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AD9763/AD9765/AD9767

Data Sheet

Rev. G | Page 28 of 44

APPLYING THE AD9763/AD9765/AD9767

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configurations

for the AD9763/AD9765/AD9767, with IOUTFS set to a nominal

20 mA, unless otherwise noted. 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 as 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 configura-

tion 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 72 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

AD9763/

AD9765/

AD9767

IOUTA

IOUTB

Mini-Circuits

T1-1T

OPTIONAL

RDIFF

Figure 72. 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 must be maintained with the output compli-

ance range of the AD9763/AD9765/AD9767 to achieve the

specified performance. 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. 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 73 to perform a

differential-to-single-ended conversion. The AD9763/AD9765/

AD9767 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 often enhances the op amp’s

distortion performance by preventing the DAC’s high-slewing

output from overloading the op amp’s input.

AD9763/

AD9765/

AD9767

500Ω

225Ω

25Ω

AD8047

IOUTA

IOUTB

225Ω

COPT

Figure 73. 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.

Select a high speed amplifier capable of preserving the

differential performance of the AD9763/AD9765/AD9767

while meeting other system level objectives (that is, cost or

power). Consider the op amp’s differential gain, gain setting

resistor values, and full-scale output swing capabilities when

optimizing this circuit.

The differential circuit shown in Figure 74 provides the

necessary level shifting required in a single-supply system.

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

the AD9763/AD9765/AD9767 and the op amp, is used to level

shift the differential output of the AD9763/AD9765/AD9767 to

midsupply (that is, AVDD/2). The AD8055 is a suitable op amp

for this application.

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