AD9763AST datasheet(14/28 Pages) AD

14 / 28 page

REV. B

AD9763

–14–

RATIO – fOUT/fCLK

0.10

0.20

0.30

0.40

0.50

125MSPS

100MSPS

65MSPS

25MSPS

5MSPS

Figure 32. IDVDD vs. Ratio @ DVDD = 3 V

APPLYING THE AD9763

Output Configurations

The following sections illustrate some typical output configura-

tions for the AD9763. 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

optimum 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,

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 convert-

ing IOUTA or IOUTB into a negative unipolar voltage. This con-

figuration provides the best dc linearity since IOUTA or IOUTB is

maintained at a virtual ground. Note that IOUTA provides slightly

better performance than IOUTB.

DIFFERENTIAL COUPLING USING A TRANSFORMER

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

single-ended signal conversion as shown in Figure 33. A

differentially coupled transformer output provides the opti-

mum distortion performance for output signals whose spectral

content lies within the transformer’s passband. An RF trans-

former such as the Mini-Circuits T1-1T provides excellent

rejection of common-mode distortion (i.e., even-order har-

monics) 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

AD9763

MINI-CIRCUITS

T1-1T

OPTIONAL

RDIFF

IOUTA

IOUTB

Figure 33. 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 symmetri-

cally around ACOM and should be maintained with the specified

output compliance range of the AD9763. A differential resistor,

RDIFF, may be inserted in applications where the output of the

transformer is connected to the load, RLOAD, via a passive recon-

struction 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 34. The AD9763 is con-

figured with two equal load resistors, RLOAD, of 25

Ω. The

differential voltage developed across IOUTA and IOUTB is con-

verted to a single-ended signal via the differential op amp con-

figuration. 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 perfor-

mance by preventing the DACs high slewing output from over-

loading the op amp’s input.

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 from a

dual supply since its output is approximately

±1.0 V. A high

speed amplifier capable of preserving the differential perfor-

mance of the AD9763, while meeting other system level

objectives (i.e., cost, power), should be selected. The op

amp’s differential gain, its gain setting resistor values, and

full-scale output swing capabilities should all be considered

when optimizing this circuit.

AD9763

IOUTA

IOUTB

500

225

500

AD8047

COPT

Figure 34. DC Differential Coupling Using an Op Amp

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