AD9833 datasheet(7/18 Pages) AD

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AD9833

– 7–

REV PrG

PRELIMINARY TECHNICAL DATA

TERMINOLOGY

Integral Nonlinearity

This is the maximum deviation of any code from a

straight line passing through the endpoints of the transfer

function.

The endpoints of the transfer function are zero

scale, a point 0.5 LSB below the first code transition

(000 . . . 00 to 000 . . . 01) and full scale, a point 0.5 LSB

above the last code transition (111 . . . 10 to 111 . . . 11).

The error is expressed in LSBs.

Differential Nonlinearity

This is the difference between the measured and ideal 1

LSB change between two adjacent codes in the DAC. A

specified differential nonlinearity of ±1 LSB maximium ensures

monotonicity.

Output Compliance

The output compliance refers to the maximum voltage

that can be generated at the output of the DAC to meet

the specifications.

When voltages greater than that speci-

fied for the output compliance are generated, the AD9833

may not meet the specifications listed in the data sheet.

Spurious Free Dynamic Range

Along with the frequency of interest, harmonics of the

fundamental frequency and images of the these frequencies

are present at the output of a DDS device.

The spurious

free dynamic range (SFDR) refers to the largest spur or

harmonic which is present in the band of interest.

The

wide band SFDR gives the magnitude of the largest har-

monic or spur relative to the magnitude of the fundamental

frequency in the 0 to Nyquist bandwidth.

The narrow band

SFDR gives the attenuation of the largest spur or harmonic

in a bandwidth of

±200 kHz about the fundamental fre-

quency.

Total Harmonic Distortion

Total Harmonic Distortion (THD) is the ratio of the rms

sum of harmonics to the rms value of the fundameltal. For

the AD9834, THD is defined as:

THD = 20 log

√(V

2 + V

2)/V

where V1 is the rms amplitude of the fundamental and V2,

V3, V4, V5 and V6 are the rms amplitudes of the second

through thre sixth harmonic.

Signal-to-Noise Ratio (SNR)

S/N is the ratio of the rms value of the measured output

signal to the rms sum of all other spectral components

below the Nyquist frequency, excluding the first six har-

monics and dc. The value for SNR is expressed in

decibels.

Clock Feedthrough

There will be feedthrough from the MCLK input to the

analog output.

Clock feedthrough refers to the magnitude

of the MCLK signal relative to the fundamental frequency

in the AD9834’s output spectrum.

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.

Figure 4. 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 = ∆Phase/δt = 2πf

Solving for f and substituting the reference clock

frequency for the reference period (1/fMCLK = δt)

f =

∆Phase x f

MCLK/2π

The AD9833 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

SIN ROM

Digital- to- Analog Convertor.

Each of these sub-circuits are discussed in the following

section.

MAGNITUDE

PHASE

-1

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