AD7623AST datasheet(28/29 Pages) AD

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AD7623

Rev. 0 | Page 27 of 28

APPLICATION

LAYOUT

While the AD7623 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 AD7623 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

AD7623, or as close as possible to the AD7623. If the AD7623 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 AD7623.

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 AD7623.

• 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 AD7623 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 AD7623, and to reduce the magnitude of the supply

spikes. Decoupling ceramic capacitors, typically 100 nF, should

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

and OVDD. 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 AD7623 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. Refer

to Figure 23 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 AD7623 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 THE AD7623 PERFORMANCE

A recommended layout for the AD7623 is outlined in the

documentation of the EVAL-AD7623CB evaluation board

for the AD7623. 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.

AD7623AST Datasheet(PDF) 28 Page - Analog Devices

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