AD1847 datasheet(25/28 Pages) AD

25 / 28 page

AD1847

REV. B

–25–

The crystals shown in the crystal connection circuitry of Figure

21 should be fundamental-mode and parallel-tuned. Two

sources for the exact crystals specified are Component Market-

ing Services in Massachusetts, U.S. at 617/762-4339 and

Cardinal Components in New Jersey, U.S. at 201/746-0333.

Note that using the exact data sheet frequencies is not required

and that external clock sources can be used to overdrive the

AD1847s internal oscillators. (See the description of the CFS2:0

control bits above.) If using an external clock source, apply it to

the crystal input pins while leaving the crystal output pins un-

connected. Attention should be paid to providing low-jitter ex-

ternal input clocks .

XTAL1I

XTAL1O

64pF

24.576MHz

XTAL2I

XTAL2O

64pF

16.9344MHz

Figure 21. Crystal Connections

Analog Devices also recommends a pull-down resistor on the

PWRDOWN

signal.

Good, standard engineering practices should be applied for

power-supply decoupling. Decoupling capacitors should be

placed as close as possible to package pins. If a separate analog

power supply is not available, the circuit shown in Figure 22 is

recommended when using a single +5 V supply. Ferrite beads

suffice for the inductors shown. This circuitry should be as close

to the supply pins as is practical.

+5V SUPPLY

0.1

µF

FERRITE

µF

0.1

µF

0.1

µF

0.1

µF

FERRITE

µF

1.6

0.1

µF

0.1

µF

0.1

µF

Figure 22. Recommended Power Supply Bypassing

Analog Devices recommends a split ground plane as shown in

Figure 23. The analog plane and the digital plane are connected

directly under the AD1847. Splitting the ground plane directly

under the SoundPort Codec is optimal because analog pins will

be located directly above the analog ground plane and digital

pins will be located directly above the digital ground plane for

the best isolation. The digital and analog grounds should be tied

together in the vicinity of the AD1847. Other schemes may also

yield satisfactory results. If the split ground plane recommended

here is not possible, the AD1847 should be entirely over the

analog ground plane with the ASIC and DSP over the digital

plane.

DIGITAL

GROUND

PLANE

ANALOG

GROUND

PLANE

PWRDOWN

L_AUX2

AD1847

Figure 23. Recommended Ground Plane

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