AD9760 datasheet(17/23 Pages) AD

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AD9760

–17–

REV. B

IOUTFS and RLOAD can be selected as long as the positive compli-

ance range is adhered to. One additional consideration in this

mode is the integral nonlinearity (INL) as discussed in the Ana-

log Output section of this data sheet. For optimum INL perfor-

mance, the single-ended, buffered voltage output configuration

is suggested.

AD9760

IOUTA

IOUTB 21

VOUTA = 0 TO +0.5V

IOUTFS = 20mA

Figure 53. 0 V to +0.5 V Unbuffered Voltage Output

SINGLE-ENDED, BUFFERED VOLTAGE OUTPUT

CONFIGURATION

Figure 54 shows a buffered single-ended output configuration

in which the op amp U1 performs an I-V conversion on the

AD9760 output current. U1 maintains IOUTA (or IOUTB) at a

virtual ground, thus minimizing the nonlinear output impedance

effect on the DAC’s INL performance as discussed in the Ana-

log Output section. Although this single-ended configuration

typically provides the best dc linearity performance, its ac distor-

tion performance at higher DAC update rates may be limited by

U1’s slewing capabilities. U1 provides a negative unipolar out-

put voltage and its full-scale output voltage is simply the product

of RFB and IOUTFS. The full-scale output should be set within

U1’s voltage output swing capabilities by scaling IOUTFS and/or

RFB. An improvement in ac distortion performance may result

with a reduced IOUTFS since the signal current U1 will be required

to sink will be subsequently reduced.

AD9760

IOUTA

IOUTB 21

COPT

200

VOUT = IOUTFS

RFB

IOUTFS = 10mA

RFB

200

Figure 54. Unipolar Buffered Voltage Output

POWER AND GROUNDING CONSIDERATIONS

In systems seeking to simultaneously achieve high speed and

high performance, the implementation and construction of the

printed circuit board design is often as important as the circuit

design. Proper RF techniques must be used in device selection,

placement and routing, and supply bypassing and grounding.

The evaluation board for the AD9760, which uses a four-layer

PC board, serves as a good example for the above-mentioned

considerations. Figures 60–65 illustrate the recommended

printed circuit board ground, power and signal plane layouts

that are implemented on the AD9760 evaluation board.

Proper grounding and decoupling should be a primary objective

in any high speed, high resolution system. The AD9760 features

separate analog and digital supply and ground pins to optimize

the management of analog and digital ground currents in a

system. In general, AVDD, the analog supply, should be de-

coupled to ACOM, the analog common, as close to the chip as

physically possible. Similarly, DVDD, the digital supply, should

be decoupled to DCOM as close as physically possible.

For those applications that require a single +5 V or +3 V supply

for both the analog and digital supply, a clean analog supply

may be generated using the circuit shown in Figure 55. The

circuit consists of a differential LC filter with separate power

supply and return lines. Lower noise can be attained using low

ESR type electrolytic and tantalum capacitors.

100 F

ELECT.

10-22 F

TANT.

0.1 F

CER.

TTL/CMOS

LOGIC

CIRCUITS

+5V OR +3V

POWER SUPPLY

FERRITE

BEADS

AVDD

ACOM

Figure 55. Differential LC Filter for Single +5 V or +3 V

Applications

Maintaining low noise on power supplies and ground is critical

to obtain optimum results from the AD9760. If properly imple-

mented, ground planes can perform a host of functions on high

speed circuit boards: bypassing, shielding, current transport,

etc. In mixed signal design, the analog and digital portions of

the board should be distinct from each other, with the analog

ground plane confined to the areas covering the analog signal

traces, and the digital ground plane confined areas covering the

digital interconnects.

All analog ground pins of the DAC, reference and other analog

components should be tied directly to the analog ground plane.

The two ground planes should be connected by a path 1/8 to

1/4 inch wide underneath or within 1/2 inch of the DAC to

maintain optimum performance. Care should be taken to ensure

that the ground plane is uninterrupted over crucial signal paths.

On the digital side, this includes the digital input lines running

to the DAC as well as any clock signals. On the analog side, this

includes the DAC output signal, reference signal and the supply

feeders.

The use of wide runs or planes in the routing of power lines is

also recommended. This serves the dual role of providing a low

series impedance power supply to the part and providing some

“free” capacitive decoupling to the appropriate ground plane. It

is essential that care be taken in the layout of signal and power

ground interconnects to avoid inducing extraneous voltage

drops in the signal ground paths. It is recommended that all

connections be short, direct and as physically close to the pack-

age as possible to minimize the sharing of conduction paths

between different currents. When runs exceed an inch in length,

strip line techniques with proper termination resistor should be

considered. The necessity and value of this resistor will be de-

pendent upon the logic family used.

For a more detailed discussion of the implementation and con-

struction of high speed, mixed signal printed circuit boards,

refer to Analog Devices’ application notes AN-280 and AN-333.

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