AD9775EB datasheet(37/48 Pages) AD

37 / 48 page

REV. 0

AD9775

–37–

APPLYING THE AD9775 OUTPUT CONFIGURATIONS

The following sections illustrate typical output configurations for

the AD9775. Unless otherwise noted, it is assumed that IOUTFS

is set to a nominal 20 mA. For applications requiring optimum

dynamic performance, a differential output configuration is

suggested. A simple differential output may be achieved by con-

verting IOUTA and IOUTB to a voltage output by terminating

them to AGND via equal value resistors. This type of configura-

tion may be useful when driving a differential voltage input

device such as a modulator. If a conversion to a single-ended

signal is desired and the application allows for ac-coupling, an RF

transformer may be useful, or if power gain is required, an op amp

may be used. The transformer configuration provides optimum

high frequency noise and distortion performance. The differen-

tial op amp configuration is suitable for applications requiring

dc-coupling, signal gain, and/or level shifting within the band-

width 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 a load resistor,

RLOAD, referred to AGND. This configuration is most 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 nega-

tive unipolar voltage. This configuration provides the best DAC

dc linearity as IOUTA or IOUTB are maintained at ground or vir-

tual ground.

UNBUFFERED DIFFERENTIAL OUTPUT, EQUIVALENT

CIRCUIT

In many applications, it may be necessary to understand the

equivalent DAC output circuit. This is especially useful when

designing output filters or when driving inputs with finite input

impedances. Figure 51 illustrates the output of the AD9775 and

the equivalent circuit. A typical application where this information

may be useful is when designing an interface filter between the

AD9775 and Analog Devices’ AD8345 quadrature modulator.

IOUTA

RA + RB

IOUTB

VOUT

(DIFFERENTIAL)

VSOURCE =

IOUTFS

(RA + RB)

p-p

VOUT+

VOUT–

Figure 51. DAC Output Equivalent Circuit

For the typical situation, where IOUTFS = 20 mA and RA and RB

both equal 50

Ω, the equivalent circuit values become:

VSOURCE = 2 V p-p

ROUT = 100

Ω

Note that the output impedance of the AD9775 DAC itself is

greater than 100 k

Ω and typically has no effect on the impedance

of the equivalent output circuit.

DIFFERENTIAL COUPLING USING A TRANSFORMER

An RF transformer can be used to perform a differential-

to-single-ended signal conversion as shown in Figure 52. A

differentially coupled transformer output provides the optimum

distortion performance for output signals whose spectral content

lies within the transformer’s pass band. 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. Trans-

formers with different impedance ratios may also be used for

impedance matching purposes.

IOUTA

IOUTB

DAC

RLOAD

MINI-CIRCUITS

T1-T2

Figure 52. Transformer-Coupled Output Circuit

The center tap on the primary side of the transformer must be

connected to AGND 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 AGND and should be maintained within the specified

output compliance range of the AD9775. A differential resistor,

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

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

struction 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 53. This has the added

benefit of providing signal gain as well. In Figure 53, the AD9775

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 configura-

tion. 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 amp’s distortion performance

by preventing the DAC’s fast slewing output from overloading

the input of the op amp.

IOUTA

IOUTB

DAC

AD8021

500

COPT

225

500

ROPT

225

AVDD

Figure 53. Op Amp-Coupled Output Circuit

The common-mode (and second order distortion) rejection of this

configuration is typically determined by the resistor matching.

The op amp used must operate from a dual supply since its

output is approximately

±1.0 V. A high speed amplifier, such as

the AD8021, capable of preserving the differential performance

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