AD7194 datasheet(52/56 Pages) AD

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AD7194

Rev. 0 | Page 52 of 56

GROUNDING AND LAYOUT

Because the analog inputs and reference inputs are differential,

most of the voltages in the analog modulator are common-

mode voltages. The high common-mode rejection of the part

removes common-mode noise on these inputs. The analog and

digital supplies to the AD7194 are independent and separately

pinned out to minimize coupling between the analog and

digital sections of the device. The digital filter provides rejection

of broadband noise on the power supplies, except at integer

multiples of the modulator sampling frequency.

Connect an R-C filter to each analog input pin to provide rejection

at the modulator sampling frequency. A 100 Ω resistor in series

with each analog input, a 0.1 μF capacitor between the analog

input pins, and a 0.01 μF capacitor from each analog input to

AGND are advised.

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 AD7194 is

more immune to noise interference than a conventional high

resolution converter. However, because the resolution of the

AD7194 is so high and the noise levels from the converter so

low, care must be taken with regard to grounding and layout.

The printed circuit board (PCB) that houses the ADC must be

designed so that the analog and digital sections are separated

and confined to certain areas of the board. This facilitates the

use of ground planes that can be easily separated. A minimum

etch technique is generally best for ground planes because it

gives the best shielding.

Although the AD7194 has separate pins for analog and digital

ground, the AGND and DGND pins are tied together internally

via the substrate. Therefore, the user must not tie these two pins

to separate ground planes unless the ground planes are connected

together near the AD7194.

In systems in which the AGND and DGND are connected

somewhere else in the system (that is, the power supply of the

system), they should not be connected again at the AD7194

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

mended that the ground pins of the AD7194 be tied to the

AGND plane.

In any layout, the user must keep in mind the flow of currents

in the system, ensuring that the 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.

Avoid running digital lines under the device because this

couples noise onto the die, and allows the analog ground plane

to run under the AD7194 to prevent noise coupling. The power

supply lines to the AD7194 must use as wide a trace as possible

to provide low impedance paths and reduce the effects of

glitches on the power supply line. Shield fast switching signals,

like clocks, with digital ground to prevent radiating noise to

other sections of the board, and never run clock signals near the

analog inputs. Avoid crossover of digital and analog signals.

Run traces on opposite sides of the board 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,

whereas signals are placed on the solder side.

Good decoupling is important when using high resolution

ADCs. Decouple all analog supplies with 10 μF tantalum

capacitors in parallel with 0.1 μF capacitors to AGND. To

achieve the best results from these decoupling components,

place them as close as possible to the device, ideally right up

against the device. Decouple all logic chips with 0.1 μF ceramic

capacitors to DGND. In systems in which a common supply

voltage is used to drive both the AVDD and DVDD of the AD7194,

it is recommended that the system AVDD supply be used. For

this supply, place the recommended analog supply decoupling

capacitors between the AVDD pin of the AD7194 and AGND

and the recommended digital supply decoupling capacitor

between the DVDD pin of the AD7194 and DGND.

AD7194 Datasheet(PDF) 52 Page - Analog Devices

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