AD9834 Analog Devices, AD9834 Datasheet - Page 8

• Manufacturer Part Number: AD9834
• Description: +2.5V to +5.5V, 50MHz, Low Power (25mW) Complete DDS With on Board Comparator in 20-pin Tssop Package
• Manufacturer: Analog Devices

T E R M I N O L O G Y
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
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.
fied for the output compliance are generated, the AD9834
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:
where V
V
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.
of the MCLK signal relative to the fundamental frequency
in the AD9834’s output spectrum.
AD9834
3
THD = 20 log V
, V
4
, V
1
5
is the rms amplitude of the fundamental and V
and V
Clock feedthrough refers to the magnitude
6
are the rms amplitudes of the second
When voltages greater than that speci-
2
2
+ V
3
2
PRELIMINARY TECHNICAL DATA
+ V
4
2
±1 LSB maximium ensures
+ V
5
2
+ V
6
2
)/V
1
2
,
–8–
THEORY OF OPERATION
Sine waves are typically thought of in terms of their
magnitude form a(t) = sin ( t). 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
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.
Solving for
Solving for f and substituting the reference clock
frequency for the reference period (1/f
The AD9834 builds the output based on this simple
equation. A simple DDS chip can implement this
equation with three major subcircuits:
Each of these sub-circuits are discussed in the following
section.
Numerical Controlled Oscillator + Phase Modulator
SIN ROM
Digital- to- Analog Convertor.
+1
- 1
2
0
0
Figure 5. Sine Wave
f = Phase x f
= 2 f.
= Phase / t = 2 f
Phase =
MAGNITUDE
PHASE
MCLK
t
/2
MCLK
= t)
REV PrL