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AD9831ASTZ

DIRECT DIGITAL SYNTGESIZER, WAVEFORM GENERATOR

Analog Devices 

AD9831

CIRCUIT DESCRIPTION

The AD9831 provides an exciting new level of integration for

the RF/Communications system designer. The AD9831 com-

bines the Numerical Controlled Oscillator (NCO), SINE Look-

Up Table, Frequency and Phase Modulators, and a Digital-to-

Analog Converter on a single integrated circuit.

The internal circuitry of the AD9831 consists of three main

sections. These are:

Numerical Controlled Oscillator (NCO) + Phase Modulator

SINE Look-Up Table

Digital-to-Analog Converter

The AD9831 is a fully integrated Direct Digital Synthesis

(DDS) chip. The chip requires one reference clock, one low

precision resistor and eight decoupling capacitors to provide

digitally created sine waves up to 12.5 MHz. In addition to the

generation of this RF signal, the chip is fully capable of a broad

range of simple and complex modulation schemes. These

modulation schemes are fully implemented in the digital domain

allowing accurate and simple realization of complex modulation

algorithms using DSP techniques.

THEORY OF OPERATION

Sine waves are typically thought of in terms of their magnitude

form a(t) = sin (ωt). However, these are nonlinear and not easy

to generate except through piece wise construction. On the

other hand, the angular information is linear in nature. That is,

the phase angle rotates through a fixed angle for each unit of

time. The angular rate depends on the frequency of the signal

by the traditional rate of ω = 2πf.

MAGNITUDE

+1

0

–1

PHASE

2π

0

Figure 21. Sine Wave

Knowing that the phase of a sine wave is linear and given a

reference interval (clock period), the phase rotation for that

period can be determined.

∆Phase = ωδt

Solving for ω

ω = ∆Phase/δt = 2πf

Solving for f and substituting the reference clock frequency for

the reference period (1/fMCLK = δt)

f = ∆Phase × fMCLK/2π

The AD9831 builds the output based on this simple equation.

A simple DDS chip can implement this equation with three

major subcircuits.

Numerical Controlled Oscillator + Phase Modulator

This consists of two frequency select registers, a phase accumu-

lator and four phase offset registers. The main component of the

NCO is a 32-bit phase accumulator which assembles the phase

component of the output signal. Continuous time signals have a

phase range of 0 to 2π. Outside this range of numbers, the

sinusoid functions repeat themselves in a periodic manner. The

digital implementation is no different. The accumulator simply

scales the range of phase numbers into a multibit digital word.

The phase accumulator in the AD9831 is implemented with 32

bits. Therefore, in the AD9831, 2π = 232. Likewise, the ∆Phase

term is scaled into this range of numbers 0 < ∆Phase < 232 – 1.

Making these substitutions into the equation above

f = ∆Phase × fMCLK/232

where 0 < ∆Phase < 232

With a clock signal of 25 MHz and a phase word of 051EB852

hex

f = 51EB852 × 25 MHz/232 = 0.500000000465 MHz

The input to the phase accumulator (i.e., the phase step) can be

selected either from the FREQ0 Register or FREQ1 Register

and this is controlled by the FSELECT pin. NCOs inherently

generate continuous phase signals, thus avoiding any output

discontinuity when switching between frequencies.

Following the NCO, a phase offset can be added to perform

phase modulation using the 12-bit PHASE Registers. The con-

tents of this register are added to the most significant bits of the

NCO. The AD9831 has four PHASE registers, the resolution

of these registers being 2π/4096.

Sine Look-Up Table (LUT)

To make the output useful, the signal must be converted from

phase information into a sinusoidal value. Since phase informa-

tion maps directly into amplitude, a ROM LUT converts the

phase information into amplitude. To do this, the digital phase

information is used to address a sine ROM LUT. Although the

NCO contains a 32-bit phase accumulator, the output of the

NCO is truncated to 12 bits. Using the full resolution of the

phase accumulator is impractical and unnecessary as this would

require a look-up table of 232 entries.

It is necessary only to have sufficient phase resolution in the

LUTs such that the dc error of the output waveform is domi-

nated by the quantization error in the DAC. This requires the

look-up table to have two more bits of phase resolution than the

10-bit DAC.

Digital-to-Analog Converter

The AD9831 includes a high impedance current source 10-bit

DAC, capable of driving a wide range of loads at different

speeds. Full-scale output current can be adjusted, for optimum

power and external load requirements, through the use of a

single external resistor (RSET).

The DAC is configured for single ended operation. The load

resistor can be any value required, as long as the full-scale volt-

age developed across it does not exceed the voltage compliance

range. Since full-scale current is controlled by RSET, adjust-

ments to RSET can balance changes made to the load resistor.

However, if the DAC full-scale output current is significantly

less than 4 mA, the DAC’s linearity may degrade.

–10–

REV. B

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