AD9831

–12–

REV. A

APPLICATIONS

The AD9831 contains functions which make it suitable for

modulation applications. The part can be used to perform

simple modulation such as FSK. More complex modulation

schemes such as GMSK and QPSK can also be implemented

using the AD9831. In an FSK application, the two frequency

registers of the AD9831 are loaded with different values; one

frequency will represent the space frequency while the other will

represent the mark frequency. The digital data stream is fed to

the FSELECT pin which will cause the AD9831 to modulate

the carrier frequency between the two values.

The AD9831 has four phase registers; this enables the part to

perform PSK. With phase shift keying, the carrier frequency is

phase shifted, the phase being altered by an amount which is

related to the bit stream being input to the modulator. The

presence of four shift registers eases the interaction needed

between the DSP and the AD9831.

The frequency and phase registers can be written to continu-

ously, if required. The maximum update rate equals the

frequency of the MCLK. However, if a selected register is

loaded with a new word, there will be a delay of 6 MCLK cycles

before the analog output will change accordingly.

The AD9831 is also suitable for signal generator applications.

With its low current consumption, the part is suitable for appli-

cations in which it can be used as a local oscillator. In addition,

the part is fully specified for operation with a +3.3 V

± 10%

power supply. Therefore, in portable applications where current

consumption is an important issue, the AD9831 is perfect.

Grounding and Layout

The printed circuit board that houses the AD9831 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 mini-

mum 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 AD9831 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 AD9831. If the AD9831 is in a system where mul-

tiple devices require 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

AD9831.

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 AD9831 to avoid noise coupling. The power

supply lines to the AD9831 should use as large a track as is

possible to provide low impedance paths and reduce the effects

of glitches on the power supply line. Fast switching signals such

as clocks should be shielded with digital ground to avoid radiat-

ing 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 tech-

nique 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 signals are placed

on the other side.

Good decoupling is important. The analog and digital supplies

to the AD9831 are independent and separately pinned out to

minimize coupling between analog and digital sections of the

device. All analog and digital supplies should be decoupled to

AGND and DGND respectively with 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 device. In

systems where a common supply is used to drive both the

AVDD and DVDD of the AD9831, it is recommended that the

system’s AVDD supply be used. This supply should have the

recommended analog supply decoupling between the AVDD

pins of the AD9831 and AGND and the recommended digital

supply decoupling capacitors between the DVDD pins and

DGND.