Datasheet

AD9763/AD9765/AD9767 Data Sheet
Rev. G | Page 28 of 44
APPLYING THE AD9763/AD9765/AD9767
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configurations
for the AD9763/AD9765/AD9767, with I
OUTFS
set to a nominal
20 mA, unless otherwise noted. For applications requiring the
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
OUTA
and/or I
OUTB
is connected to an appropriately
sized load resistor (R
LOAD
) referred to as ACOM. This configuration
may be 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
OUTA
or I
OUTB
into a negative unipolar voltage. This configura-
tion provides the best dc linearity because I
OUTA
or I
OUTB
is
maintained at a virtual ground. Note that I
OUTA
provides slightly
better performance than I
OUTB
.
DIFFERENTIAL COUPLING USING A
TRANSFORMER
An RF transformer can be used as shown in Figure 72 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.
R
LOAD
AD9763/
AD9765/
AD9767
I
OUTA
I
OUTB
Mini-Circuits
T1-1T
OPTIONAL
R
DIFF
00617-072
Figure 72. 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 I
OUTA
and I
OUTB
. The complementary voltages appearing
at I
OUTA
and I
OUTB
(that is, V
OUTA
and V
OUTB
) swing symmetrically
around ACOM and must be maintained with the output compli-
ance range of the AD9763/AD9765/AD9767 to achieve the
specified performance. A differential resistor (R
DIFF
) can 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. Approximately half the signal power will be dissipated
across R
DIFF
.
DIFFERENTIAL COUPLING USING AN OP AMP
An op amp can also be used as shown in Figure 73 to perform a
differential-to-single-ended conversion. The AD9763/AD9765/
AD9767 is configured with two equal load resistors (R
LOAD
) of
25  each. The differential voltage developed across I
OUTA
and
I
OUTB
is converted to a single-ended signal via the differential
op amp configuration. An optional capacitor can be installed
across I
OUTA
and I
OUTB
, forming a real pole in a low-pass filter.
The addition of this capacitor often enhances the op amp’s
distortion performance by preventing the DAC’s high-slewing
output from overloading the op amp’s input.
AD9763/
AD9765/
AD9767
500
Ω
500Ω
225Ω
25Ω25Ω
AD8047
I
OUTA
I
OUTB
225Ω
C
OPT
0
0617-073
Figure 73. 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
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.
Select a high speed amplifier capable of preserving the
differential performance of the AD9763/AD9765/AD9767
while meeting other system level objectives (that is, cost or
power). Consider the op amp’s differential gain, gain setting
resistor values, and full-scale output swing capabilities when
optimizing this circuit.
The differential circuit shown in Figure 74 provides the
necessary level shifting required in a single-supply system.
In this case, AVDD, which is the positive analog supply for both
the AD9763/AD9765/AD9767 and the op amp, is used to level
shift the differential output of the AD9763/AD9765/AD9767 to
midsupply (that is, AVDD/2). The AD8055 is a suitable op amp
for this application.