ICS8440258-46 Datasheet, PDF (9/16 Pages) Integrated Device Technology – FEMTOCLOCK™ CRYSTAL/LVCMOS-TOLVDS/LVCMOS FREQUENCY SYNTHESIZER

ICS8440258-46
FEMTOCLOCK™ CRYSTAL/LVCMOS-TO-LVDS/ LVCMOS FREQUENCY SYNTHESIZER
PRELIMINARY
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS
signal through an AC couple 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 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 50Ω applications, R1
and R2 can be 100Ω. This can also be accomplished by
removing R1 and making R2 50Ω.
VDD
VDD
R1
Ro
Rs
.1uf
Zo = 50
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
Figure 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
RECOMMENDATIONS FOR UNUSED INPUT AND OUTPUT PINS
INPUTS:
OUTPUTS:
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 1kW resistor can be tied
from XTAL_IN to ground.
REF_CLK INPUT
For applications not requiring the use of the reference clock, it
can be left floating. Though not required, but for additional
protection, a 1kΩ resistor can be tied from the REF_CLK to
ground.
LVCMOS OUTPUTS
All unused LVCMOS output can be left floating. There should be
no trace attached.
LVDS OUTPUTS
All unused LVDS output pairs can be either left floating or
terminated with 100Ω across. If they are left floating, there should
be no trace attached.
LVCMOS CONTROL PINS
All control pins have internal pull-downs; additional resistance is
not required but can be added for additional protection. A 1kΩ
resistor can be used.
IDT^™ / ICS^™ LVDS/LVCMOS FREQUENCY SYNTHESIZER
9
ICS8440258AK-46 REV B JANUARY 15, 2008