Rainbow-electronics MAX3625В Manual de usuario descargar pdf (Pagina 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 50Ω trans-

mission lines terminated with 50Ω to V

= V

- 2V. If a

separate termination voltage (V

) 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

82Ω

= 50Ω

82Ω

130Ω 130Ω

+3.3V

HIGH

IMPEDANCE

Figure 4. Thevenin Equivalent of Standard PECL Termination

MAX3625B

150Ω

100Ω

= 50Ω

HIGH

IMPEDANCE

150Ω

0.1μF

NOTE: AC-COUPLING IS OPTIONAL.

0.1μF

Figure 5. AC-Coupled PECL Termination

Table 4. Crystal Selection Parameters

PARAMETER SYMBOL MIN TYP MAX UNITS

Crystal Oscillation

Frequency

OSC

24.8 27 MHz

Shunt Capacitance C

2.0 7.0 pF

Load Capacitance C

18 pF

Equivalent Series

Resistance (ESR)

50 

Maximum Crystal

Drive Level

300 μW

Figure 2. Crystal Layout

X_IN

X_OUT

27pF

CRYSTAL

= 18pF)

33pF

Figure 3. Crystal, Capacitors Connection

1 2 3 4 5 6 7 8 9 10

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Rainbow-electronics MAX3625В Manual de usuario Pagina 8

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