Datasheet

LTC6957-1/LTC6957-2/
LTC6957-3/LTC6957-4
20
6957f
For more information www.linear.com/LTC6957-1
applicaTions inForMaTion
Figure 2
Figure 3. Single-Ended 50Ω Input Source
Figure 4
Figure 4 shows the interface between current mode logic
(CML) signals and the LTC6957 inputs. The specifics of
terminating will be dependent on the particular CML driver
used; Figure4 shows terminations only at the load end
of the line, but the same LTC6957 interface is appropri-
ate for applications with the source end of the line also
terminated. In Figure 4a, a differential signal interface to
the LTC6957 is shown, which must be AC-coupled due to
the DC input levels required at the LTC6957.
Figure 4b shows a single-ended CML signal driving the
LTC6957. This is not commonly used because of noise and
immunity weaknesses compared to the differential CML
case. Because the signal is created by a current pulled
through the termination resistor, the signal is inherently
referenced to the supply voltage to which R
TERM
is tied. For
that reason, the other LTC6957 should be AC-referenced
to that supply voltage as shown.
The polarity change shown here is for graphic clarity
only, and can be reversed by swapping the LTC6957
input terminals.
R
TERM
s are shown in a "Y" configuration that creates a pas-
sive V
TT
at the common point. Most 3.3V LVPECL devices
have differential outputs and can be terminated with three
50Ω resistors as shown.
Figure 3 shows a 50Ω RF signal source interface to the
LTC6957. For a pure tone (sine wave) input, Figure 3 can
handle up to 10dBm maximum. A broadband 50Ω match
as shown should suffice for most applications, though
for small amplitude input signals a narrow band reactive
matching network may offer incremental improvements
in performance.
6957 F02b
6957 F02a
10nF
R
TERM
–
+
LTC6957
Figure 2a. Single-Ended LVPECL Input
3.3V
–
+
LTC6957
Figure 2b. Differential LVPECL Input
3.3V
3×
R
TERM
6957 F04a
R
TERM
R
TERM
–
+
LTC6957
10nF
10nF
6957 F04b
R
TERM
–
+
LTC6957
Figure 4b. Single-Ended CML Input
Figure 4a. Differential CML Input
10nF
10nF
6957 F03
10nF
50Ω
–
+
LTC6957
50Ω
SOURCE
10nF