
IDT / ICS LVDS FREQUENCY SYNTHESIZER
8
ICS844008BYI-15 REV. A AUGUST 14, 2007
ICS844008I-15
FEMTOCLOCKS CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER
CRYSTAL INPUT INTERFACE
The ICS844008I-15 has been characterized with an 18pF
parallel resonant crystals. The capacitor values shown in
FIGURE 2. CRYSTAL INPUt INTERFACE
Figure 2 below were determined using a 25MHz parallel
resonant crystal and were chosen to minimize the ppm error.
APPLICATION INFORMATION
As in any high speed analog circuitry, the power supply pins
are vulnerable to random noise. The ICS844008I-15 provides
separate power supplies to isolate any high switching noise
from the outputs to the internal PLL. V
DD and VDDA
should be
individually connected to the power supply plane through vias,
and bypass capacitors should be used for each pin. To achieve
optimum jitter performance, power supply isolation is required.
Figure 1 illustrates how a 10
Ω resistor along with a 10F and
a 0.01
μF bypass capacitor should be connected to each V
DDA.
POWER SUPPLY FILTERING TECHNIQUES
FIGURE 1. POWER SUPPLY FILTERING
10
Ω
V
DDA
10
μF
.01
μF
3.3V
.01
μF
V
DD
C1
27p
X1
18pF Parallel Crystal
C2
27p
XTAL_OUT
XTAL_IN
INPUTS:
LVCMOS CONTROL PINS:
All control pins have internal pull-ups or pull-downs; additional
resistance is not required but can be added for additional
protection. A 1k
Ω resistor can be used.
RECOMMENDATIONS FOR UNUSED INPUT AND OUTPUT PINS
OUTPUTS:
LVDS
All unused LVDS outputs should be terminated with 100
Ω resistor
between the differential pair.