AD9773-EB datasheet(46/60 Pages) AD

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AD9773

Rev. D | Page 46 of 60

APPLYING THE OUTPUT CONFIGURATIONS

The following sections illustrate typical output configurations

for the AD9773. Unless otherwise noted, it is assumed that

IOUTFS is set to a nominal 20 mA. For applications requiring

optimum dynamic performance, a differential output

configuration is suggested. A simple differential output can be

achieved by converting IOUTA and IOUTB to a voltage output by

terminating them to AGND via equal value resistors. This type of

configuration may be useful when driving a differential voltage

input device such as a modulator. If a conversion to a single-

ended signal is desired and the application allows for ac coupling,

an RF transformer may be useful, or if power gain is required, an

op amp may be used. The transformer configuration provides

optimum high frequency noise and distortion performance. The

differential op amp configuration is suitable for applications

requiring dc coupling, 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 a load resistor, RLOAD,

referred to AGND. This configuration is most 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

DAC dc linearity as IOUTA or IOUTB are maintained at ground or

virtual ground.

UNBUFFERED DIFFERENTIAL OUTPUT,

EQUIVALENT CIRCUIT

In many applications, it may be necessary to understand the

equivalent DAC output circuit. This is especially useful when

designing output filters or when driving inputs with finite input

impedances. Figure 97 illustrates the output of the AD9773 and

the equivalent circuit. A typical application where this information

may be useful is when designing an interface filter between

the AD9773 and the Analog Devices AD8345 quadrature

modulator.

IOUTA

IOUTB

AD9773

VOUT+

VOUT–

VOUT

(DIFFERENTIAL)

VSOURCE =

IOUTFS × (RA + RB)

p-p

RA + RB

Figure 97. DAC Output Equivalent Circuit

For the typical situation, where IOUTFS = 20 mA and RA and RB

both equal 50 Ω, the equivalent circuit values become

VSOURCE

= 2 V p-p

ROUT

= 100 Ω

Note that the output impedance of the AD9773 DAC itself

is greater than 100 kΩ and typically has no effect on the

impedance of the equivalent output circuit.

DIFFERENTIAL COUPLING USING A

TRANSFORMER

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

single-ended signal conversion, as shown in Figure 98. 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 may also be used for impedance

matching purposes.

IOUTA

IOUTB

DAC

RLOAD

MINI-CIRCUITS

T1-1T

Figure 98. Transformer-Coupled Output Circuit

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

connected to AGND to provide the necessary dc current path

for both IOUTA and IOUTB. The complementary voltages appearing

at IOUTA and IOUTB (that is, VOUTA and VOUTB) swing symmetrically

around AGND and should be maintained within the specified

output compliance range of the AD9773. A differential resistor,

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

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

reconstruction 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 dissipates across RDIFF.

AD9773-EB Datasheet(PDF) 46 Page - Analog Devices

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