impedance probe, direct
probing on the EVM is supported via installation of R2, R3, and R4. LVDS
outputs can be observed by installing R4, a 100-
Ω
resistor. LVPECL outputs
can be observed by installing R2 and R3 (49.9-
Ω
resistors), and setting power
supply 2 to 1.3 V. CML outputs can be observed by setting power supply 2 to
V
TT
and installing 49.9-
Ω
resistors at R2 and R3 for single termination, or
24.9-
Ω
resistors at R2 and R3 for dual termination (Note that power supply 2
must be able to sink current.)
2.4 Typical Test Results
Figure 2-5 shows a typical test result obtained with the EVM. Figure 2-5
shows the output of an SN65LVDS100 being driven directly into a 50-
Ω
oscilloscope. For this figure, the SN65LVDS100 was stimulated with an HP
3-GBPS BERT. The input data was pseudorandom data at 2 GBPS and with
a random record length of 2
23
-1. The BERT drove two electrically matched
one-meter cables with an electrical length of 3.667 ns. These cables were then
connected to the EVM inputs. The EVM outputs were connected through
another set of electrically matched one-meter cables and terminated by a
TDS8000 oscilloscope’s 50-
Ω
resistors to ground.