
87354AMI
www.idt.com
REV. A AUGUST 5, 2010
8
ICS87354I
÷4/÷5 DIFFERENTIAL-TO-
3.3V LVPECL CLOCK GENERATOR
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 3A and
3B 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 3.3V LVPECL OUTPUT
FIGURE 3B. LVPECL OUTPUT TERMINATION
FIGURE 3A. LVPECL OUTPUT TERMINATION