83918 Datasheet, PDF (12/19 Pages) Integrated Device Technology – Maximum output frequency

83918 DATA SHEET
Applications Information
Crystal Input Interface
The 83918 has been characterized with 18pF parallel resonant
crystals. The capacitor values, C1 and C2, shown in Figure 1 below
were determined using an 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.
X1
18pF Parallel Crystal
C1
27pF
XTAL_IN
C2
27pF
XTAL_OUT
Figure 1. Crystal Input Interface
Overdriving the XTAL Interface
The XTAL_IN input can accept a single-ended LVCMOS signal
through an AC coupling capacitor. A general interface diagram is
shown in Figure 2A. The XTAL_OUT pin can be left floating. The
maximum amplitude of the input signal should not exceed 2V and the
input edge rate can be as slow as 10ns. This configuration requires
that the output impedance of the driver (Ro) plus the series
resistance (Rs) equals the transmission line impedance. In addition,
matched termination at the crystal input will attenuate the signal in
half. This can be done in one of two ways. First, R1 and R2 in parallel
should equal the transmission line impedance. For most 50
applications, R1 and R2 can be 100. This can also be
accomplished by removing R1 and making R2 50. By overdriving
the crystal oscillator, the device will be functional, but note, the device
performance is guaranteed by using a quartz crystal.
3.3V
Ro ~ 7 Ohm
RS 43
Driv er_LVCMOS
3.3V
R1
100
Zo = 50 Ohm
R2
100
C1
0.1uF
XTAL_IN
XTAL_OUT
Cry stal Input Interf ace
Figure 2A. General Diagram for LVCMOS Driver to XTAL Input Interface
VCC=3.3V
LVPECL
Zo = 50 Ohm
Zo = 50 Ohm
C1
XTAL_IN
R1
0.1uF
50
XTAL_OUT
Cry stal Input Interf ace
R2
50
R3
50
Figure 2B. General Diagram for LVPECL Driver to XTAL Input Interface
Rev B 3/25/15
12
LOW SKEW, 1:18 CRYSTAL-TO-LVCMOS/LVTTL FANOUT BUFFER