AD9830

REV. A

–10–

CIRCUIT DESCRIPTION

The AD9830 provides an exciting new level of integration

for the RF/Communications system designer. The AD9830

combines 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 AD9830 consists of three main

sections. These are:

Numerical Controlled Oscillator (NCO) + Phase Modulator

SINE Look-Up Table

Digital-to-Analog Converter

The AD9830 is a fully integrated Direct Digital Synthesis

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

precision resistors and eight decoupling capacitors to provide

digitally created sine waves up to 25 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 do-

main 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

PHASE

+1

0

–1

2

π

0

Figure 22. Sine Wave

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

erence 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

δt)

f =

∆Phase × f

π

The AD9830 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 2

π. 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 AD9830 is implemented

with 32 bits. Therefore, in the AD9830, 2

the

∆Phase term is scaled into this range of numbers 0 < ∆Phase

< 2

f =

∆Phase × f

32

where 0 <

With a clock signal of 50 MHz and a phase word of 051EB852

hex

f = 51EB852

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. More com-

plex frequency modulation schemes can be implemented by up-

dating the contents of these registers. This facilitates complex

frequency modulation schemes, such as GMSK.

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 AD9830 has four PHASE registers. The resolution

of the phase registers equals 2

π/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 2

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 AD9830 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

The DAC can be configured for single or differential ended op-

eration. IOUT can be tied directly to AGND for single ended

operation or through a load resistor to develop an output volt-

age. The load resistor can be any value required, as long as the