AD7191 datasheet(19/21 Pages) AD

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AD7191

Rev. A | Page 18 of 20

In systems where the AGND and DGND pins are connected

somewhere else in the system(that is, at the system power

supply), they should not be connected again at the AD7191

because a ground loop results. In these situations, it is

recommended that the AD7191 AGND and DGND pins be tied

to the AGND plane. In any layout, it is important that the user

keep in mind the flow of currents in the system, ensuring that

the return paths for all currents are as close as possible to the

paths the currents took to reach their destinations. Avoid

forcing digital currents to flow through the AGND sections of

the layout.

Avoid running digital lines under the device because these

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

allowed to run under the AD7191 to prevent noise coupling.

The power supply lines to the AD7191 should use as wide 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 but 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 important when using high resolution

ADCs. All analog supplies should be decoupled with 10 μF

tantalum capacitors in parallel with 0.1 μF capacitors to AGND.

To achieve the best from these decoupling components, they

have to be placed as close as possible to the device, ideally right

up against the device. All logic chips should be decoupled with

0.1 μF ceramic capacitors to DGND. In systems where a

common supply voltage is used to drive both the AVDD and

DVDD of the AD7191, it is recommended that the system AVDD

supply be used. This supply should have the recommended

analog supply decoupling capacitors between the AVDD pin of

the AD7191 and AGND, and the recommended digital supply

decoupling capacitor between the DVDD pin of the AD7191 and

DGND.

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