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參數(shù)資料
| 型號(hào)： | AD9744-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 14-Bit, 165 MSPS TxDAC D/A Converter |
| 中文描述： | 14位，165 MSPS的TxDAC系列D / A轉(zhuǎn)換 |
| 文件頁數(shù)： | 14/28頁 |
| 文件大?。?/td> | 1142K |
| 代理商： | AD9744-EB |
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REV. A
–14–
AD9744
APPLYING THE AD9744
The following sections illustrate some typical output configura-
tions for the AD9744. Unless otherwise noted, it is assumed
that I
OUTFS
is set to a nominal 20 mA. For applications requir-
ing the optimum dynamic performance, a differential output
configuration is suggested. A differential output configuration
may consist of either an RF transformer or a differential op amp
configuration. The transformer configuration provides the opti-
mum high frequency performance and is recommended for any
application that allows ac coupling. The differential op amp
configuration is suitable for applications requiring dc coupling,
a bipolar output, signal gain, and/or level shifting within the
bandwidth of the chosen op amp.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if IOUTA and/or IOUTB is connected to an appropriately
sized load resistor, R
LOAD
, referred to ACOM. This configura-
tion may be more suitable for a single-supply system requiring a
dc-coupled, ground referred output voltage. Alternatively, an
amplifier could be configured as an I-V converter, thus convert-
ing IOUTA or IOUTB into a negative unipolar voltage. This
configuration provides the best dc linearity since IOUTA or
IOUTB is maintained at a virtual ground.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-single-
ended signal conversion, as shown in Figure 12. A differentially
coupled transformer output provides the optimum distortion
performance for output signals whose spectral content lies within
the transformer’s pass band. An RF transformer, such as the
Mini-Circuits T1–1T, provides excellent rejection of common-
mode distortion (i.e., even-order harmonics) and noise over a
wide frequency range. It also provides electrical isolation and the
ability to deliver twice the power to the load. Transformers with
different impedance ratios may also be used for impedance match-
ing purposes. Note that the transformer provides ac coupling only.
R
LOAD
AD9744
22
21
MINI-CIRCUITS
T1-1T
OPTIONAL R
DIFF
IOUTA
IOUTB
Figure 12. Differential Output Using a Transformer
The center tap on the primary side of the transformer must be
connected to ACOM to provide the necessary dc current path
for both IOUTA and IOUTB. The complementary voltages
appearing at IOUTA and IOUTB (i.e., V
OUTA
and V
OUTB
)
swing symmetrically around ACOM and should be maintained
with the specified output compliance range of the AD9744. A
differential resistor, R
DIFF
, may be inserted in applications where
the output of the transformer is connected to the load, R
LOAD
,
via a passive reconstruction filter or cable. R
DIFF
is determined
by the transformer’s impedance ratio and provides the proper
source termination that results in a low VSWR. Note that approxi-
mately half the signal power will be dissipated across R
DIFF
.
DIFFERENTIAL COUPLING USING AN OP AMP
An op amp can also be used to perform a differential-to-single-
ended conversion, as shown in Figure 13. The AD9744 is
configured with two equal load resistors, R
LOAD
, of 25
W
. The
differential voltage developed across IOUTA and IOUTB is conver-
ted to a single-ended signal via the differential op amp configuration.
An optional capacitor can be installed across IOUTA and IOUTB,
forming a real pole in a low-pass filter. The addition of this
capacitor also enhances the op amp’s distortion performance by
preventing the DAC’s high slewing output from overloading the
op amp’s input.
AD9744
22
IOUTA
IOUTB
21
C
OPT
500
225
225
500
25
25
AD8047
Figure 13. DC Differential Coupling Using an Op Amp
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the differ-
ential op amp circuit using the AD8047 is configured to provide
some additional signal gain. The op amp must operate off a dual
supply since its output is approximately
±
1 V. A high speed
amplifier capable of preserving the differential performance of the
AD9744 while meeting other system level objectives (e.g., cost
or power) should be selected. The op amp’s differential gain, gain
setting resistor values, and full-scale output swing capabilities
should all be considered when optimizing this circuit.
The differential circuit shown in Figure 14 provides the neces-
sary level shifting required in a single-supply system. In this case,
AVDD, which is the positive analog supply for both the AD9744
and the op amp, is also used to level-shift the differential output
of the AD9744 to midsupply (i.e., AVDD/2). The AD8041 is a
suitable op amp for this application.
AD9744
22
IOUTA
IOUTB
21
C
OPT
500
225
225
1k
25
25
AD8041
1k
AVDD
Figure 14. Single Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 15 shows the AD9744 configured to provide a unipolar
output range of approximately 0 V to 0.5 V for a doubly termi-
nated 50
W
cable since the nominal full-scale current, I
OUTFS
, of
20 mA flows through the equivalent R
LOAD
of 25
W
. In this case,
R
LOAD
represents the equivalent load resistance seen by IOUTA
or IOUTB. The unused output (IOUTA or IOUTB) can be
connected to ACOM directly or via a matching R
LOAD
. Different
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