AD7441/AD7451

Rev. D | Page 22 of 24

GROUNDING AND LAYOUT HINTS

The printed circuit board that houses the AD7441/AD7451

must be designed so that the analog and digital sections are

separated and confined to certain areas of the board. This

facilitates the use of ground planes that can be easily separated.

A minimum etch technique is generally best for ground planes,

as it gives the best shielding. Digital and analog ground planes

must be joined in only one place: a star ground point estab-

lished as close to the GND pin on the AD7441/AD7451 as

possible.

Avoid running digital lines under the device, as this couples

noise onto the die. The analog ground plane must be allowed to

run under the AD7441/AD7451 to avoid noise coupling. The

power supply lines to the AD7441/AD7451 must use as large

a trace as possible to provide low impedance paths and reduce

the effects of glitches on the power supply line.

Fast switching signals like clocks must be shielded with digital

grounds to avoid radiating noise to other sections of the board,

and clock signals must never run near the analog inputs. Avoid

crossover of digital and analog signals. Traces on opposite sides

of the board must run at right angles to each other. This reduces

the effects of feedthrough on the board. A microstrip technique is

by far the best but is not always possible with a double-sided board.

In this technique, the component side of the board is dedicated

to ground planes while signals are placed on the solder side.

Good decoupling is also important. All analog supplies must

be decoupled with 10 μF tantalum capacitors in parallel with

0.1 μF capacitors to GND. To achieve the best from these

decoupling components, they must be placed as close as

possible to the device.

EVALUATING PERFORMANCE

The evaluation board package includes a fully assembled and

tested evaluation board, documentation, and software for con-

trolling the board from a PC via the evaluation board controller.

The evaluation board controller can be used in conjunction with

the AD7441 and the AD7451 evaluation boards, as well as with

many other Analog Devices, Inc. evaluation boards ending with

the CB designator, to demonstrate and evaluate the ac and dc

performance of the AD7441 and the AD7451.

The software allows the user to perform ac (fast Fourier transform)

and dc (histogram of codes) tests on the AD7441/AD7451. See

the AD7441/AD7451 application note that accompanies the

evaluation kit for more information.