AD7725 datasheet(27/28 Pages) AD

27 / 28 page

REV. A

AD7725

–27–

A minimum etch technique is generally best for ground planes

as it gives the best shielding. Noise can be minimized by paying

attention to the system layout and preventing different signals

from interfering with each other. High level analog signals should

be separated from low level analog signals, and both should be

kept away from digital signals. In waveform sampling and recon-

struction systems, the sampling clock (CLKIN) is as vulnerable

to noise as any analog signal. CLKIN should be isolated from the

analog and digital systems. Fast switching signals like clocks

should be shielded with their associated ground to avoid radiating

noise to other sections of the board, and clock signals should

never be routed near the analog inputs.

Avoid running digital lines under the device, as these will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7725 to shield it from noise coupling. The

power supply lines to the AD7725 should use as large a trace as

possible (preferably a plane) to provide a low impedance path

and reduce the effects of glitches on the power supply line.

Avoid crossover of digital and analog signals. Traces on oppo-

site sides of the board should run at right angles to each other.

This will reduce the effects of feedthrough through the board.

0.1 F

AVDD

AGND

10nF

DVDD

DGND

10 F +

10nF

AVDD1

AGND1

AVDD

AGND

10nF

AGND

0.1 F

10 F

AGND

REF2

REF1

AGND2

220nF

10nF

1 F

ANALOG

GROUND

PLANE

DIGITAL

GROUND

PLANE

Figure 36. Reference and Supply Decoupling

OPTIMIZING HEAT REMOVAL BY PCB

CONSTRUCTION AND DEVICE MOUNTING

For normal still air conditions, the primary heat dissipation path

from the chip to the ambient is via the component leads into the

PCB. The thermal resistance of the board is then a significant

variable. This can be lowered by maximizing the use of ground

planes as heat sinks and also by optimizing the way in which the

heat can be dissipated, for example conduction into the board

mounting chassis. The greater the percentage of copper in the

board, especially in the region of the device, the lower the ther-

mal resistance. The use of wide tracks and thermal vias to the

ground plane will have a significant effect. Placing critical com-

ponents close to where the edge of the board is attached to the

chassis can provide additional cooling without the use of heat

sinks or forced air. Avoid close spacing of high power devices in

order to ensure that the heat is dissipated over the maximum

possible area.

EVALUATING THE AD7725 PERFORMANCE

There is an AD7725 evaluation package available that includes

a fully assembled and tested evaluation board, documentation,

and software for controlling the board from a PC via the Evalu-

ation Board Controller. The Evaluation Board Controller can

be used in conjunction with the AD7725 Evaluation Board (as

well as with many other Analog Devices evaluation boards ending

in the CB designator) to demonstrate/evaluate the performance of

the AD7725. The software allows the user to perform ac (Fast

Fourier Transform) and dc (Histogram of Codes) tests on the

AD7725. By downloading the filter design package, Filter

Wizard, user-defined filter files can be loaded into the AD7725

to program the postprocessor via the Evaluation Board Controller.

See the ADI website for more information.

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