AD9764
APPLYING THE AD9764
OUTPUT CONFIGURATIONS
DIFFERENTIAL USING AN OP AMP
The following sections illustrate some typical output configura-
tions for the AD9764. Unless otherwise noted, it is assumed
that I
OUTFS
is set to a nominal 20 mA. For applications requir-
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 allowing for ac coupling. The differential op amp
configuration is suitable for applications requiring dc coupling, a
bipolar output, signal gain and/or level shifting.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if I
OUTA
and/or I
OUTB
is connected to an appropriately
sized load resistor, R
LOAD
, referred to ACOM. This configura-
tion 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 convert-
ing I
OUTA
or I
OUTB
into a negative unipolar voltage. This con-
figuration provides the best dc linearity since I
OUTA
or I
OUTB
is
maintained at a virtual ground. Note, I
OUTA
provides slightly
better performance than I
OUTB
.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An op amp can also be used to perform a differential-to-single-
ended conversion as shown in Figure 34. The AD9764 is con-
figured with two equal load resistors, R
LOAD
, of 25
Ω.
The
differential voltage developed across I
OUTA
and I
OUTB
is con-
verted to a single-ended signal via the differential op amp con-
figuration. 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 also enhances the op amp’s distortion perfor-
mance by preventing the DAC’s high slewing output from over-
loading the op amp’s input.
500
AD9764
225
I
OUTA 22
225
I
OUTB 21
C
OPT
500
25
25
AD8047
Figure 34. DC Differential Coupling Using an Op Amp
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 33. A
differentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s passband. An RF transformer such
as the Mini-Circuits T1-1T provides excellent rejection of com-
mon-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. Trans-
formers with different impedance ratios may also be used for
impedance matching purposes. Note that the transformer
provides ac coupling only.
MINI-CIRCUITS
T1-1T
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 from a
dual supply since its output is approximately
±
1.0 V. A high
speed amplifier capable of preserving the differential perform-
ance of the AD9764 while meeting other system level objectives
(i.e., cost, power) should be selected. The op amps differential
gain, its gain setting resistor values and full-scale output swing
capabilities should all be considered when optimizing this circuit.
The differential circuit shown in Figure 35 provides the neces-
sary level-shifting required in a single supply system. In this
case, AVDD, which is the positive analog supply for both the
AD9764 and the op amp, is also used to level-shift the differ-
ential output of the AD9764 to midsupply (i.e., AVDD/2). The
AD8041 is a suitable op amp for this application.
500
I
OUTA 22
AD9764
AD9764
I
OUTB 21
OPTIONAL R
DIFF
I
OUTB 21
C
OPT
25
25
R
LOAD
I
OUTA 22
225
AD8041
1k
AVDD
1k
225
Figure 33. 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 appear-
ing at I
OUTA
and I
OUTB
(i.e., V
OUTA
and V
OUTB
) swing sym-
metrically around ACOM and should be maintained with the
specified output compliance range of the AD9764. A differential
resistor, R
DIFF
, may be inserted in applications in which 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. Note that approxi-
mately half the signal power will be dissipated across R
DIFF
.
Figure 35. Single-Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 36 shows the AD9764 configured to provide a unipolar
output range of approximately 0 V to +0.5 V for a doubly termi-
nated 50
Ω
cable since the nominal full-scale current, I
OUTFS
, of
20 mA flows through the equivalent R
LOAD
of 25
Ω.
In this case,
R
LOAD
represents the equivalent load resistance seen by I
OUTA
or
I
OUTB
. The unused output (I
OUTA
or I
OUTB
) can be connected to
ACOM directly or via a matching R
LOAD
. Different values of
–14–
REV. B
AD [ ANALOG DEVICES ]
