AD9767 datasheet(14/27 Pages) AD

14 / 27 page

REV. B

AD9767

–14–

APPLYING THE AD9767

Output Configurations

The following sections illustrate some typical output configura-

tions for the AD9767. 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, 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 will

result 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 could be configured as an I-V converter, thus convert-

ing IOUTA or IOUTB into a negative unipolar voltage. This con-

figuration provides the best dc linearity since 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 to perform a differential-to-

single-ended signal conversion as shown in Figure 33. A dif-

ferentially coupled transformer output provides the optimum

distortion performance for output signals whose spectral con-

tent 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.

Transformers with different impedance ratios may also be used

for impedance matching purposes. Note that the transformer

provides ac coupling only.

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 appear-

ing at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing symmetri-

cally around ACOM and should be maintained with the specified

output compliance range of the AD9767. A differential resistor,

RDIFF, may 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 trans-

former’s impedance ratio and provides the proper source termi-

nation that results in a low VSWR. Note that approximately half

the signal power will be dissipated across RDIFF.

RLOAD

AD9767

MINI-CIRCUITS

T1-1T

OPTIONAL

RDIFF

IOUTA

IOUTB

Figure 33. Differential Output Using a Transformer

DIFFERENTIAL COUPLING USING AN OP AMP

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

ended conversion as shown in Figure 34. The AD9767 is con-

figured with two equal load resistors, RLOAD, of 25

Ω. The

differential voltage developed across IOUTA and IOUTB is con-

verted to a single-ended signal via the differential op amp con-

figuration. 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 amps distortion perfor-

mance by preventing the DACs high slewing output from over-

loading 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 using the AD8047 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 perfor-

mance of the AD9767, while meeting other system level

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

amp’s differential gain, its gain setting resistor values, and

full-scale output swing capabilities should all be considered

when optimizing this circuit.

AD9767

IOUTA

IOUTB

500

225

500

AD8047

COPT

Figure 34. DC Differential Coupling Using an Op Amp

The differential circuit shown in Figure 35 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 AD9767

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

of the AD9767 to midsupply (i.e., AVDD/2). The AD8055 is a

suitable op amp for this application.

AD9767

IOUTA

IOUTB

COPT

500

225

500

AD8055

AVDD

Figure 35. Single Supply DC Differential Coupled Circuit

SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT

Figure 36 shows the AD9767 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 or

IOUTB. The unused output (IOUTA or IOUTB) can be connected to

ACOM directly or via a matching RLOAD. Different values of

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