8731CY-01
www.idt.com
REV. B JULY 27, 2010
9
ICS8731-01
LOW SKEW, 1-TO-11 DIFFERENTIAL-TO-3.3V LVPECL
CLOCK MULTIPLIER / ZERO DELAY BUFFER
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 termi-
nation for LVPECL outputs. The two different layouts men-
tioned 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
designed to drive 50
Ω transmission lines. Matched imped-
ance techniques should be used to maximize operating
frequency and minimize signal distortion.
Figures 4A and
4B 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.
TERMINATION FOR LVPECL OUTPUTS
FIGURE 4B. LVPECL OUTPUT TERMINATION
FIGURE 4A. LVPECL OUTPUT TERMINATION
INPUTS:
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 OUTPUT
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.