AD7274 datasheet(25/28 Pages) AD

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AD7273/AD7274

Rev. 0 | Page 24 of 28

APPLICATION HINTS

GROUNDING AND LAYOUT

The printed circuit board that houses the AD7273/AD7274

should be designed so that the analog and digital sections are

separated and confined to certain areas of the board. This design

facilitates using ground planes that can be easily separated.

To provide optimum shielding for ground planes, a minimum

etch technique is generally best. All AGND pins of the AD7273/

AD7274 should be sunk into the AGND plane. Digital and

analog ground planes should be joined in only one place. If the

AD7273/AD7274 are in a system where multiple devices require

an AGND-to-DGND connection, the connection should be

made at only one point, a star ground point, established as close

as possible to the ground pin on the AD7273/AD7274.

Avoid running digital lines under the device, because this

couples noise onto the die. However, the analog ground plane

should be allowed to run under the AD7273/AD7274 to avoid

noise coupling. The power supply lines to the AD7273/AD7274

should use as large a trace as possible to provide low impedance

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

To avoid radiating noise to other sections of the board,

components with fast-switching signals, such as clocks, should

be shielded with digital ground, and they should never be run

near the analog inputs. Avoid crossover of digital and analog

signals. To reduce the effects of feedthrough within the board,

traces on opposite sides of the board should run at right angles

to each other. A microstrip technique is by far the best method,

but it is not always possible to use this approach with a double-

sided board. In this technique, the component side of the board

is dedicated to ground planes, and signals are placed on the

solder side.

Good decoupling is also important. All analog supplies should

be decoupled with 10 μF ceramic capacitors in parallel with

0.1 μF capacitors to AGND/DGND. To achieve the best results

from these decoupling components, they must be placed as close

as possible to the device, ideally right up against the device. The

0.1 μF capacitors should have low effective series resistance

(ESR) and low effective series inductance (ESI), such as is typical

of common ceramic or surface-mount types of capacitors.

Capacitors with low ESR and low ESI provide a low impedance

path to ground at high frequencies, which allows them to

handle transient currents due to internal logic switching.

EVALUATING THE AD7273/AD7274 PERFORMANCE

The recommended layout for the AD7273/AD7274 is outlined

in the evaluation board documentation. The evaluation board

package includes a fully assembled and tested evaluation board,

documentation, and software for controlling the board from a

PC via the evaluation board controller. The evaluation board

controller can be used in conjunction with the AD7273/AD7274

evaluation board, as well as many other Analog Devices evaluation

boards ending in the CB designator, to demonstrate/evaluate the

ac and dc performance of the AD7273/AD7274.

The software allows the user to perform ac (fast Fourier trans-

form) and dc (histogram of codes) tests on the AD7273/AD7274.

The software and documentation are on a CD shipped with the

evaluation board.

AD7274 Datasheet(PDF) 25 Page - Analog Devices

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