Rainbow Electronics MAX3625В - Manual (Page 8)

MAX3625B
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.
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 50
Ω trans-
mission lines terminated with 50 Ω to V
TT
= V
CC
- 2V. If a
separate termination voltage (V TT ) 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 .
Low-Jitter, Precision Clock
Generator with Three Outputs
8 ____________________________________________________
MAX3625B
Qx
82 Ω
Z
0
= 50 Ω
Qx Z 0 = 50 Ω
82 Ω
130 Ω 130 Ω
+3.3V
HIGH
IMPEDANCE
Figure 4. Thevenin Equivalent of Standard PECL Termination
MAX3625B
Qx
150 Ω
100 Ω
Qx Z 0 = 50 Ω
Z
0
= 50 Ω
HIGH
IMPEDANCE
150 Ω
0.1 μF
NOTE: AC-COUPLING IS OPTIONAL.
0.1 μF
Figure 5. AC-Coupled PECL Termination
Table 4. Crystal Selection Parameters
PARAMETER SYMBOL MIN TYP MAX UNITS
Crystal Oscillation
Frequency
f OSC 24.8 27 MHz
Shunt Capacitance C O 2.0 7.0 pF
Load Capacitance C L 18 pF
Equivalent Series
Resistance (ESR)
R S 50

Maximum Crystal
Drive Level
300 μW
Figure 2. Crystal Layout
X_IN
X_OUT
27pF
CRYSTAL
(C
L
= 18pF)
33pF
Figure 3. Crystal, Capacitors Connection