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XO

or

2LVCMOS

or

Differential

Input

CLK_IN0

CLK_IN1

Input source select

(from EEPROM)

C

X0

C

X1

crystal

unit

C

ICB

C

ICB

0

10

20

30

40

50

60

70

80

90

100

5

10

15

20

25

30

35

40

45

50

55

C = 18 pF

Frequency − MHz

~

21 mW

ERS = 60 W

ERS = 50 W

ERS = 40 W

ERS = 30 W

ERS = 25 W

ERS = 15 W

CDCE706

SCAS815A – OCTOBER 2005 – REVISED OCTOBER 2005

Crystal Oscillator Inputs

The input frequency range in crystal mode is 8 MHz to 54 MHz. The CDCE706 uses a Pierce-type oscillator

circuitry with included feedback resistance for the inverting amplifier. The user, however, has to add external

calculated:

device including the board capacitance (stray capacitance of PCB).

It is important to use a short PCB trace from the device to the crystal unit to keep the stray capacitance of the

oscillator loop to a minimum.

Figure 9. Crystal Input Circuitry

In order to ensure a stable oscillating, a certain drive power must be applied. The CDCE706 features an input

oscillator with adaptive gain control which relieves the user to manually program the gain. The drive level is the

amount of power dissipated by the oscillating crystal unit and is usually specified in terms of power dissipated by

the resonator (equivalent series resistance (ESR)). Figure 10 gives the resulting drive level vs crystal frequency

and ESR.

Figure 10. Crystal Drive Power

19