AD9750-EB Analog Devices Inc, AD9750-EB Datasheet - Page 16

• Manufacturer Part Number: AD9750-EB
• Description: BOARD EVAL FOR AD9750
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9750-EB

• Rohs Status: RoHS non-compliant
• Number Of Dac's: 1
• Number Of Bits: 10
• 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: AD9750

AD9750
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 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.
APPLICATIONS
Using the AD9750 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 independently
modulating two carriers of identical frequency but with a 90
phase difference. This results in an in-phase (I) carrier compo-
nent 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.
–16–
A common and traditional implementation of a QAM modu-
lator is shown in Figure 35. 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 36 helps improve
upon the matching and temperature stability characteristics
between the I and Q channels, as well as showing a path for up-
conversion using the AD8346 quadrature modulator. 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 stabil-
ity. R
between the two channels. This mismatch may be attributed to
the mismatch between R
of each channel, and/or the voltage offset of the control ampli-
fier in each DAC. The differential voltage outputs of U1 and
U2 are fed into the respective differential inputs of the AD8346
via matching networks.
ASIC
DSP
OR
CAL
Figure 35. Typical Analog QAM Architecture
can be used to compensate for any mismatch in gain
12
12
AD9750
AD9750
SET1
FREQUENCY
CARRIER
NYQUIST
and R
FILTERS
SET2
, effective load resistance
QUADRATURE
MODULATOR
0
90
TO
MIXER
REV. 0