Texas Instruments SLLU040A, User Manual

Page: 16 / 27

Applying an Input
2-4
with 50-
Ω
pullups to V
TT
. When using external terminations, the onboard
termination resistor R1 should be removed from the EVM. It should be noted
that the signal quality at the receiver input may be degraded when external
terminations are used, as a significant stub exists from the external termination
network to the receiver input. The user needs to verify that the transition time
of the input signal, coupled with the stub length, does not lead to reflection
problems. These concerns would be addressed in a real application where the
terminations are placed close to the receiver input.
Figure 2-3. External Termination for Differential CML or LVPECL Inputs to EVM
Select V
T
for LVPECL
or
Select V
TT
for CML
Select V
T
for LVPECL
or
Select V
TT
for CML
50
Ω
50
Ω
OUT
OUT
Signal Source EVM BOARD
NOTES: A. Locate 50-
Ω
resistors as close to the EVM as possible
B. Remove R1
A
B
Y
Z
Finally, as mentioned above, the SN65LVDS100, SN65LVDS101, and
SN65CML100 devices provide a V
BB
reference voltage output. This output
can be used with an externally terminated, single-ended, LVPECL input to
convert from a single-ended input to a differential output. The same cautions
that are mentioned above concerning signal quality and reflections apply.
When using V
BB
as a single-ended reference, R1 should be removed while R5
and JMP2 should be installed. The single-ended input signal is applied to J4.
This setup directly connects the V
BB
output to the DUT receiver B input via a
0-
Ω
connection (see Figure 2-4).