AD9744ARURL7 datasheet(17/32 Pages) AD

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AD9744

Rev. B | Page 17 of 32

100

150

fCLOCK (MSPS)

250

200

PECL

DIFF

Figure 33. ICLKVDD vs. fCLOCK and Clock Mode

APPLYING THE AD9744

Output Configurations

The following sections illustrate some typical output configura-

tions for the AD9744. 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 configu-

ration is suggested. A differential output configuration may

consist of either an RF transformer or a differential op amp

configuration. The transformer configuration provides the

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

results if IOUTA and/or IOUTB are connected to an appro-

priately 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. Alter-

natively, 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 TRANS-

FORMER

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

single-ended signal conversion, as shown in Figure 34. A

differentially coupled transformer output provides the optimum

distortion performance for output signals whose spectral con-

tent 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

AD9744

MINI-CIRCUITS

T1-1T

OPTIONAL RDIFF

IOUTA

IOUTB

Figure 34. 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 ap-

pearing at IOUTA and IOUTB (that is, VOUTA and VOUTB) swing

symmetrically around ACOM and should be maintained with

the specified output compliance range of the AD9744. A differ-

ential 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 approxi-

mately 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 35. The AD9744 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.

AD9744

IOUTA

IOUTB

COPT

500

Ω

225

Ω

225

Ω

500

Ω

AD8047

Figure 35. 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 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 am-

plifier capable of preserving the differential performance of the

AD9744 while meeting other system level objectives (such as,

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

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

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