AD9835

–10–

REV. 0

CIRCUIT DESCRIPTION

The AD9835 provides an exciting new level of integration for

the RF/Communications system designer. The AD9835 com-

bines the Numerical Controlled Oscillator (NCO), COS Look-Up

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

Analog Converter on a single integrated circuit.

The internal circuitry of the AD9835 consists of three main

sections. These are:

Numerical Controlled Oscillator (NCO) + Phase Modulator

COS Look-Up Table

Digital-to-Analog Converter

The AD9835 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 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 domain allowing

accurate and simple realization of complex modulation algo-

rithms using DSP techniques.

THEORY OF OPERATION

Cos waves are typically thought of in terms of their magnitude

form a(t) = cos (

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 = 2 πf.

MAGNITUDE

PHASE

+1

0

–1

2

π

0

Figure 19. Cos Wave

Knowing that the phase of a cos wave is linear and given a refer-

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

can be determined.

∆Phase = ωδt

Solving for

ω

ω = ∆Phase/δt = 2 πf

Solving for f and substituting the reference clock frequency for

δt)

f =

∆Phase × f

π

The AD9835 builds the output based on this simple equation.

A simple DDS chip can implement this equation with three

major subcircuits.

Numerical Controlled Oscillator and 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

π 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 AD9835 is implemented with

32 bits. Therefore, in the AD9835, 2

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

< 2

f =

∆Phase × f

32

where 0 <

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 or the FSELECT

bit. 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 AD9835 has four PHASE registers, the resolution

of these registers being 2

π/4096.

COS 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 COS 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 AD9835 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 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

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

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

DSP and MPU Interfacing

The AD9835 has a serial interface, with 16 bits being loaded

during each write cycle. SCLK, SDATA and FSYNC are used

to load the word into the AD9835. When FSYNC is taken low,

the AD9835 is informed that a word is being written to the

device. The first bit is read into the device on the next SCLK

falling edge with the remaining bits being read into the device

on the subsequent SCLK falling edges. FSYNC frames the

16 bits; therefore, when 16 SCLK falling edges have occurred,