IDT / ICS LVCMOS/LVTTL FREQUENCY SYNTHESIZER
9
ICS840245AGI REV. B NOVEMBER 16, 2006
ICS840245I
FEMTOCLOCKS 1:5, CRYSTAL-TO-LVCMOS/LVTTL FREQUENCY SYNTHESIZER
CRYSTAL INPUT INTERFACE
The ICS840245I has been characterized with 18pF parallel
resonant crystals. The capacitor values shown in
Figure 2 below
FIGURE 2. CRYSTAL INPUt INTERFACE
were determined using a 25MHz, 18pF parallel resonant crystal
and were chosen to minimize the ppm error.
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS signal
through an AC coupling capacitor. A general interface diagram is
shown in
Figure 3. The XTAL_OUT pin can be left floating. The
input edge rate can be as slow as 10ns. For LVCMOS inputs, it is
recommended that the amplitude be reduced from full swing to
half swing in order to prevent signal interference with the power
rail and to reduce noise. This configuration requires that the output
FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
impedance of the driver (Ro) plus the series resistance (Rs) equals
the transmission line impedance. In addition, matched termination
at the crystal input will attenuate the signal in half. This can be
done in one of two ways. First, R1 and R2 in parallel should equal
the transmission line impedance. For most 50
applications, R1
and R2 can be 100
. This can also be accomplished by removing
R1 and making R2 50
.
R2
Zo = 50
VDD
Ro
Zo = Ro + Rs
R1
VDD
XTAL_IN
XTAL_OUT
.1uf
Rs
C1
27p
XTAL_OUT
XTAL_IN
C2
33p
X1
18pF Parallel Crystal