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AD9833

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REV PrG

PRELIMINARY TECHNICAL DATA

APPLICATIONS

Because of the various output options available from the

part, the AD9833 can be configured to suit a wide variety

of applications.

One of the areas where the AD9833 is suitable is in modu-

lation applications.

The part can be used to perform

simple modulation such as FSK.

More complex modula-

tion schemes such as GMSK and QPSK can also be

implemented using the AD9833.

In an FSK application, the two frequency registers of the

AD9833 are loaded with different values; one frequency

will represent the space frequency while the other will

represent the mark frequency. Using the FSELECT bit in

the control register of the AD9833, the user can modulate

the carrier frequency between the two values.

The AD9833 has two phase registers; this enables the part

to perform PSK. With phase shift keying, the carrier fre-

quency is phase shifted, the phase being altered by an

amount which is related to the bit stream being input to

the modulator.

The AD9833 is also suitable for signal generator applica-

tions. Because the MSB of the DAC data is available at

the VOUT pin, the device can be used to generate a

square wave.

With its low current consumption, the part is suitable for

applications in which it can be used as a local oscillator.

GROUNDING AND LAYOUT

The printed circuit board that houses the AD9833 should

be designed so that the analog and digital sections are

separated and confined to certain areas of the board.

This

facilitates the use of ground planes which can be separated

easily. A minimum etch technique is generally best for

ground planes as it gives the best shielding. Digital and

analog ground planes should only be joined in one place.

If the AD9833 is the only device requiring an AGND to

DGND connection, then the ground planes should be

connected at the AGND and DGND pins of the AD9833.

If the AD9833 is in a system where multiple devices re-

quire AGND to DGND connections, the connection

should be made at one point only, a star ground point that

should be established as close as possible to the AD9833.

Avoid running digital lines under the device as these will

couple noise onto the die. The analog ground plane should

be allowed to run under the AD9833 to avoid noise cou-

pling. The power supply lines to the AD9833 should use

as large a track as is possible to provide low impedance

paths and reduce the effects of glitches on the power sup-

ply line. Fast switching signals such as clocks should be

shielded with digital ground to avoid radiating noise to

other sections of the board. Avoid crossover of digital and

analog signals. Traces on opposite sides of the board

should run at right angles to each other. This will reduce

the effects of feedthrough through the board.

A microstrip

technique is by far the best but is not always possible with

a double-sided board. In this technique, the component

side of the board is dedicated to ground planes while sig-

nals are placed on the other side.

Good decoupling is important. The AD9833 should have

supply bypassing of 0.1 µF ceramic capacitors in parallel

with 10 µF tantalum capacitors. To achieve the best from

the decoupling capacitors, they should be placed as close

as possible to the device, ideally right up against the de-

vice.

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