AD9760AR50 datasheet(16/23 Pages) AD

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AD9760

–16–

REV. B

APPLYING THE AD9760

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9760. 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 appropriately

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 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 50. 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 com-

mon-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

AD9760

MINI-CIRCUITS

T1-1T

OPTIONAL RDIFF

IOUTA

IOUTB

Figure 50. 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 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 AD9760. A differential resistor,

RDIFF, may be inserted in applications where the output of the

transformer is connected to the load, RLOAD, via a passive re-

construction 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 approxi-

mately 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 51. The AD9760 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.

AD9760

IOUTA

IOUTB 21

COPT

500

225

500

AD8047

Figure 51. 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 AD8047 is configured to provide

some additional signal gain. The op amp must operate off of a

dual supply since its output is approximately

±1.0 V. A high

speed amplifier capable of preserving the differential perfor-

mance of the AD9760 while meeting other system level objec-

tives (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 52 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

AD9760 and the op amp is also used to level-shift the differ-

ential output of the AD9760 to midsupply (i.e., AVDD/2). The

AD8041 is a suitable op amp for this application.

AD9760

IOUTA

IOUTB 21

COPT

500

225

AD8041

AVDD

Figure 52. Single-Supply DC Differential Coupled Circuit

SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT

Figure 53 shows the AD9760 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, IOUTFS, 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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