AD9772EB datasheet(19/30 Pages) AD

19 / 30 page

REV. 0

AD9772

–19–

RATIO – fOUT/fDATA

120

0.45

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

100

fDATA = 25MSPS

fDATA = 65MSPS

fDATA = 150MSPS

fDATA = 125MSPS

fDATA = 100MSPS

fDATA = 50MSPS

Figure 39. IDVDD vs. Ratio @ DVDD = 3 V

fDATA – MSPS

160

100

120

140

ICLKVDD

IPLLVDD

Figure 40. IPLLVDD and ICLKVDD vs. fDATA

APPLYING THE AD9772

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9772. Unless otherwise noted, it is assumed

that IOUTFS is set to a nominal 20 mA for optimum performance.

For applications requiring the optimum dynamic performance, a

differential output configuration is highly recommended. A

differential output configuration may consist of either an RF

transformer or a differential op amp configuration. The trans-

former configuration provides the optimum 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

configuration provides the best dc linearity since IOUTA or

IOUTB is maintained at a virtual ground.

DIFFERENTIAL COUPLING USING A TRANSFORMER

An RF transformer can be used to perform a differential-to-

single-ended signal conversion as shown in Figure 41. 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. Trans-

formers with different impedance ratios may also be used for

impedance matching purposes. Note that the transformer

provides ac coupling only and its linearity performance degrades

at the low end of its frequency range due to core saturation.

OPTIONAL

RDIFF

RLOAD

MINI-CIRCUITS

T1-1T

AD9772

IOUTA

IOUTB

Figure 41. 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 AD9772. A

differential resistor, RDIFF, may be inserted in applications in

which the output of the transformer is connected to the load,

RLOAD, via a passive reconstruction filter or cable. RDIFF is deter-

mined by the transformer’s impedance ratio and provides the

proper source termination that results in a low VSWR (Voltage

Standing Wave Ratio). 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 42. The AD9772 is con-

figured 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 amp’s dis-

tortion performance by preventing the DAC’s high slewing

output from overloading the op amp’s input.

AD9772

IOUTA

IOUTB

AD8055

COPT

225

500

Figure 42. 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 AD8055 is configured to provide

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