AD9705BCPZ datasheet(40/52 Pages) AD

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AD9704/AD9705/AD9706/AD9707

Rev. 0 | Page 40 of 52

APPLICATIONS

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD970x. 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 90. 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 90. 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.

It should be noted 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 AD970x.

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 approximately half the signal

power is dissipated across RDIFF.

SINGLE-ENDED BUFFERED OUTPUT

USING AN OP AMP

An op amp can be used to perform a single-ended current-

to-voltage conversion, as shown in

Figure 91. u

. T

he AD970x is

configured with a pair of series resistors, RS, off each output.

The feedback resistor, RFB, determines the peak single-ended

output voltage by the formula

OUT

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

formula

OUT

REF

−

)

(

The maximum and minimum voltages out of the amplifier are

)

(

REF

MAX

MIN

−

respectively.

+5V

AD9704/AD9705

AD9706/AD9707

IOUTA

IOUTB 20

RFB

ADA4841

VOUT

REFIO 23

OTCM 19

Figure 91. Single-Supply Single-Ended Buffer

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