AD9761ARS datasheet(15/23 Pages) AD

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AD9761

–15–

REV. A

5 MSPS

RATIO – fOUT/fCLK

0.05

0.15

40 MSPS

0.1

20 MSPS

10 MSPS

0.2

2.5 MSPS

Figure 34. IDVDD vs. Ratio @ DVDD = 3 V

APPLYING THE AD9761

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9761. Unless otherwise noted, it is assumed

that IOUTFS is set to a nominal 10 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.

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 appropri-

ately 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. Alterna-

tively, 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 35. A differen-

tially coupled transformer output provides the optimum distortion

performance for output signals whose spectral content lies within

the transformers 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 cou-

pling only.

OPTIONAL

RDIFF

RLOAD

MINI-CIRCUITS

T1-1T

IOUTA

IOUTB

AD9761

Figure 35. 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 appear-

ing at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing sym-

metrically around ACOM and should be maintained with the

specified output compliance range of the AD9761. A differential

resistor, RDIFF, may be inserted in applications in which the

output of the transformer is connected to the load, RLOAD, via a

passive reconstruction filter or cable requiring double termina-

tion. RDIFF is determined by the transformer’s impedance ratio

and provides the proper source termination which results in a

low VSWR. Note that approximately half the signal power will

be dissipated across RDIFF.

DIFFERENTIAL USING AN OP AMP

An op amp can also be used to perform a differential to single-

ended conversion as shown in Figure 36. The AD9761 is config-

ured with two equal load resistors, RLOAD, of 50

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.

COPT

200

500

IOUTA

IOUTB

AD9761

RLOAD

200

AD8042

500

RLOAD

Figure 36. 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 differen-

tial op amp circuit using the AD8042 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 performance

of the AD9761 while meeting other system level objectives (i.e.,

cost, power) should be selected. The op amps differential gain, its

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

ties should all be considered when optimizing this circuit.

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