AD9742 datasheet(12/20 Pages) AD

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REV. 0

AD9742

–12–

IOUTFS – mA

468

Figure 8. IAVDD vs. IOUTFS

RATIO –

fOUT/fCLOCK

0.01

0.1

165MSPS

125MSPS

65MSPS

Figure 9. IDVDD vs. Ratio @ DVDD = 3.3 V

APPLYING THE AD9742

Output Configurations

The following sections illustrate some typical output configura-

tions for the AD9742. Unless otherwise noted, it is assumed

that IOUTFS is set to a nominal 20 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 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 is 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 10. A differentially

coupled transformer output provides the optimum distortion per-

formance 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 (i.e., even-order harmonics) and noise over a wide fre-

quency 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 pur-

poses. Note that the transformer provides ac coupling only.

RLOAD

AD9742

MINI-CIRCUITS

T1-1T

OPTIONAL RDIFF

IOUTA

IOUTB

Figure 10. 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 11. The AD9742 is configured with

two equal load resistors, RLOAD, of 25

W. The differential voltage

developed across IOUTA and IOUTB is converted to a single-ended

signal via the differential op amp configuration. An optional capaci-

tor 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 DACs high

slewing output from overloading the op amp’s input.

AD9742

IOUTA

IOUTB

COPT

500

225

500

AD8047

Figure 11. 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 of a dual

supply since its output is approximately

±1.0 V. A high-speed

amplifier capable of preserving the differential performance of the

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