AD9705 datasheet(40/42 Pages) AD

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AD9704/AD9705/AD9706/AD9707

Data Sheet

Rev. D | Page 40 of 42

APPLICATIONS INFORMATION

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output

configurations for the AD9704/AD9705/AD9706/AD9707.

Unless otherwise noted, it is assumed that IOUTFS is set to a

nominal 2 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, signal gain,

and/or a low output impedance.

A single-ended output is suitable for applications where low

cost and low power consumption are primary concerns.

DIFFERENTIAL COUPLING USING A TRANSFORMER

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

ended signal conversion, as shown in Figure 88. The distortion

performance of a transformer typically exceeds that available from

standard op amps, particularly at higher frequencies. Transformer

coupling provides excellent rejection of common-mode distortion

(that is, even-order harmonics) over a wide frequency range. It

also provides electrical isolation and can deliver voltage gain

without adding noise. Transformers with different impedance

ratios can also be used for impedance matching purposes. The

main disadvantages of transformer coupling are the low frequency

roll-off, lack of power gain, and the higher output impedance.

AD9704/AD9705

AD9706/AD9707

IOUTB

IOUTA

OPTIONAL RDIFF

RLOAD

Figure 88. Differential Output Using a Transformer

The center tap on the primary side of the transformer must be

connected to a voltage that keeps the voltages on IOUTA and

IOUTB within the output common voltage range of the device.

Note that the dc component of the DAC output current is equal

to IFS/2 and flows out of both IOUTA and IOUTB. The center

tap of the transformer should provide a path for this dc current.

In many applications, AGND provides the most convenient

voltage for the transformer center tap. The complementary

voltages appearing at IOUTA and IOUTB (that is, VIOUTA and

VIOUTB) swing symmetrically around AGND and should be

maintained with the specified output compliance range of the

AD9704/AD9705/AD9706/AD9707.

A differential resistor, RDIFF, can be inserted in applications

where the output of the transformer is connected to the load,

RLOAD, via a passive reconstruction filter or cable. RDIFF, as

reflected by the transformer, is chosen to provide a source

termination that results in a low VSWR. Note that approxi-

mately half the signal power is dissipated across RDIFF.

SINGLE-ENDED BUFFERED OUTPUT USING AN OP

AMP

An op amp, such as the ADA4899-1, can be used to perform a

single-ended current-to-voltage conversion, as shown in Figure 89.

The AD9704/AD9705/AD9706/AD9707 are configured with a

pair of series resistors, RS, off each output. The feedback resistor,

RFB, determines the peak signal swing by the following formula:

OUT

The common-mode voltage of the output is determined by the

following formula:

OUT

REF

−













The maximum and minimum voltages out of the amplifier are,

respectively, the following:













REF

MAX

VMIN

= VMAX − IFS × RFB

+5V

AD9704/AD9705

AD9706/AD9707

IOUTA

IOUTB 20

RFB

VOUT

REFIO 23

OTCM 19

–

ADA4899-1

–5V

Figure 89. Single-Supply Single-Ended Buffer

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