AD7612_15 datasheet(31/32 Pages) AD

Part #	AD7612

Description	16-Bit, 750 kSPS, Unipolar/Bipolar Programmable Input PulSAR

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Manufacturer	AD [Analog Devices]

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Data Sheet

AD7612

Rev. A | Page 31 of 32

APPLICATION INFORMATION

LAYOUT GUIDELINES

While the AD7612 has very good immunity to noise on the

power supplies, exercise care with the grounding layout. To facil-

itate the use of ground planes that can be easily separated, design

the printed circuit board that houses the AD7612 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 AD7612, or

as close as possible to the AD7612. If the AD7612 is in a system

where multiple devices require analog-to-digital ground connec-

tions, the connections should still be made at one point only, a

star ground point, established as close as possible to the AD7612.

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

and to reduce feedthrough:

 Do not run digital lines under the device.

 Do run the analog ground plane under the AD7612.

 Do shield fast switching signals, like 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 AD7612 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 AD7612, 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, and 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

in the vicinity of the ADC to further reduce low frequency ripple.

The DVDD supply of the AD7612 can be either a separate supply

or come from the analog supply, AVDD, or from the digital inter-

face 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 AD7612 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 AD7612 is outlined in the EVAL-

AD7612EDZ 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-CED1Z.

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