
ICS843251-04
FEMTOCLOCKCRYSTAL-TO-3.3V LVPECL CLOCK GENERATOR
IDT / ICS 3.3V LVPECL CLOCK GENERATOR
7
ICS843S51AG-04 REV. A JANUARY 12, 2009
Application Information
Power Supply Filtering Technique
As in any high speed analog circuitry, the power supply pins are
vulnerable to random noise. To achieve optimum jitter perform-
ance, power supply isolation is required. The ICS843251-04
provides separate power supplies to isolate any high switching
noise from the outputs to the internal PLL. VCC and VCCA should
be individually connected to the power supply plane through vias,
and 0.01F bypass capacitors should be used for each pin. Figure
1 illustrates this for a generic VCC pin and also shows that VCCA
requires that an additional 10
resistor along with a 10F bypass
capacitor be connected to the VCCA pin.
Figure 1. Power Supply Filtering
Crystal Input Interface
The ICS843251-04 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
VCC
VCCA
3.3V
10
10F
.01F
XTAL_IN
XTAL_OUT
X1
18pF Parallel Crystal
C1
33p
C2
27p