AD9754

–14–

REV. A

APPLYING THE AD9754

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9754. Unless otherwise noted, it is assumed

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 appropri-

figuration may be more suitable for a single-supply system

requiring a dc coupled, ground referred output voltage. Alterna-

tively, 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 dc linearity since IOUTA

or IOUTB is maintained at a virtual ground. Note, IOUTA

provides slightly better performance than IOUTB.

DIFFERENTIAL COUPLING USING A TRANSFORMER

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

single-ended signal conversion as shown in Figure 27. 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.

AD9754

22

21

MINI-CIRCUITS

T1-1T

IOUTA

IOUTB

Figure 27. 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

swing symmetrically around ACOM and should be maintained

with the specified output compliance range of the AD9754. A

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

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

DIFFERENTIAL USING AN OP AMP

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

ended conversion as shown in Figure 28. The AD9754 is con-

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 dis-

tortion performance by preventing the DAC’s high slewing

output from overloading the op amp’s input.

The common-mode rejection of this configuration is typically

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

ential op amp circuit 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 such

as the AD8055 or AD9632 capable of preserving the differential

AD9754

22

IOUTA

500

225

225

500

25

25

AD8055

Figure 28. DC Differential Coupling Using an Op Amp

performance of the AD9754 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 opti-

mizing this circuit.

The differential circuit shown in Figure 29 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

AD9754 and the op amp, is also used to level-shift the differ-

ential output of the AD9754 to midsupply (i.e., AVDD/2). The

AD8041 is a suitable op amp for this application.

AD9754

22

IOUTA

500

225

225

1k

25

25

AD8041

1k

AVDD

Figure 29. Single-Supply DC Differential Coupled Circuit