AD9915 AD [Analog Devices], AD9915 Datasheet - Page 20

• Manufacturer Part Number: AD9915
• Description: 2.5 GSPS Direct Digital Synthesizer with 12-Bit DAC
• Manufacturer: AD [Analog Devices]

12-BIT DAC OUTPUT
The
DAC. The output current is delivered as a balanced signal using
two outputs. The use of balanced outputs reduces the potential
amount of common-mode noise present at the DAC output,
offering the advantage of an increased signal-to-noise ratio. An
external resistor (R
and AGND establishes the reference current. The recommended
value of R
Attention should be paid to the load termination to keep the
output voltage within the specified compliance range; voltages
developed beyond this range cause excessive distortion and can
damage the DAC output circuitry.
DAC CALIBRATION OUTPUT
The DAC CAL enable bit in the CFR4 control register (0x03[24])
must be manually set and then cleared after each power-up and
every time the REF CLK or internal system clock is changed.
This initiates an internal calibration routine to optimize the
setup and hold times for internal DAC timing. Failure to
calibrate may degrade performance and even result in loss of
functionality. The length of time to calibrate the DAC clock is
calculated from the following equation:
RECONSTRUCTION FILTER
The DAC output signal appears as a sinusoid sampled at f
frequency of the sinusoid is determined by the frequency tuning
word (FTW) that appears at the input to the DDS. The DAC
AD9915
AD9915
t
CAL
(
SET
f
S
)
is 3.3 kΩ.
incorporates an integrated 12-bit, current output
=
531
SET
f
,
S
840
) connected between the DAC_RSET pin
MAGNITUDE
–100
–20
–40
–60
–80
0
(dB)
BASE BAND
IMAGE 0
SPURS
PRIMARY
SIGNAL
Figure 31. DAC Spectrum vs. Reconstruction Filter Response
f
s
/2
RESPONSE
FILTER
IMAGE 1
S
. The
f
Rev. A | Page 20 of 48
s
IMAGE 2
3
f
output is typically passed through an external reconstruction
filter that serves to remove the artifacts of the sampling process
and other spurs outside the filter bandwidth.
Because the DAC constitutes a sampled system, its output must
be filtered so that the analog waveform accurately represents the
digital samples supplied to the DAC input. The unfiltered DAC
output contains the desired baseband signal, which extends
from dc to the Nyquist frequency (f
of the baseband signal that theoretically extend to infinity. Notice
that the odd numbered images (shown in Figure 31) are mirror
images of the baseband signal. Furthermore, the entire DAC
output spectrum is affected by a sin(x)/x response, which is
caused by the sample-and-hold nature of the DAC output signal.
For applications using the fundamental frequency of the DAC
output, the response of the reconstruction filter should preserve
the baseband signal (Image 0), while completely rejecting all
other images. However, a practical filter implementation
typically exhibits a relatively flat pass band that covers the
desired output frequency plus 20%, rolls off as steeply as
possible, and then maintains significant (though not complete)
rejection of the remaining images. Depending on how close
unwanted spurs are to the desired signal, a third-, fifth-, or
seventh-order elliptic low-pass filter is common.
Some applications operate from an image above the Nyquist
frequency, and those applications use a band-pass filter instead
of a low-pass filter. The design of the reconstruction filter has a
significant impact on the overall signal performance. Therefore,
good filter design and implementation techniques are important
for obtaining the best possible jitter results.
s
/2
ENVELOPE
SIN(x)/x
IMAGE 3
2
f
s
IMAGE 4
5
f
s
/2
f
S
/2). It also contains images
Data Sheet