AD9742ARURL7 datasheet(16/32 Pages) AD

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AD9742

Data Sheet

Rev. C | Page 16 of 32

APPLYING THE AD9742

Output Configurations

The following sections illustrate some typical output configurations

for the AD9742. Unless otherwise noted, it is assumed that IOUTFS is

set to a nominal 20 mA. For applications requiring 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 optimum high frequency

performance and is recommended for any application that allows

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.

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 are 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 converting

IOUTA or IOUTB into a negative unipolar voltage. This

configuration provides the best dc linearity since IOUTA or

IOUTB is maintained at a virtual ground.

DIFFERENTIAL COUPLING USING A TRANSFORMER

An RF transformer can be used to perform a differential-to-single-

ended signal conversion, as shown in Figure 31. A differentially

coupled transformer output provides the optimum distortion

performance for output signals whose spectral content lies within

the transformer’s pass band. 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 may also be used for impedance matching

purposes. Note that the transformer provides ac coupling only.

RLOAD

AD9742

MINI-CIRCUITS

T1-1T

OPTIONAL RDIFF

IOUTA

IOUTB

Figure 31. 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 IOUTA and IOUTB. The complementary voltages

appearing at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing

symmetrically around ACOM and should be maintained with the

specified output compliance range of the AD9742. 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

transformer’s impedance ratio and provides the proper source

termination that results in a low VSWR. Note that approximately

half the signal power will be dissipated across RDIFF.

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 32. The AD9742 is configured

with two equal load resistors, RLOAD, 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 amp’s distortion performance by

preventing the DAC’s high slewing output from overloading the

op amp’s input.

AD9742

IOUTA

IOUTB

COPT

500

Ω

225

Ω

225

Ω

500

Ω

AD8047

Figure 32. 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

off a dual supply since its output is approximately ±1 V. A high

speed amplifier capable of preserving the differential performance

of the AD9742 while meeting other system level objectives (e.g.,

cost or power) should be selected. The op amp’s differential gain,

gain setting resistor values, and full-scale output swing capabilities

should all be considered when optimizing this circuit.

The differential circuit shown in Figure 33 provides the necessary

level shifting required in a single-supply system. In this case,

AVDD, which is the positive analog supply for both the AD9742

and the op amp, is also used to level shift the differential output

of the AD9742 to midsupply (i.e., AVDD/2). The AD8041 is a

suitable op amp for this application.

AD9742

IOUTA

IOUTB

COPT

500

Ω

225

Ω

225

Ω

AD8041

Ω

AVDD

Figure 33. Single-Supply DC Differential Coupled Circuit

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