AD9709 Analog Devices, AD9709 Datasheet - Page 19

• Manufacturer Part Number: AD9709
• Description: 8-Bit, 125 MSPS Dual TxDAC+ Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9709

• Resolution (bits): 8bit
• Dac Update Rate: 125MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

APPLYING THE AD9709
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9709. Unless otherwise noted, it is assumed that
I
optimum dynamic performance, a differential output
configuration is suggested. A differential output configuration
can 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 allowing for ac coupling. The differential op
amp configuration is suitable for applications requiring dc
coupling, 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 results
if I
resistor, R
more suitable for a single-supply system requiring a dc-coupled,
ground-referred output voltage. Alternatively, an amplifier can be
configured as an I-V converter, thus converting I
negative unipolar voltage. This configuration provides the best dc
linearity because I
Note that I
DIFFERENTIAL COUPLING USING A
TRANSFORMER
An RF transformer can be used as shown in Figure 36 to perform
a differential-to-single-ended signal conversion. A differentially
coupled transformer output provides the optimum distortion
performance for output signals whose spectral content lies within
the pass band of the transformer. An RF transformer such as the
Mini-Circuits® T1-1T provides excellent rejection of common-
mode distortion (that is, 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 can 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 I
at I
around ACOM and should be maintained with the specified
output compliance range of the AD9709. A differential resistor,
OUTFS
OUTA
OUTA
is set to a nominal 20 mA. For applications requiring the
and/or I
and I
OUTA
LOAD
OUTA
Figure 36. Differential Output Using a Transformer
AD9709
OUTB
, referred to ACOM. This configuration may be
and I
provides slightly better performance than I
OUTB
I
I
OUTA
OUTB
(that is, V
OUTA
OUTB
is connected to an appropriately sized load
or I
. The complementary voltages appearing
OUTB
OPTIONAL
R
OUTA
DIFF
is maintained at a virtual ground.
and V
Mini-Circuits
T1-1T
OUTB
) swing symmetrically
OUTA
R
LOAD
or I
OUTB
into a
OUTB
Rev. B | Page 19 of 32
.
R
transformer is connected to the load, R
reconstruction filter or cable. R
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 as shown in Figure 37 to perform a
differential-to-single-ended conversion. The AD9709 is configured
with two equal load resistors, R
voltage developed across I
ended signal via the differential op amp configuration. An optional
capacitor can be installed across I
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 differential
op amp circuit using the AD8047 is configured to provide some
additional signal gain. The op amp must operate from a dual
supply because its output is approximately ±1.0 V. A high speed
amplifier capable of preserving the differential performance of
the AD9709 while meeting other system level objectives (that is,
cost and power) should be selected. The op amp’s differential
gain, gain setting resistor values, and full-scale output swing
capabilities should be considered when optimizing this circuit.
The differential circuit shown in Figure 38 provides the
necessary level shifting required in a single-supply system. In
this case, AVDD, which is the positive analog supply for both
the AD9709 and the op amp, is used to level shift the differential
output of the AD9709 to midsupply (that is, AVDD/2). The
AD8041
DIFF
, can be inserted in applications where the output of the
AD9709
AD9709
is a suitable op amp for this application.
Figure 38. Single-Supply DC Differential Coupled Circuit
Figure 37. DC Differential Coupling Using an Op Amp
I
I
I
I
OUTA
OUTB
OUTA
OUTB
25Ω
25Ω
C
C
OPT
OPT
OUTA
LOAD
and I
25Ω
DIFF
25Ω
OUTA
225Ω
225Ω
, of 25 Ω each. The differential
225Ω
225Ω
is determined by the
OUTB
and I
LOAD
is converted to a single-
500Ω
OUTB
500Ω
, via a passive
, forming a real pole
AD8041
500Ω
AD8047
1kΩ
DIFF
500Ω
.
AD9709
AVDD