ICS840004I-01 Datasheet, PDF (11/16 Pages) Integrated Circuit Systems – FEMTOCLOCKS™ CRYSTAL-TO LVCMOS/LVTTL FREQUENCY SYNTHESIZER

ICS840004I-01
FEMTOCLOCKS™ CRYSTAL-TO-
LVCMOS/LVTTL FREQUENCY SYNTHESIZER
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:
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 1kΩ
resistor can be tied from XTAL_IN to ground.
OUTPUTS:
LVCMOS OUTPUTS
All unused LVCMOS output can be left floating. We
recommend that there is no trace attached.
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 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 1kΩ resistor can be used.
840004AGI-01
11
REV. A OCTOBER 22, 2007