AD7887ARM datasheet(20/24 Pages) AD

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AD7887

Rev. D | Page 20 of 24

APPLICATION HINTS

Grounding and Layout

The AD7887 has very good immunity to noise on the power

supplies, as can be seen in Figure 7. However, care should still

be taken with regard to grounding and layout.

The printed circuit board that houses the AD7887 should 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 because it

results in the best shielding. Digital and analog ground planes

should be joined in only one place, as close as possible to the

GND pin of the AD7887. If the AD7887 is in a system where

multiple devices require AGND-to-DGND connections, the

connection should still be made at one point only, a star ground

point, which should be established as close as possible to the

AD7887.

Avoid running digital lines under the device because these will

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

allowed to run under the AD7887 to avoid noise coupling. The

power supply lines to the AD7887 should 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 should be shielded with digital ground to avoid radiating

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

never be run near the analog inputs. Avoid crossover of digital

and analog signals. Traces on opposite sides of the board should

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

feedthrough through the board. A microstrip technique is by far

the best approach, but it is not always possible 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 tantalum in parallel with 0.1 μF

capacitors to AGND. To achieve the best from these decoupling

components, they must be placed as close as possible to the

device, ideally right up against the device.

Evaluating the AD7887 Performance

The recommended layout for the AD7887 is outlined in the

evaluation board for the AD7887. The evaluation board

package includes a fully assembled and tested evaluation board,

documentation, and software for controlling the board from the

PC via the EVAL-CONTROL BOARD. The EVAL-CONTROL

BOARD can be used in conjunction with the AD7887

evaluation board, as well as many other Analog Devices, Inc.,

evaluation boards ending in the CB designator, to

demonstrate/evaluate the ac and dc performance of the

AD7887.

The software allows the user to perform ac (fast Fourier

transform) and dc (histogram of codes) tests on the AD7887.

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