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參數(shù)資料
| 型號： | AD976CRS |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 16-Bit, 100 kSPS/200 kSPS BiCMOS A/D Converters |
| 中文描述： | 1-CH 16-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, PDSO28 |
| 封裝： | SSOP-28 |
| 文件頁數(shù)： | 17/23頁 |
| 文件大?。?/td> | 577K |
| 代理商： | AD976CRS |
AD9761
–17–
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: 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 main-
tain 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. Its 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 con-
struction 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 modu-
lated both in amplitude (i.e., AM modulation) and in phase (i.e.,
PM modulation). It can be generated by independently modu-
lating 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 modula-
tor is shown in Figure 41. 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 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 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.
Σ
0
90
CARRIER
FREQ
NYQUIST
FILTERS
QUADRATURE
MODULATOR
TO
MIXER
IOUT
QOUT
AD9761
10
DSP
OR
ASIC
Figure 41. Typical Analog QAM Architecture
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 Minicircuit transformer (i.e., Model T2-1T) into
the available socket.
The digital inputs are designed to be driven directly from vari-
ous word generators with the onboard 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.
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