AD9708

–11–

REV. B

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

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-

maintained within the specified output compliance range of the

applications in which the output of the transformer is connected

provides the proper source termination. Note that approxi-

AD9708

22

21

MINI-CIRCUITS

T1-1T

IOUTA

IOUTB

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

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.

AD9708

22

IOUTA

500

225

225

500

25

25

AD8072

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).

AD9708

22

IOUTA

500

225

225

1k

25

25

AD8072

1k

AVDD

Figure 25. Single-Supply DC Differential Coupled Circuit

AD9708 EVALUATION BOARD

General Description

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.