Datasheet

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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 I

OUTFS

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 I

OUTA

and/or I

OUTB

is connected to an appropriately

sized load resistor, R

LOAD

, 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 I

OUTA

or I

OUTB

into a negative unipolar voltage. This con-

figuration provides the best dc linearity since I

OUTA

or I

OUTB

maintained at a virtual ground. Note that I

OUTA

provides slightly

better performance than I

OUTB

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.

LOAD

AD9760

MINI-CIRCUITS

T1-1T

OPTIONAL R

DIFF

OUTA

OUTB

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 I

OUTA

and I

OUTB

. The complementary voltages appear-

ing at I

OUTA

and I

OUTB

(i.e., V

OUTA

and V

OUTB

) swing symmetri-

cally around ACOM and should be maintained with the specified

output compliance range of the AD9760. A differential resistor,

DIFF

, may be inserted in applications where the output of the

transformer is connected to the load, R

LOAD

, via a passive re-

construction filter or cable. R

DIFF

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 R

DIFF

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, R

LOAD

, of 25 Ω. The

differential voltage developed across I

OUTA

and I

OUTB

is con-

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

figuration. An optional capacitor can be installed across I

OUTA

and I

OUTB

, 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

OUTA

OUTB

OPT

500⍀

225⍀

500⍀

25⍀25⍀

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

OUTA

OUTB

OPT

500⍀

225⍀

1k⍀

25⍀25⍀

AD8041

1k⍀

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, I

OUTFS

, of

20 mA flows through the equivalent R

LOAD

of 25 Ω. In this case,

LOAD

represents the equivalent load resistance seen by I

OUTA

OUTB

. The unused output (I

OUTA

or I

OUTB

) can be connected

to ACOM directly or via a matching R

LOAD

. Different values of