IDT / ICS 3.3V LVPECL FREQUENCY SYNTHESIZER
8
ICS843256BG REV. A MAY 23, 2007
ICS843256
FEMTOCLOCKS CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER
APPLICATION INFORMATION
POWER SUPPLY FILTERING TECHNIQUES
As in any high speed analog circuitry, the power supply pins
are vulnerable to random noise. The ICS843256 provides sepa-
rate power supplies to isolate any high switching noise from
the outputs to the internal PLL. V
CC, VCCA, and VCCO 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 10μF and a
.01
μF bypass capacitor should be connected to each V
CCA pin.
FIGURE 1. POWER SUPPLY FILTERING
10
Ω
V
CCA
10
μF
.01
μF
3.3V
.01
μF
V
CC
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:
LVPECL OUTPUTS
All unused LVPECL outputs can be left floating. We recommend
that there is no trace attached. Both sides of the differential output
pair should either be left floating or terminated.
FIGURE 2. CRYSTAL INPUt INTERFACE
C1
18p
X1
18pF Parallel Crystal
C2
22p
XTAL_IN
XTAL_OUT
CRYSTAL INPUT INTERFACE
The ICS843256 has been characterized with 18pF parallel
resonant crystals. The capacitor values shown in Figure 2 below
were determined using an 18pF parallel resonant crystal and
were chosen to minimize the ppm error.