AD7171

Rev. 0 | Page 12 of 16

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 part 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 conven-

tional 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.

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.

parallel with 0.1 μF capacitors to GND, with the system’s analog

ground to digital ground (DGND) connection 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.