AD9739A Analog Devices, AD9739A Datasheet - Page 60

• Manufacturer Part Number: AD9739A
• Description: 14-Bit, 2.5 GSPS, RF D/A Converter
• Manufacturer: Analog Devices

Specifications of AD9739A

• Resolution (bits): 14bit
• Dac Update Rate: 2.5GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): n/a
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9737A/AD9739A
NONIDEAL SPECTRAL ARTIFACTS
The
artifacts that are not part of the original digital input waveform.
These nonideal artifacts include harmonics (including alias
harmonics), images, and clock spurs. Figure 186 shows a spectral
plot of the
(that is, dc to f
at 2.4 GSPS. Besides the desired fundamental tone at the −7.8 dBm
level, the spectrum also reveals these nonideal artifacts that also
appear as spurs above the measurement noise floor. Because
these nonideal artifacts are also evident in the second and third
Nyquist zones during mix-mode operation, the effects of these
artifacts should also be considered when selecting the DAC
clock rate for a target RF band.
Note the following important observations pertaining to these
nonideal spectral artifacts:
1.
2.
AD9737A/AD9739A
A full-scale sine wave (that is, single-tone) typically represents
the worst case condition because it is has a peak-to-rms
ratio of 3 dB and is unmodulated. Harmonics and aliased
harmonics of a sine wave are easy to identify because they
also appear as discrete spurs. Significant characterization of
a high speed DAC is performed using single (or multitone)
signals for this reason.
Modulated signals (that is, AM, PM, or FM) do not appear
as spurs but rather as signals whose power spectral density
is spread over a defined bandwidth determined by the
modulation parameters of the signals. Any harmonics from
the DAC spread over a wider bandwidth determined by the
order of the harmonic and bandwidth of the modulated signal.
For this reason, harmonics often appear as slight bumps in
the measurement noise floor and can be difficult to discern.
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
AD9737A/AD9739A
f
DAC
f
OUT
DAC
/4 –
HD4
200
/2) reconstructing a 650 MHz, 0 dBFS sine wave
f
DAC
f
Figure 186. Spectral Plot
OUT
400
/2 –
FUND AT
–7.6dBm
HD3
FREQUENCY (MHz)
output spectrum contains spectral
f
DAC
600
/4
within the first Nyquist zone
800
HD5
HD6
3/4 ×
1000
HD9
f
f
OUT
DAC
HD2
/4 –
1200
Rev. | Page 60 of 64
C
3.
4.
5.
6.
Images appear as replicas of the original signal, hence, can
be easier to identify. In the case of the AD9737A/AD9739A,
internal modulation of the sampling clock at intervals
related to f
and ¾ × f
associated with ¼ × f
whereas only the lower image of ½ × f
back. Note that the lower images appear frequency inverted.
The ratio between the fundamental and various images (that
is, dBc) remains mostly signal independent because the
mechanism causing these images is related to corruption of
the sampling clock.
The magnitude of these images for a given device depends
on several factors, including DAC clock rate, output
frequency, and Mu controller phase setting. Because the
image magnitude is repeatable between power-up cycles
(assuming the same conditions), a one-time factory
calibration procedure can be used to improve suppression.
Calibration consists of additional dedicated DSP resources in
the host that can generate a replica of the image with proper
amplitude, phase, and frequency scaling to cancel the image
from the DAC. Because the image magnitude can vary
among devices, each device must be calibrated.
A clock spur appears at f
Similar to images, the spur magnitude also depends on the
same factors that cause variations in image levels. However,
unlike images and harmonics, clock spurs always appear
as discrete spurs, albeit their magnitude shows a slight
dependency on the digital waveform and output frequency.
The calibration method is similar to image calibration;
however, only a digital tone of equal amplitude and
opposite phase at f
A large clock spur also appears at 2 × f
or mix-mode operation. This clock spur is due to the quad
switch DAC architecture causing switching events to occur
on both edges of f
DAC
DAC
. Both upper and lower sideband images
/4 generate image pairs at ¼ × f
DAC
DAC
.
DAC
/4 need be generated.
fall within the first Nyquist zone,
DAC
/4 and integer multiples of it.
DAC
DAC
and ¾ × f
Data Sheet
in either normal
DAC
, ½ × f
DAC
fall
DAC
,