AD9765AST(RevB) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD9765AST
(Rev.:RevB)

12-Bit, 125 MSPS Dual TxDAC+® D/A Converter

Analog Devices 

AD9765

18

16

125MSPS

14

100MSPS

12

10

65MSPS

8

6

25MSPS

4

5MSPS

2

0

0

0.10

0.20

0.30

0.40

0.50

RATIO – fOUT/fCLK

Figure 32. IDVDD vs. Ratio @ DVDD = 3 V

APPLYING THE AD9765

Output Configurations

The following sections illustrate some typical output configura-

tions for the AD9765. 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

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, 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 configura-

tion may be more suitable for a single-supply system requir-

ing 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 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 33. A

differentially coupled transformer output provides the opti-

mum distortion performance for output signals whose spectral

content lies within the transformer’s passband. An RF trans-

former such as the Mini-Circuits T1-1T provides excellent

rejection of common-mode distortion (i.e., even-order harmon-

ics) 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 trans-

former provides ac coupling only.

AD9765

IOUTA

MINI-CIRCUITS

T1-1T

RLOAD

IOUTB

OPTIONAL

RDIFF

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 appear-

ing at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing symmetri-

cally around ACOM and should be maintained with the specified

output compliance range of the AD9765. 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 34. The AD9765 is con-

figured with two equal load resistors, RLOAD, of 25 Ω. The

differential voltage developed across IOUTA and IOUTB is con-

verted to a single-ended signal via the differential op amp con-

figuration. 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 perfor-

mance by preventing the DACs high slewing output from over-

loading the op amp’s input.

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 AD8047 is configured to provide

some additional signal gain. The op amp must operate from a

dual supply since its output is approximately ± 1.0 V. A high

speed amplifier capable of preserving the differential perfor-

mance of the AD9765 while meeting other system level ob-

jectives (i.e., cost, power) should be selected. The op amp’s

differential gain, its gain setting resistor values, and full-scale

output swing capabilities should all be considered when opti-

mizing this circuit.

–14–

REV. B

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