ICS844071

FEMTOCLOCK™CRYSTAL-TO-LVDS CLOCK GENERATOR

IDT™ / ICS™ LVDS CLOCK GENERATOR

9

ICS844071AG SEPTEMBER 26, 2007

Crystal Input Interface

The ICS844071 has been characterized with 18pF parallel

resonant crystals. The capacitor values, C1 and C2, shown in

Figure 2 below were determined using a 25MHz, 18pF parallel

resonant crystal and were chosen to minimize the ppm error. The

optimum C1 and C2 values can be slightly adjusted for different

board layouts.

Figure 2. Crystal Input Interface

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

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.

Figure 3. General Disgram for LVCMOS Driver to XTAL Input Interface

XTAL_IN

XTAL_OUT

X1

18pF Parallel Crystal

C1

33p

C2

22p

XTAL_IN

XTAL_OUT

Ro

Rs

Zo = Ro + Rs

50

Ω

0.1µf

R1

R2