AD7870LP datasheet(22/28 Pages) AD

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AD7870/AD7875/AD7876

Rev. C | Page 22 of 2

APPLICATIONS INFORMATION

Good printed circuit board (PCB) layout is as important as

the overall circuit design itself in achieving high speed analog-

to-digital performance. The designer has to be conscious of

noise both in the ADC itself and in the preceding analog

circuitry. Switching mode power supplies are not recommended

because the switching spikes feed through to the comparator

causing noisy code transitions. Other causes of concern are

ground loops and digital feedthrough from microprocessors.

These are factors which influence any ADC, and a proper

PCB layout which minimizes these effects is essential for best

performance.

LAYOUT HINTS

Ensure that the layout for the printed circuit board has the

digital and analog signal lines separated as much as possible.

Take care not to run any digital track alongside an analog signal

track. Guard (screen) the analog input with AGND.

Establish a single point analog ground (star ground) separate

from the logic system ground at the AGND pin or as close

as possible to the ADC. Connect all other grounds and the

AD7870/AD7875/AD7876 DGND to this single analog

ground point. Do not connect any other digital grounds to

this analog ground point.

Low impedance analog and digital power supply common

returns are essential to low noise operation of the ADC, so

make the foil width for these tracks as wide as possible. The

use of ground planes minimizes impedance paths and also

guards the analog circuitry from digital noise. The circuit

layout has both analog and digital ground planes which are

kept separated and only joined together at the AD7870/

AD7875/AD7876 AGND pin.

NOISE

Keep the input signal leads to VIN and signal return leads from

AGND as short as possible to minimize input noise coupling.

In applications where this is not possible, use a shielded cable

between the source and the ADC. Reduce the ground circuit

impedance as much as possible since any potential difference

in grounds between the signal source and the ADC appears as

an error voltage in series with the input signal.

AD7870LP Folha de dados(PDF) 22 Page - Analog Devices