AD9709AST datasheet(14/27 Pages) AD

14 / 27 page

REV. 0

AD9709

–14–

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 converting

IOUTA or IOUTB into a negative unipolar voltage. This conﬁguration

provides the best dc linearity since IOUTA or IOUTB is maintained at

a virtual ground. Note that IOUTA provides slightly better perfor-

mance 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

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

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.

RLOAD

AD9709

MINI-CIRCUITS

T1-1T

OPTIONAL

RDIFF

IOUTA

IOUTB

Figure 33. 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 (i.e., VOUTA and VOUTB) swing symmetrically

around ACOM and should be maintained with the speciﬁed

output compliance range of the AD9709. 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

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 to perform a differential to single-

ended conversion as shown in Figure 34. The AD9709 is

conﬁgured with two equal load resistors, RLOAD, of 25

Ω. 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 amps distortion performance by

preventing the DACs high-slewing output from overloading the

op amp’s input.

The common-mode rejection of this conﬁguration is typically

determined by the resistor matching. In this circuit, the differen-

tial 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 performance

of the AD9709 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 capa-

bilities should all be considered when optimizing this circuit.

The differential circuit shown in Figure 35 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 also used to level-shift the differential output

of the AD9709 to midsupply (i.e., AVDD/2). The AD8041 is a

suitable op amp for this application.

SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT

Figure 36 shows the AD9709 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

IOUTFS and RLOAD can be selected as long as the positive compli-

ance range is adhered to. One additional consideration in this

mode is the integral nonlinearity (INL) as discussed in the

Analog Output section of this data sheet. For optimum INL

performance, the single-ended, buffered voltage output conﬁgu-

ration is suggested.

AD9709

500

225

500

AD8047

COPT

IOUTA

IOUTB

Figure 34. DC Differential Coupling Using an Op Amp

AD9709

IOUTA

IOUTB

COPT

500

225

500

AD8041

AVDD

Figure 35. Single Supply DC Differential Coupled Circuit

AD9709

IOUTA

IOUTB

Figure 36. 0 V to 0.5 V Unbuffered Voltage Output

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