ICS844256DGI REVISION A AUGUST5, 2010

10

©2010 Integrated Device Technology, Inc.

ICS844256DI Data Sheet

FEMTOCLOCK® CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER W/INTEGRATED FANOUT BUFFER

Crystal Input Interface

The ICS844256DI has been characterized with 18pF parallel

resonant crystals. The capacitor values shown in Figure 2 were

determined using an 18pF parallel resonant crystal and were chosen

to minimize the ppm error.

Figure 2. Crystal Input Interface

Overdriving the 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 3A. The XTAL_OUT pin can be left floating. The

maximum amplitude of the input signal should not exceed 2V and the

input edge rate can be as slow as 10ns. 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 50

Ω

applications, R1 and R2 can be 100

Ω. This can also be

accomplished by removing R1 and making R2 50

Ω. By overdriving

the crystal oscillator, the device will be functional, but note, the device

performance is guaranteed by using a quartz crystal.

Figure 3A. General Diagram for LVCMOS Driver to XTAL Input Interface

Figure 3B. General Diagram for LVPECL Driver to XTAL Input Interface

XTAL_IN

XTAL_OUT

X1

18pF Parallel Crystal

C1

27pF

C2

27pF

R2

100

R1

100

RS

43

Ro ~ 7 Ohm

Driv er_LVCMOS

Zo = 50 Ohm

C1

0.1uF

3.3V

3.3V

Cry stal Input Interf ace

XTAL_IN

XTAL_OUT

Cry stal Input Interf ace

XTAL_IN

XTAL_OUT

R3

50

C1

0.1uF

R2

50

R1

50

Zo = 50 Ohm

LVPECL

Zo = 50 Ohm

VCC=3.3V