AD9753AST datasheet(16/28 Pages) AD

16 / 28 page

REV. B

–16–

AD9753

and is insensitive to fCLOCK. Conversely, IDVDD is dependent on

both the digital input waveform, fCLOCK, and digital supply,

DVDD. Figure 18 shows IDVDD as a function of the ratio (fOUT/

fDAC) for various update rates. In addition, Figure 19 shows the

effect that the speed of fDAC has on the PLLVDD current, given

the PLL divider ratio.

IOUTFS (mA)

20.0

10.0

2.5

5.0

7.5

12.5

15.0

17.5

Figure 17. IAVDD vs. IOUTFS

RATIO

(fOUT /fDAC

0.01

0.001

0.1

300MSPS

200MSPS

100MSPS

50MSPS

25MSPS

)

Figure 18. IDVDD vs. fOUT/fDAC Ratio

fDAC (MHz)

300

150

100

200

250

175

125

225

275

DIV SETTING 00

DIV SETTING 11

DIV SETTING 10

DIV SETTING 01

Figure 19. PLLVDD vs. fDAC

APPLYING THE AD9753

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9753. 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, within the

bandwidth 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 an appropriately

sized load resistor, RLOAD, referred to ACOM. This configu-

ration 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 con-

verting 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 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 20. 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. When IOUTA and IOUTB are termi-

nated to ground with 50

Ω, this configuration provides 0 dBm

power to a 50

Ω load on the secondary with a DAC full-scale

current of 20 mA. A 2:1 transformer, such as the Coilcraft

WB2040-PC, can also be used in a configuration in which IOUTA

and IOUTB are terminated to ground with 75

Ω. This configura-

tion improves load matching and increases power to 2 dBm into

a 50

Ω load on the secondary. Transformers with different imped-

ance ratios may also be used for impedance matching purposes.

Note that the transformer provides ac coupling only.

RLOAD

AD9753

MINI-CIRCUITS

T1-1T

IOUTA

IOUTB

Figure 20. 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 AD9753. A differ-

ential resistor, RDIFF, may be inserted in applications where the

output of the transformer is connected to the load, RLOAD, via a

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