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ICS844256BG REV. B NOVEMBER 19, 2007

ICS844256

FEMTOCLOCKS™ CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER W/INTEGRATED FANOUT BUFFER

PRELIMINARY

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 50

Ω applications, R1 and R2 can be 100Ω.

This can also be accomplished by removing R1 and making R2

50

Ω.

XTAL_IN

XTAL_OUT

VCC

R2

Ro

R1

Zo = 50

Rs

VCC

.1uf

Zo = Ro + Rs

Reference Document: JEDEC Publication 95, MO-153

A general LVDS interface is shown in Figure 4. In a 100

Ω

differential transmission line environment, LVDS dr ivers

require a matched load termination of 100

Ω across near

the receiver input.

2.5V or 3.3V

+

-

VDD

100 Ohm Differential Transmission Line

R1

100

LVDS_Driv er