AD9762-EBZ Analog Devices Inc, AD9762-EBZ Datasheet - Page 17

• Manufacturer Part Number: AD9762-EBZ
• Description: 12-BIT 100 MSPS+ TxDAC D/A Converter
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9762-EBZ

• Number Of Dac's: 1
• Number Of Bits: 12
• Outputs And Type: 1, Differential
• Sampling Rate (per Second): 125M
• Data Interface: Parallel
• Settling Time: 35ns
• Dac Type: Current
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9762
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

For those applications that require a single +5 V or +3 V supply
for both the analog and digital supply, a clean analog supply
may be generated using the circuit shown in Figure 55. The
circuit consists of a differential LC filter with separate power
supply and return lines. Lower noise can be attained using low
ESR type electrolytic and tantalum capacitors.
Maintaining low noise on power supplies and ground is critical
to obtaining optimum results from the AD9762. If properly
implemented, ground planes can perform a host of functions on
high speed circuit boards: bypassing, shielding, current trans-
port, etc. 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 appropriate 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 conduc-
tion 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.
REV. B
TTL/CMOS
Figure 55. Differential LC Filter for Single +5 V or +3 V
Applications
CIRCUITS
LOGIC
POWER SUPPLY
+5V OR +3V
FERRITE
BEADS
100 F
ELECT.
10-22 F
TANT.
0.1 F
CER.
AVDD
ACOM
–17–
APPLICATIONS
Using the AD9762 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 spreadspectrum (i.e.,
CDMA) based systems. A QAM signal is a carrier frequency
which is both modulated 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.
A common and traditional implementation of a QAM modu-
lator is shown in Figure 56. 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.
In this implementation, it is much more difficult to maintain
proper gain and phase matching between the I and Q channels.
The circuit implementation shown in Figure 57 helps improve
upon the matching and temperature stability characteristics
between the I and Q channels. Using a single voltage reference
derived from U1 to set the gain for both the I and Q channels
will improve the gain matching and stability. Further enhance-
ments in gain matching and stability are achieved by using
separate matching resistor networks for both R
Additional trim capability via R
compensate for any initial mismatch in gain between the two
channels. This may be attributed to any mismatch between U1
and U2’s gain setting resistor, (R
(R
The differential voltage outputs of U1 and U2 are fed into their
respective differential inputs of a quadrature mixer via matching
50 Ω filter networks.
LOAD
ASIC
DSP
OR
); and/or voltage offset of each DAC’s control amplifier.
Figure 56. Typical Analog QAM Architecture
12
12
AD9762
AD9762
FREQUENCY
CARRIER
NYQUIST
FILTERS
CAL1
SET
); effective load resistance,
and R
QUADRATURE
MODULATOR
CAL2
0
90
SET
can be added to
AD9762
Σ
and R
TO
MIXER
LOAD
.