AD9740-EB Analog Devices, AD9740-EB Datasheet - Page 12

• Manufacturer Part Number: AD9740-EB
• Description: 10-Bit/ 165 MSPS TxDAC D/A Converter
• Manufacturer: Analog Devices

AD9740
APPLYING THE AD9740
Output Configurations
The following sections illustrate some typical output configurations
for the AD9740. Unless otherwise noted, it is assumed that I
set to a nominal 20 mA. For applications requiring 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 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 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.
35
30
25
20
15
10
16
14
12
10
0
0.01
8
6
4
2
0
Figure 9. I
2
4
Figure 8. I
LOAD
6
DVDD
, referred to ACOM. This configuration
8
vs. Ratio @ DVDD = 3.3 V
RATIO –
I
OUTFS
AVDD
165MSPS
125MSPS
10
65MSPS
0.1
f
OUT
– mA
vs. I
12
/
f
CLOCK
OUTFS
14
16
18
20
OUTFS
1
is
–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
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 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
cally around ACOM and should be maintained with the specified
output compliance range of the AD9740. A differential resistor,
R
transformer is connected to the load, R
struction filter or cable. R
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 AD9740 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
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
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
DIFF
Figure 11. DC Differential Coupling Using an Op Amp
Figure 10. Differential Output Using a Transformer
, may be inserted in applications where the output of the
AD9740
IOUTA
IOUTB
AD9740
IOUTA
IOUTB
25
LOAD
DIFF
C
OPT
, of 25 Ω. The differential voltage
is determined by the transformer’s
OPTIONAL R
OUTA
DIFF
25
MINI-CIRCUITS
and V
225
225
.
T1–1T
LOAD
DIFF
OUTB
, via a passive recon-
500
) swing symmetri-
AD8047
500
R
LOAD
REV. 0