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AD9709

Rev. B | Page 19 of 32

APPLYING THE AD9709

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9709. Unless otherwise noted, it is assumed that

IOUTFS is set to a nominal 20 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 allowing for ac coupling. The differential op

amp configuration is suitable for applications requiring dc

coupling, bipolar output, 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 an appropriately sized load

resistor, RLOAD, referred to ACOM. This configuration may be

more suitable for a single-supply system requiring a dc-coupled,

ground-referred output voltage. Alternatively, an amplifier can be

configured as an I-V converter, thus converting IOUTA or IOUTB into a

negative unipolar voltage. This configuration provides the best dc

linearity because IOUTA or IOUTB is maintained at a virtual ground.

Note that IOUTA provides slightly better performance than IOUTB.

DIFFERENTIAL COUPLING USING A

TRANSFORMER

An RF transformer can be used as shown in Figure 36 to perform

a differential-to-single-ended signal conversion. A differentially

coupled transformer output provides the optimum distortion

performance for output signals whose spectral content lies within

the pass band of the transformer. 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. Note that the transformer provides ac

coupling only.

RLOAD

AD9709

IOUTA

IOUTB

Mini-Circuits

T1-1T

OPTIONAL

RDIFF

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 (that is, VOUTA and VOUTB) swing symmetrically

around ACOM and should be maintained with the specified

output compliance range of the AD9709. 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 approximately

half the signal power will be dissipated across RDIFF.

DIFFERENTIAL COUPLING USING AN OP AMP

An op amp can also be used as shown in Figure 37 to perform a

differential-to-single-ended conversion. The AD9709 is configured

with two equal load resistors, RLOAD, of 25 Ω each. 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 distortion performance by preventing the DAC’s high-

slewing output from overloading the op amp’s input.

AD9709

500

Ω

500

Ω

225

Ω

AD8047

IOUTA

IOUTB

225

Ω

COPT

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 differential

op amp circuit using the AD8047 is configured to provide some

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

supply because its output is approximately ±1.0 V. A high speed

amplifier capable of preserving the differential performance of

the AD9709 while meeting other system level objectives (that is,

cost and power) should be selected. The op amp’s differential

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

capabilities should be considered when optimizing this circuit.

The differential circuit shown in Figure 38 provides the

necessary level shifting required in a single-supply system. In

this case, AVDD, which is the positive analog supply for both

the AD9709 and the op amp, is used to level shift the differential

output of the AD9709 to midsupply (that is, AVDD/2). The

AD8041 is a suitable op amp for this application.

AD9709

500

Ω

500

Ω

225

Ω

AD8041

IOUTA

IOUTB

225

Ω

COPT

AVDD

Ω

Figure 38. Single-Supply DC Differential Coupled Circuit

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