ICS843002I Datasheet, PDF (9/16 Pages) Integrated Circuit Systems – FEMTOCLOCKS-TM CRYSTAL-TO-3.3V, 2.5V LVPECL FREQUENCY SYNTHESIZER

Integrated
Circuit
Systems, Inc.
ICS843002I
FEMTOCLOCKS™ CRYSTAL-TO-3.3V, 2.5V
LVPECL FREQUENCY SYNTHESIZER
CRYSTAL INPUT INTERFACE
The ICS843002I has been characterized with 18pF parallel
resonant crystals. The capacitor values shown in Figure 2
below were determined using a 26.5625MHz, 18pF parallel
resonant crystal and were chosen to minimize the ppm error.
X1
18pF Parallel Crystal
XTAL_OUT
C1
33p
XTAL_IN
C2
27p
ICS843002I
Figure 2. CRYSTAL INPUt INTERFACE
TERMINATION FOR 3.3V LVPECL OUTPUT
The clock layout topology shown below is a typical ter-
mination 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. There-
fore, terminating resistors (DC current path to ground)
or current sources must be used for functionality. These
outputs are designed to drive 50Ω transmission lines.
Matched impedance techniques should be used to maxi-
mize operating frequency and minimize signal distor-
tion. 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 compat-
ibility across all printed circuit and clock component pro-
cess variations.
Z_o = 50Ω
FOUT
FIN
Z_o = 50Ω
50Ω
RTT =
1
((V_OH + V_OL) / (V_CC – 2)) – 2
Z_o
50Ω
V_CC - 2V
RTT
FOUT
3.3V
125Ω
125Ω
Z_o = 50Ω
FIN
Z_o = 50Ω
84Ω
84Ω
FIGURE 3A. LVPECL OUTPUT TERMINATION
FIGURE 3B. LVPECL OUTPUT TERMINATION
843002AGI
www.icst.com/products/hiperclocks.html
9
REV. A JANUARY 4, 2006