
ICS843021AG REVISION D OCTOBER 12, 2010
6
2010 Integrated Device Technology, Inc.
ICS843021 Data Sheet
FEMTOCLOCK CRYSTAL-TO-3.3V LVPECL CLOCK GENERATOR
Applications 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 performance,
power supply isolation is required. The ICS843021 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 ICS843021 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
33pF
C2
27pF