AD9752 Analog Devices, AD9752 Datasheet - Page 16

• Manufacturer Part Number: AD9752
• Manufacturer: Analog Devices

Specifications of AD9752

• Resolution (bits): 12bit
• Dac Update Rate: 125MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

AD9752
maintain optimum performance. Care should be taken to ensure
that the ground plane is uninterrupted over crucial signal paths.
On the digital side, this includes the digital input lines running
to the DAC as well as any clock signals. On the analog side, this
includes the DAC output signal, reference signal and the supply
feeders.
The use of wide runs or planes in the routing of power lines is
also recommended. This serves the dual role of providing a low
series impedance power supply to the part, as well as providing
some “free” capacitive decoupling to the appropriate ground
plane. It is essential that care be taken in the layout of signal and
power ground interconnects to avoid inducing extraneous volt-
age drops in the signal ground paths. It is recommended that all
connections be short, direct and as physically close to the pack-
age as possible in order to minimize the sharing of conduction
paths between different currents. When runs exceed an inch in
length, strip line techniques with proper termination resistor
should be considered. The necessity and value of this resistor
will be dependent upon the logic family used.
For a more detailed discussion of the implementation and
construction of high speed, mixed signal printed circuit boards,
refer to Analog Devices’ application notes AN-280 and
AN-333.
Figure 35b. Notch in Missing Bin at 5 MHz is Down
Figure 35a. Notch in Missing Bin at 750 kHz is Down
>60 dB. (Peak Amplitude + 0 dBm).
>60 dB. (Peak Amplitude + 0 dBm).
–100
–110
–100
–30
–40
–50
–60
–70
–80
–90
–30
–40
–50
–60
–70
–80
–90
4.85
600k
FREQUENCY – MHz
FREQUENCY – Hz
800k
5
5.15
1M
–16–
APPLICATIONS
VDSL Applications Using the AD9752
Very High Frequency Digital Subscriber Line (VDSL) technol-
ogy is growing rapidly in applications requiring data transfer
over relatively short distances. By using QAM modulation and
transmitting the data in multiple discrete tones, high data rates
can be achieved.
As with other multitone applications, each VDSL tone is ca-
pable of transmitting a given number of bits, depending on the
signal-to-noise ratio (SNR) in a narrow band around that tone.
The tones are evenly spaced over the range of several kHz to
10 MHz. At the high frequency end of this range, performance
is generally limited by cable characteristics and environmental
factors, such as external interferers. Performance at the lower
frequencies is much more dependent on the performance of the
components in the signal chain. In addition to in-band noise,
intermodulation from other tones can also potentially interfere
with the recovery of data for a given tone. The two graphs in
Figure 35 represent a 500 tone missing bin test vector, with
frequencies evenly spaced from 400 Hz to 10 MHz. This test is
very commonly done to determine if distortion will limit the
number of bits which can be transmitted in a tone. The test
vector has a series of missing tones around 750 kHz, which is
represented in Figure 35a and a series of missing tones around
5 MHz which is represented in Figure 35b. In both cases, the
spurious free range between the transmitted tones and the empty
bins is greater than 60 dB.
Using the AD9752 for Quadrature Amplitude Modulation
(QAM)
QAM is one of the most widely used digital modulation
schemes in digital communication systems. This modulation
technique can be found in FDM as well as spreadspectrum (i.e.,
CDMA) based systems. A QAM signal is a carrier frequency
that is modulated in both amplitude (i.e., AM modulation) and
phase (i.e., PM modulation). It can be generated by indepen-
dently modulating two carriers of identical frequency but with a
90 phase difference. This results in an in-phase (I) carrier com-
ponent and a quadrature (Q) carrier component at a 90 phase
shift with respect to the I component. The I and Q components
are then summed to provide a QAM signal at the specified car-
rier frequency.
A common and traditional implementation of a QAM modu-
lator is shown in Figure 36. The modulation is performed in the
analog domain in which two DACs are used to generate the
baseband I and Q components, respectively. Each component is
then typically applied to a Nyquist filter before being applied to
a quadrature mixer. The matching Nyquist filters shape and
limit each component’s spectral envelope while minimizing
intersymbol interference. The DAC is typically updated at the
QAM symbol rate or possibly a multiple of it if an interpolating
filter precedes the DAC. The use of an interpolating filter typi-
cally eases the implementation and complexity of the analog
filter, which can be a significant contributor to mismatches in
gain and phase between the two baseband channels. A quadra-
ture mixer modulates the I and Q components with in-phase
and quadrature phase carrier frequency and then sums the two
outputs to provide the QAM signal.
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