AD9712B/AD9713B

REV. B

–8–

NUMERICALLY-CONTROLLED OSCILLATOR

TUNING

WORD

12

PHASE

ACCUMULATOR

14

32

OUTPUT

SINE DATA

TTL

REGISTER

12

SYSTEM

CLOCK

LATCH

ENABLE

PHASE-TO-AMPLITUDE

CONVERSION

D

D

1

12

AD9712B

AD9713B

D/A CONVERTER

Figure 6. Direct Digital Synthesizer Block Diagram

90

100

10

0%

5mV/div

5ns/div

Figure 7. AD9712B/AD9713B Glitch Impulse

1ns/div

90

100

10

0%

200mV/div

Figure 8. Rise and Fall Characteristics

integer, the DDS continually uses a small subset of the available

DAC codes. The DNL of the converter is effectively the DNL

error of the codes used, and is typically worse than the error

measured against all available DAC codes. This increase in

DNL is translated into higher harmonic and noise levels at the

output.

Glitch impulse, often considered a figure of merit in DDS appli-

cations, is simply the initial transient response of the DAC as it

moves between two output levels. This nonlinearity is com-

monly associated with external data skew, but this effect is mini-

mized by using the on-board registers of the AD9712B/AD9713B

converters (see Digital Inputs/Timing section). The majority of

the glitch impulse, shown below, is produced as the current in

the R-2R ladder network settles, and is fairly constant over the

full-scale range of the DAC. The fast transients which form the

glitch impulse appear as high-frequency spurs in the output

spectrum.

While it is difficult to predict the effects of glitch on the output

waveform, slew rate limitations translate directly into harmonics.

This makes slew rate the dominant effect in ac linearity of the

high slew rate and low output capacitance of the AD9712B/

AD9713B devices.

Another concern in DDS applications is the presence of aliased

harmonics in the output spectrum. Aliased harmonics appear as

spurs in the output spectrum at frequencies which are deter-

mined by:

MfA

± Nf

C

where M and N are integers.

The effects of these spurs are most easily observed in applica-

rate. This condition causes the aliased harmonics to fold near

the fundamental output frequency (see Performance Curves.)