ICS844001-21 Datasheet, PDF (9/17 Pages) Integrated Device Technology – FEMTOCLOCKS™ CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER

ICS844001-21
FEMTOCLOCKS™ CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER
APPLICATION INFORMATION
PRELIMINARY
POWER SUPPLY FILTERING TECHNIQUES
As in any high speed analog circuitry, the power supply pins
are vulnerable to random noise. The ICS844001-21 provides
separate power supplies to isolate any high switching
noise from the outputs to the internal PLL. V_DD, V_DDA, and V_{DDO_x}
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 10Ω resistor along with a 10µF and a 0.01μF bypass
capacitor should be connected to each V_DDA.
3.3V
V_DD
.01μF 10Ω
V_DDA
.01μF
10μF
FIGURE 1. POWER SUPPLY FILTERING
RECOMMENDATIONS FOR UNUSED INPUT AND OUTPUT PINS
INPUTS:
CRYSTAL INPUTS
For applications not requiring the use of the crystal oscillator input,
both XTAL_IN and XTAL_OUT can be left floating. Though not
required, but for additional protection, a 1kΩ resistor can be tied
from XTAL_IN to ground.
REF_CLK INPUT
For applications not requiring the use of the reference clock, it
can be left floating. Though not required, but for additional
protection, a 1kΩ resistor can be tied from the REF_CLK to
ground.
OUTPUTS:
LVDS OUTPUT
All unused LVDS output pairs can be either left floating or
terminated with 100Ω across. If they are left floating, there should
be no trace attached.
LVCMOS OUTPUT
All unused LVCMOS output can be left floating. There should be
no trace attached.
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 1kΩ resistor can be used.
CRYSTAL INPUT INTERFACE
The ICS844001-21 has been characterized with 18pF parallel
resonant crystals. The capacitor values shown in Figure 2 below
were determined using a 25MHz 18pF parallel resonant crystal
and were chosen to minimize the ppm error.
X1
18pF Parallel Crystal
XTAL_IN
C1
22p
XTAL_OUT
C2
22p
FIGURE 2. CRYSTAL INPUt INTERFACE
IDT^™ / ICS^™ INSERT PRODUCT NAME
9
ICS844001AG-21 REV. A SEPTEMBER 14, 2007