AD9761 Analog Devices, AD9761 Datasheet - Page 17

• Manufacturer Part Number: AD9761
• Description: 10-Bit, Complete, 40 MSPS, dual Transmit D/A Converter
• Manufacturer: Analog Devices

Specifications of AD9761

• Resolution (bits): 10bit
• Dac Update Rate: 40MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

Maintaining low noise on power supplies and ground is critical
to obtaining optimum results from the AD9761. If properly
implemented, ground planes can perform a host of functions
on high speed circuit boards such as bypassing, shielding,
current transport. In mixed signal design, the analog and
digital portions of the board should be distinct from each
other, with the analog ground plane confined to the areas
covering the analog signal traces and the digital ground plane
confined to areas covering the digital interconnects.
All analog ground pins of the DAC, reference, and other
analog components should be tied directly to the analog
ground plane. The two ground planes should be connected
by a path 1/8 to 1/4 inch wide underneath, or within 1/2 inch
of the DAC to 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 appropri-
ate ground plane. It is essential that care be taken in the
layout of signal and power ground interconnects to avoid
inducing extraneous voltage drops in the signal ground
paths. It is recommended that all connections be short, direct,
and as physically close to the package as possible in order
to minimize the sharing of conduction paths between differ-
ent currents. When runs exceed an inch in length, strip line
techniques with a 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.
APPLICATIONS
Using the AD9761 for QAM Modulation
QAM is one of the most widely used digital modulation
schemes in digital communication systems. This modulation
technique can be found in both FDM as well as spread
spectrum (i.e., CDMA) based systems. A QAM signal is a
carrier frequency that is modulated both in amplitude (i.e.,
AM modulation) and in phase (i.e., PM modulation). It can
be generated by independently modulating two carriers of
identical frequency but with a 90° phase difference. This results
in an in-phase (I) carrier component 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 carrier frequency.
REV. C
–17–
A common and traditional implementation of a QAM
modulator is shown in Figure 23. The modulation is performed
in the analog domain in which two DACs are used to gen-
erate 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 filter shapes and limits 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 typically eases the imple-
mentation and complexity of the analog filter, which can be
a significant contributor to mismatches in gain and phase
between the two baseband channels. A quadrature 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.
EVALUATION BOARD
The AD9761-EB is an evaluation board for the AD9761
dual 10-bit, 40 MSPS DAC. Careful attention to layout
and circuit design along with prototyping area allows the
user to easily and effectively evaluate the AD9761. This
board allows the user the flexibility to operate each of the
AD9761 DACs in a single-ended or differential output
configuration. Each of the DACs’ single-ended outputs are
terminated in a 50  resistor. Evaluation using a transformer
coupled output can be accomplished simply by installing
a Mini-Circuits transformer (i.e., Model T2-1T) into the
available socket.
The digital inputs are designed to be driven directly from
various word generators with the on-board option to add
a resistor network for proper load termination. Separate
50  terminated SMA connectors are also provided for the
CLOCK, WRITE, and SELECT inputs. Provisions are also
made to operate the AD9761 with either the internal or an
external reference as well as to exercise the power-down
feature.
ASIC
DSP
OR
Figure 23.Typical Analog QAM Architecture
10
AD9761
QOUT
IOUT
CARRIER
NYQUIST
FILTERS
FREQ
QUADRATURE
MODULATOR
0
AD9761
90
S
MIXER
TO