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AD9774

–16–

REV. B

APPLYING THE AD9774

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9774. Unless otherwise noted, it is assumed

that IOUTFS is set to a nominal 20 mA. For applications requir-

ing the optimum dynamic performance, a differential output

configuration is suggested. A differential output configuration

may consist of either an RF transformer or a differential op amp

configuration. The transformer configuration provides the opti-

mum high frequency performance and is recommended for any

application allowing for ac coupling. The differential op amp

configuration is suitable for applications requiring dc coupling, a

bipolar output, signal gain and/or level shifting.

A single-ended output is suitable for applications requiring a

unipolar voltage output. A positive unipolar output voltage will

result if IOUTA and/or IOUTB is connected to an approximately

sized load resistor, RLOAD, referred to ACOM. This configura-

tion may 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 convert-

ing IOUTA or IOUTB into a negative unipolar voltage. This

configuration provides the best dc linearity since 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 36. A

differentially coupled transformer output provides the optimum

distortion performance for output signals whose spectral content

lies within the transformer’s passband. An RF transformer such

as the Mini-Circuits T1-1T provides excellent rejection of

common-mode distortion (i.e., 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. Trans-

formers with different impedance ratios may also be used for

impedance matching purposes. Note that the transformer

provides ac coupling only.

RLOAD

AD9774

MINI-CIRCUITS

T1-1T

OPTIONAL RDIFF

IOUTA

IOUTB

Figure 36. Differential Output Using a Transformer

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

connected to ACOM to provide the necessary dc current path

for both IOUTA and IOUTB. The complementary voltages

appearing at IOUTA and IOUTB (i.e., VOUTA and VOUTB)

swing symmetrically around ACOM and should be maintained

with the specified output compliance range of the AD9774. A

differential resistor, RDIFF, may be inserted in applications in

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

RLOAD, via a passive reconstruction filter or cable. RDIFF is deter-

mined 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 will be dissipated across RDIFF.

DIFFERENTIAL USING AN OP AMP

An op amp can also be used to perform a differential-to-single-

ended conversion as shown in Figure 37. The AD9774 is

configured with two equal load resistors, RLOAD, of 25 Ω. The

differential voltage developed across IOUTA and IOUTB is

converted to a single-ended signal via the differential op amp

configuration. An optional capacitor can be installed across

IOUTA and IOUTB, forming a real pole in a low-pass filter.

The addition of this capacitor also enhances the op amp’s distor-

tion performance by preventing the DAC’s high slewing output

from overloading the op amp’s input.

AD9774

IOUTA

IOUTB 21

COPT

500

225

500

AD8055

Figure 37. DC Differential Coupling Using an Op Amp

The common-mode rejection of this configuration is typically

determined by the resistor matching. In this circuit, the differ-

ential op amp circuit using the AD8055 is configured to provide

some additional signal gain. The op amp must operate from a

dual supply since its output is approximately

±1.0 V. A high

speed amplifier capable of preserving the differential perform-

ance of the AD9774 while meeting other system level objectives

(i.e., cost, power) should be selected. The op amps differential

gain, its gain setting resistor values and full-scale output swing

capabilities should all be considered when optimizing this circuit.

The differential circuit shown in Figure 38 provides the neces-

sary level-shifting required in a single supply system. In this case,

AVDD, which is the positive analog supply for both the AD9774

and the op amp, is also used to level-shift the differential output

of the AD9774 to midsupply (i.e., AVDD/2). The AD8041 is a

suitable op amp for this application.

AD9774

IOUTA

IOUTB 21

COPT

500

225

AD8041

AVDD

Figure 38. Single-Supply DC Differential Coupled Circuit

SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT

Figure 39 shows the AD9774 configured to provide a unipolar

output range of approximately 0 V to +0.5 V for a doubly termi-

nated 50

Ω cable since the nominal full-scale current, I

OUTFS, of

20 mA flows through the equivalent RLOAD of 25 Ω. In this case,

RLOAD represents the equivalent load resistance seen by IOUTA.

The unused output (IOUTB) can be connected to ACOM di-

rectly. Different values of IOUTFS and RLOAD can be selected as

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