AD9751AST datasheet(15/26 Pages) AD

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REV. 0

AD9751

–15–

IOUTFS – mA

2.5

7.5

12.5

17.5

Figure 17. IAVDD vs. IOUTFS

RATIO – fOUT/fDAC

0.01

0.001

0.1

300MSPS

200MSPS

100MSPS

50MSPS

25MSPS

Figure 18. IDVDD vs. fOUT/fDAC Ratio

fDAC – MHz

300

150

100

200

250

175

125

225

275

DIV SETTING 00

DIV SETTING 11

DIV SETTING 10

DIV SETTING 01

Figure 19. PLLVDD vs. fDAC

APPLYING THE AD9751

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9751. 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 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 configura-

tion 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. Note that IOUTA provides slightly better per-

formance 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 20. A

differentially-coupled transformer output provides the optimum

distortion performance for output signals whose spectral content

lies within the transformer’s 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. When IOUTA and IOUTB are

terminated to ground with 50

Ω, this configuration provides

0 dBm power to a 50

Ω load on the secondary with a DAC full-

scale current of 20 mA. A 2:1 transformer such as the Coilcraft

WB2040-PC can also be used in a configuration in which IOUTA

and IOUTB are terminated to ground with 75

Ω. This configura-

tion improves load matching and increases power to 2 dBm into

a 50

Ω load on the secondary. Transformers with different

impedance ratios may also be used for impedance matching

purposes. Note that the transformer provides ac-coupling only.

RLOAD

AD9751

MINI-CIRCUITS

T1-1T

IOUTA

IOUTB

Figure 20. Differential Output Using a Transformer

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