MAX3625B Ver la hoja de datos (PDF) - Maxim Integrated
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MAX3625B Datasheet PDF : 10 Pages
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Low-Jitter, Precision Clock
Generator with Three Outputs
Crystal Selection
The crystal oscillator is designed to drive a fundamental
mode, AT-cut crystal resonator. See Table 4 for recom-
mended crystal specifications. See Figure 3 for external
capacitance connection.
Table 4. Crystal Selection Parameters
PARAMETER SYMBOL MIN
Crystal Oscillation
Frequency
fOSC
24.8
Shunt Capacitance CO
Load Capacitance
CL
Equivalent Series
Resistance (ESR)
RS
Maximum Crystal
Drive Level
TYP
2.0
18
MAX UNITS
27 MHz
7.0 pF
pF
50
300 μW
Crystal Input Layout
The crystal, trace, and two external capacitors should
be placed on the board as close as possible to the
MAX3625B’s X_IN and X_OUT pins to reduce crosstalk
of active signals into the oscillator. The example layout
shown in Figure 2 gives approximately 3pF of trace
plus footprint capacitance per side of the crystal. The
dielectric material is FR4 and dielectric thickness of the
reference board is 15 mils. Using a 25MHz crystal and
the capacitor values of C10 = 27pF and C9 = 33pF, the
measured output frequency accuracy is -14ppm at
+25°C ambient temperature.
Interfacing with LVPECL Outputs
The equivalent LVPECL output circuit is given in Figure 7.
These outputs are designed to drive a pair of 50Ω trans-
mission lines terminated with 50Ω to VTT = VCC - 2V. If a
separate termination voltage (VTT) is not available, other
termination methods can be used such as shown in
Figures 4 and 5. Unused outputs should be disabled and
may be left open. For more information on LVPECL termi-
nations and how to interface with other logic families, refer
to Application Note 291: HFAN-01.0: Introduction to
LVDS, PECL, and CML.
+3.3V
Figure 2. Crystal Layout
27pF
CRYSTAL
(CL = 18pF)
33pF
X_IN
X_OUT
MAX3625B Qx
Qx
130Ω
Z0 = 50Ω
Z0 = 50Ω
82Ω
130Ω
HIGH
IMPEDANCE
82Ω
Figure 4. Thevenin Equivalent of Standard PECL Termination
Qx
MAX3625B
Qx
0.1μF
Z0 = 50Ω
0.1μF
100Ω
Z0 = 50Ω
150Ω 150Ω
HIGH
IMPEDANCE
Figure 3. Crystal, Capacitors Connection
NOTE: AC-COUPLING IS OPTIONAL.
Figure 5. AC-Coupled PECL Termination
8 ____________________________________________________
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