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IDT / ICS 3.3V LVPECL FREQUENCY SYNTHESIZER
8
ICS843204AGI-01 REV. A OCTOBER 18, 2007
ICS843204I-01
FEMTOCLOCKS CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER
TERMINATION FOR 3.3V LVPECL OUTPUT
V
CC - 2V
50
Ω
50
Ω
RTT
Z
o = 50Ω
Z
o = 50Ω
FOUT
FIN
RTT =
Z
o
1
((V
OH + VOL) / (VCC – 2)) – 2
3.3V
125
Ω
125
Ω
84
Ω
84
Ω
Z
o = 50Ω
Z
o = 50Ω
FOUT
FIN
The clock layout topology shown below is a typical ter mina-
tion for LVPECL outputs. The two different layouts mentioned
are recommended only as guidelines.
FOUT and nFOUT are low impedance follower outputs that
generate ECL/LVPECL compatible outputs. Therefore, ter-
minating resistors (DC current path to ground) or current
sources must be used for functionality. These outputs are
FIGURE 5B. LVPECL OUTPUT TERMINATION
FIGURE 5A. LVPECL OUTPUT TERMINATION
designed to drive 50
Ω transmission lines. Matched imped-
ance techniques should be used to maximize operating fre-
quency and minimize signal distor tion. Figures 5A and 5B
show two different layouts which are recommended only as
guidelines. Other suitable clock layouts may exist and it
would be recommended that the board designers simulate
to guarantee compatibility across all printed circuit and clock
component process variations.
INPUTS:
CRYSTAL INPUTS
For applications not requiring the use of the crystal oscillator input,
both XTAL_IN and XTAL_OUT can be left floating. Though not
required, but for additional protection, a 1k
Ω resistor can be tied
from XTAL_IN to ground.
CLK INPUT
For applications not requiring the use of a clock input, it can be
left floating. Though not required, but for additional protection, a
1k
Ω resistor can be tied from the CLK input to ground.
CLK/nCLK INPUTS
For applications not requiring the use of the differential input,
both CLK and nCLK can be left floating. Though not required, but
for additional protection, a 1k
Ω resistor can be tied from CLK to
ground.
LVCMOS CONTROL PINS
All control pins have internal pull-ups or pull-downs; additional
resistance is not required but can be added for additional
protection. A 1k
Ω resistor can be used.
RECOMMENDATIONS FOR UNUSED INPUT AND OUTPUT PINS
OUTPUTS:
LVPECL OUTPUTS
All unused LVPECL outputs can be left floating. We recommend
that there is no trace attached. Both sides of the differential output
pair should either be left floating or terminated.