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AD9775

Rev. E | Page 42 of 56

APPLYING THE OUTPUT CONFIGURATIONS

The following sections illustrate typical output configurations

for the AD9775. Unless otherwise noted, it is assumed that

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

optimum dynamic performance, a differential output configu-

ration is suggested. A simple differential output may 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 configu-

ration 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 AD9775 and

the equivalent circuit. A typical application where this

information may be useful is when designing an interface filter

between the AD9775 and Analog Devices’ AD8345 quadrature

modulator.

IOUTA

IOUTB

VOUT+

VOUT

(DIFFERENTIAL)

VSOURCE =

IOUTFS × (RA + RB)

p-p

VOUT–

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

SOURCE

V p-p

100

OUT

Note that the output impedance of the AD9775 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 dif-

ferentially 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.

MINI-CIRCUITS

T1-1T

RLOAD

IOUTA

IOUTB

DAC

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 AD9775. 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 is determined by the

transformer’s impedance ratio and provides the proper source

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

mately half the signal power dissipates across RDIFF.

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