AD9750 Просмотр технического описания (PDF) - Analog Devices

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AD9750

10-Bit, 125 MSPS High Performance TxDAC® D/A Converter

Analog Devices 

POWER DISSIPATION

The power dissipation, PD, of the AD9750 is dependent on

several factors which include: (1) AVDD and DVDD, the

power supply voltages; (2) IOUTFS, the full-scale current output;

(3) fCLOCK, the update rate; (4) and the reconstructed digital

input waveform. The power dissipation is directly proportional

to the analog supply current, IAVDD, and the digital supply cur-

rent, IDVDD. IAVDD is directly proportional to IOUTFS as shown in

Figure 25 and is insensitive to fCLOCK.

Conversely, IDVDD is dependent on both the digital input wave-

form, fCLOCK, and digital supply DVDD. Figures 26 and 27

show IDVDD as a function of full-scale sine wave output ratios

(fOUT/fCLOCK) for various update rates with DVDD = 5 V and

DVDD = 3 V, respectively. Note, how IDVDD is reduced by more

than a factor of 2 when DVDD is reduced from 5 V to 3 V.

35

30

25

20

15

10

5

2

4

6

8 10 12 14 16 18 20

IOUTFS – mA

Figure 25. IAVDD vs. IOUTFS

18

125MSPS

16

14

100MSPS

12

10

8

50MSPS

6

4

2

0

0.01

0.1

RATIO (fCLOCK/fOUT)

25MSPS

5MSPS

1

Figure 26. IDVDD vs. Ratio @ DVDD = 5 V

AD9750

8

125MSPS

6

100MSPS

4

50MSPS

2

25MSPS

0

0.01

0.1

RATIO (fCLOCK/fOUT)

5MSPS

1

Figure 27. IDVDD vs. Ratio @ DVDD = 3 V

APPLYING THE AD9750

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configura-

tions for the AD9750. 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.

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 appropri-

ately sized load resistor, RLOAD, referred to ACOM. This con-

figuration may be more suitable for a single-supply system

requiring a dc coupled, ground referred output voltage. Alterna-

tively, 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, 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 28. 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.

REV. 0

–13–

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