AD9708
should be considered. The necessity and value of this resistor
will be dependent upon the logic family used.
For a more detailed discussion of the implementation and
construction of high speed, mixed signal printed circuit boards,
refer to Analog Devices’ application notes AN-280 and AN-333.
DIFFERENTIAL OUTPUT CONFIGURATIONS
500
AD9708
IOUTA
22
225
225
IOUTB
21
C
OPT
500
25
25
AD8072
For applications requiring the optimum dynamic performance
and/or a bipolar output swing, a differential output configura-
tion is suggested. A differential output configuration may con-
sists of either an RF transformer or a differential op amp
configuration. The transformer configuration is well suited for
ac coupling applications. It provides the optimum high fre-
quency performance due to its 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 (i.e., assuming
no source termination). The differential op amp configuration is
suitable for applications requiring dc coupling, a bipolar output,
signal gain, and/or level shifting.
Figure 23 shows the AD9708 in a typical transformer coupled
output configuration. 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 complemen-
tary voltages appearing at IOUTA and IOUTB (i.e., V
OUTA
and
V
OUTB
) swing symmetrically around ACOM and should be
maintained within the specified output compliance range of the
AD9708. 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. Note that approxi-
mately half the signal power will be dissipated across R
DIFF
.
MINI-CIRCUITS
T1-1T
IOUTA
22
Figure 24. 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 differ-
ential op amp circuit is configured to provide some additional
signal gain. The op amp must operate off a dual supply since its
output is approximately
±
1.0 V. A high speed amplifier capable
of preserving the differential performance of the AD9708 while
meeting other system level objectives (i.e., cost, power) should
be selected. The op amps differential gain, its gain setting resis-
tor values and full-scale output swing capabilities should all be
considered when optimizing this circuit.
The differential circuit shown in Figure 25 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
AD9708 and the op amp, is also used to level-shift the differ-
ential output of the AD9762 to midsupply (i.e., AVDD/2).
500
AD9708
IOUTA
22
225
225
IOUTB
21
C
OPT
25
25
1k
AD8072
1k
AVDD
Figure 25. Single-Supply DC Differential Coupled Circuit
R
LOAD
AD9708
IOUTB
21
OPTIONAL R
DIFF
AD9708 EVALUATION BOARD
General Description
Figure 23. Differential Output Using a Transformer
An op amp can also be used to perform a differential to single-
ended conversion as shown in Figure 24. The AD9708 is
configured with two equal load resistors, R
LOAD
, of 25
Ω.
The
differential voltage 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 amps distortion
performance by preventing the DACs high slewing output from
overloading the op amp’s input.
The AD9708-EB is an evaluation board for the AD9708 8-bit
D/A converter. Careful attention to layout and circuit design,
combined with a prototyping area, allows the user to easily and
effectively evaluate the AD9708 in any application where high
resolution, high speed conversion is required.
This board allows the user the flexibility to operate the AD9708
in various configurations. Possible output configurations include
transformer coupled, resistor terminated, inverting/noninverting
and differential amplifier outputs. The digital inputs are
designed to be driven directly from various word generators,
with the on-board option to add a resistor network for proper
load termination. Provisions are also made to operate the
AD9708 with either the internal or external reference, or to
exercise the power-down feature.
Refer to the application note AN-420 “Using the AD9760/
AD9762/AD9764-EB Evaluation Board” for a thorough
description and operating instructions for the AD9708 evalua-
tion board.
REV. B
–11–
AD [ ANALOG DEVICES ]
