AD9767
APPLYING THE AD9767
Output Configurations
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 34. The AD9767 is con-
figured with two equal load resistors, RLOAD, of 25 Ω. The
differential voltage developed across IOUTA and IOUTB is con-
verted to a single-ended signal via the differential op amp con-
figuration. 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 amps distortion perfor-
mance by preventing the DACs high slewing output from over-
loading the op amp’s input.
The following sections illustrate some typical output configura-
tions for the AD9767. Unless otherwise noted, it is assumed
that IOUTFS 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, within the
bandwidth of the chosen op amp.
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 perfor-
mance of the AD9767, while meeting other system level
objectives (i.e., cost, power), should be selected. The op
amp’s differential gain, its gain setting resistor values, and
full-scale output swing capabilities should all be considered
when optimizing this circuit.
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, RLOAD, referred to ACOM. This configuration
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 IOUTA or IOUTB into a negative unipolar voltage. This con-
figuration provides the best dc linearity since IOUTA or IOUTB
is maintained at a virtual ground. Note that IOUTA provides
500⍀
slightly better performance than IOUTB
.
AD9767
225⍀
I
OUTA
AD8047
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 33. A dif-
ferentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral con-
tent lies within the transformer’s passband. 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.
225⍀
I
OUTB
C
OPT
500⍀
25⍀
25⍀
Figure 34. DC Differential Coupling Using an Op Amp
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 AD9767
and the op amp, is also used to level-shift the differential output
of the AD9767 to midsupply (i.e., AVDD/2). The AD8055 is a
suitable op amp for this application.
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 appear-
ing at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing symmetri-
cally around ACOM and should be maintained with the specified
output compliance range of the AD9767. A differential resistor,
RDIFF, may be inserted in applications where the output of the
transformer is connected to the load, RLOAD, via a passive
reconstruction filter or cable. RDIFF is determined by the trans-
former’s impedance ratio and provides the proper source termi-
nation that results in a low VSWR. Note that approximately half
500⍀
AD9767
225⍀
I
OUTA
AD8055
1k⍀
225⍀
I
OUTB
C
OPT
AVDD
25⍀
500⍀
25⍀
the signal power will be dissipated across RDIFF
.
Figure 35. Single Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 36 shows the AD9767 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, IOUTFS, of
20 mA flows through the equivalent RLOAD of 25 Ω. In this case,
RLOAD represents the equivalent load resistance seen by IOUTA or
IOUTB. The unused output (IOUTA or IOUTB) can be connected to
ACOM directly or via a matching RLOAD. Different values of
MINI-CIRCUITS
AD9767
T1-1T
I
OUTA
R
LOAD
I
OUTB
OPTIONAL
DIFF
R
Figure 33. Differential Output Using a Transformer
–14–
REV. B
ADI [ ADI ]
