AD7631BSTZ datasheet(31/32 Pages) AD

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AD7631

Rev. A | Page 31 of 32

APPLICATION INFORMATION

LAYOUT GUIDELINES

While the AD7631 has very good immunity to noise on the

power supplies, exercise care with the grounding layout. To

facilitate the use of ground planes that can be easily separated,

design the printed circuit board that houses the AD7631 so that

the analog and digital sections are separated and confined to

certain areas of the board. Digital and analog ground planes

should be joined in only one place, preferably underneath the

AD7631, or as close as possible to the AD7631. If the AD7631 is

in a system where multiple devices require analog-to-digital

ground connections, the connections should still be made at

one point only, a star ground point, established as close as

possible to the AD7631.

To prevent coupling noise onto the die, avoid radiating noise,

and reduce feedthrough:

• Do not run digital lines under the device.

• Do run the analog ground plane under the AD7631.

• Do shield fast switching signals, such as CNVST or clocks,

with digital ground to avoid radiating noise to other sections

of the board and never run them near analog signal paths.

• Avoid crossover of digital and analog signals.

• Run traces on different but close layers of the board, at right

angles to each other, to reduce the effect of feedthrough through

the board.

The power supply lines to the AD7631 should use as large a

trace as possible to provide low impedance paths and reduce the

effect of glitches on the power supply lines. Good decoupling is

also important to lower the impedance of the supplies presented

to the AD7631 and to reduce the magnitude of the supply

spikes. Decoupled ceramic capacitors, typically 100 nF, should

be placed on each of the power supplies pins, AVDD, DVDD,

OVDD, VCC, and VEE. The capacitors should be placed close

to, and ideally right up against, these pins and their corresponding

ground pins. Additionally, low ESR 10 μF capacitors should be

located near the ADC to further reduce low frequency ripple.

The DVDD supply of the AD7631 can be either a separate

supply or come from the analog supply, AVDD, or from the

digital interface supply, OVDD. When the system digital supply

is noisy, or fast switching digital signals are present and no

separate supply is available, it is recommended to connect the

DVDD digital supply to the analog supply AVDD through an

RC filter, and to connect the system supply to the interface

digital supply OVDD and the remaining digital circuitry. See

Figure 27 for an example of this configuration. When DVDD is

powered from the system supply, it is useful to insert a bead to

further reduce high frequency spikes.

The AD7631 has four different ground pins: REFGND, AGND,

DGND, and OGND.

•

REFGND senses the reference voltage and, because it carries

pulsed currents, should be a low impedance return to the

reference.

•

AGND is the ground to which most internal ADC analog

signals are referenced; it must be connected with the least

resistance to the analog ground plane.

•

DGND must be tied to the analog or digital ground plane

depending on the configuration.

•

OGND is connected to the digital system ground.

The layout of the decoupling of the reference voltage is important.

To minimize parasitic inductances, place the decoupling capacitor

close to the ADC and connect it with short, thick traces.

EVALUATING PERFORMANCE

A recommended layout for the AD7631 is outlined in the

EVAL-AD7631CBZ evaluation board documentation. The

evaluation board package includes a fully assembled and tested

evaluation board, documentation, and software for controlling

the board from a PC via the EVAL-CONTROL BRD3.

AD7631BSTZ Datasheet(PDF) 31 Page - Analog Devices

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