AD9742-EB Analog Devices, AD9742-EB Datasheet - Page 12

• Manufacturer Part Number: AD9742-EB
• Description: 12-Bit/ 165 MSPS TxDAC D/A Converter
• Manufacturer: Analog Devices

AD9742
APPLYING THE AD9742
Output Configurations
The following sections illustrate some typical output configura-
tions for the AD9742. Unless otherwise noted, it is assumed
that I
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
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 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.
OUTFS
35
30
25
20
15
10
16
14
12
10
0.01
0
8
6
4
2
0
Figure 9. I
2
is set to a nominal 20 mA. For applications requir-
4
Figure 8. I
LOAD
6
DVDD
, referred to ACOM. This configuration
8
vs. Ratio @ DVDD = 3.3 V
RATIO –
I
125MSPS
OUTFS
165MSPS
AVDD
10
65MSPS
0.1
f
OUT
– mA
vs. I
12
/
f
CLOCK
OUTFS
14
16
18
20
1
–12–
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-single-
ended signal conversion as shown in Figure 10. A differentially
coupled transformer output provides the optimum distortion per-
formance 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 fre-
quency 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 matching pur-
poses. 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
swing symmetrically around ACOM and should be maintained
with the specified output compliance range of the AD9742. A
differential resistor, R
the output of the transformer is connected to the load, R
via a passive reconstruction filter or cable. R
the transformer’s impedance ratio and provides the proper source
termination that results in a low VSWR. Note that approximately
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 11. The AD9742 is configured with
two equal load resistors, R
developed across IOUTA and IOUTB is converted to a single-ended
signal via the differential op amp configuration. An optional capaci-
tor 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 DACs 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 differential
op amp circuit using the AD8047 is configured to provide some
additional signal gain. The op amp must operate off of a dual
supply since its output is approximately ± 1.0 V. A high-speed
amplifier capable of preserving the differential performance of the
Figure 11. DC Differential Coupling Using an Op Amp
Figure 10. Differential Output Using a Transformer
AD9742
IOUTA
IOUTB
AD9742
IOUTA
IOUTB
25
DIFF
LOAD
, may be inserted in applications where
C
OPT
, of 25 W. The differential voltage
OPTIONAL R
25
MINI-CIRCUITS
225
225
T1-1T
DIFF
OUTA
DIFF
500
is determined by
AD8047
and V
DIFF
500
R
LOAD
.
OUTB
REV. 0
LOAD
)
,