AD7798 datasheet(25/28 Pages) AD

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Data Sheet

AD7798/AD7799

Rev. B | Page 25 of 28

GROUNDING AND LAYOUT

Because the analog inputs and reference inputs of the ADC are

differential, most of the voltages in the analog modulator are

common-mode voltages. The excellent common-mode reject-

ion of the parts removes common-mode noise on these inputs.

The digital filter provides rejection of broadband noise on the

power supply, except at integer multiples of the modulator

sampling frequency. The digital filter also removes noise from

the analog and reference inputs, provided that these noise

sources do not saturate the analog modulator. As a result, the

AD7798/AD7799 are more immune to noise interference than

conventional high resolution converters. However, because the

resolution of the AD7798/AD7799 is so high and the noise

levels from the AD7798/AD7799 are so low, care must be taken

with regard to grounding and layout.

The printed circuit board that houses the AD7798/AD7799

should be designed such that the analog and digital sections are

separated and confined to certain areas of the board. A mini-

mum etch technique is generally best for ground planes because

it provides the best shielding.

It is recommended that the GND pin be tied to the AGND plane

of the system. 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.

The ground planes should be allowed to run under the AD7798/

AD7799 to prevent noise coupling. The power supply lines to

the AD7798/AD7799 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, such as clock

signals, 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 works best, but it is not always

possible to use this method 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. AVDD should be decoupled with 10 µF tantalum in

parallel with 0.1 µF capacitors to GND. DVDD should be

decoupled with 10 µF tantalum in parallel with 0.1 µF

capacitors to the system’s DGND plane, with the system’s

AGND-to-DGND connection being close to the AD7798/

AD7799. To achieve the best from these decoupling components,

they should 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.

AD7798 Datasheet(PDF) 25 Page - Analog Devices

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