AD7171 데이터 시트보기 (PDF) - Analog Devices

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AD7171

16-Bit, Low Power, Sigma-Delta ADC

Analog Devices 

AD7171

GROUNDING AND LAYOUT

Because the analog input and reference input 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 device 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 AD7171 is

more immune to noise interference than conventional high

resolution converters. However, because the noise levels from

the AD7171 are so low, care must be taken with regard to

grounding and layout.

The printed circuit board that houses the AD7171 should be

designed such that the analog and digital sections are separated

and confined to certain areas of the board. A minimum etch

technique is generally best for ground planes because it gives

the best shielding.

It is recommended that the GND pin of the AD7171 be tied to

the analog ground (AGND) plane of the system. In any layout,

it is important that the user pay attention to the flow of currents

in the system, and ensure 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.

Data Sheet

The ground plane of the AD7171 should be allowed to run under

the AD7171 to prevent noise coupling. The power supply lines

to the AD7171 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 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 it 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 important when using high resolution ADCs.

VDD should be decoupled with 10 µF tantalum capacitors in parallel

with 0.1 µF capacitors to GND, with the analog ground to digital

ground (DGND) connection of the system being close to the

AD7171. To achieve the best results 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.

Rev. C | Page 12 of 16

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