AD9742 Analog Devices, AD9742 Datasheet - Page 16

• Manufacturer Part Number: AD9742
• Manufacturer: Analog Devices

Specifications of AD9742

• Resolution (bits): 12bit
• Dac Update Rate: 210MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

AD9742
APPLYING THE AD9742
Output Configurations
The following sections illustrate some typical output
configurations for the AD9742. Unless otherwise noted, it is
assumed that I
requiring the optimum dynamic performance, a differential
output configuration is suggested. A differential output configu-
ration may consist of either an RF transformer or a differential
op amp configuration. The transformer configuration provides
the optimum 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 are connected to an appropri-
ately sized load resistor, R
configuration may be more suitable for a single-supply system
requiring a dc-coupled, ground-referred output voltage. Alter-
natively, an amplifier could be configured as an I-V converter,
thus converting 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 31. A
differentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral con-
tent 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
12
10
8
6
4
2
0
0
OUTFS
PECL
Figure 30. I
50
is set to a nominal 20 mA. For applications
SE
CLKVDD
LOAD
100
f
DIFF
CLOCK
vs. f
, referred to ACOM. This
CLOCK
(MSPS)
and Clock Mode
150
200
250
Rev. B | Page 16 of 32
for impedance matching purposes. Note that the transformer
provides ac coupling only.
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
symmetrically around ACOM and should be maintained with
the specified output compliance range of the AD9742. A differ-
ential resistor, R
output of the transformer is connected to the load, R
passive reconstruction filter or cable. R
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
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 32. The AD9742 is
configured with two equal load resistors, R
differential voltage developed across IOUTA and IOUTB is
converted 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.
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 AD9742 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 capabili-
ties should all be considered when optimizing this circuit.
AD9742
Figure 32. DC Differential Coupling Using an Op Amp
Figure 31. Differential Output Using a Transformer
IOUTA
IOUTB
AD9742
DIFF
IOUTA
IOUTB
22
21
, may be inserted in applications where the
25Ω
22
21
C
OPT
OPTIONAL R
25Ω
MINI-CIRCUITS
225Ω
225Ω
T1-1T
DIFF
DIFF
OUTA
is determined by the
LOAD
500Ω
and V
, of 25 Ω. The
AD8047
R
500Ω
LOAD
OUTB
LOAD
) swing
DIFF
, via a
.