AD9740-EB datasheet(17/32 Pages) AD

17 / 32 page

AD9740

Rev. B | Page 17 of 32

IOUTFS (mA)

121416

1820

Figure 29. IAVDD vs. IOUTFS

RATIO (fOUT/fCLOCK)

0.01

0.1

165MSPS

210MSPS

65MSPS

125MSPS

Figure 30. IDVDD vs. Ratio @ DVDD = 3.3 V

fCLOCK (MSPS)

150

100

200

250

PECL

DIFF

Figure 31. ICLKVDD vs. fCLOCK and Clock Mode

APPLYING THE AD9740

Output Configurations

The following sections illustrate some typical output

configurations for the AD9740. 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 configuration is suggested. A differential output

configuration can 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, 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 is connected to an

appropriately sized load resistor, RLOAD, referred to ACOM.

This configuration can 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

because 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 32. A

differentially coupled transformer output provides the

optimum distortion performance 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 (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

can also be used for impedance matching purposes. Note that

the transformer provides ac coupling only.

RLOAD

AD9740

MINI-CIRCUITS

T1-1T

OPTIONAL RDIFF

IOUTA

IOUTB

Figure 32. Differential Output Using a Transformer

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