AD7654ASTZ datasheet(25/28 Pages) AD

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AD7654

Rev. B | Page 25 of 28

APPLICATION HINTS

LAYOUT

The AD7654 has very good immunity to noise on the power

supplies. However, care should still be taken with regard to

grounding layout.

The printed circuit board that houses the AD7654 should be

designed so 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 separated easily. Digital and analog

ground planes should be joined in only one place, preferably

underneath the AD7654, or as close as possible to the AD7654.

If the AD7654 is in a system where multiple devices require

analog-to-digital ground connections, the connection should

still be made at only a star ground point established as close as

possible to the AD7654.

Running digital lines under the device should be avoided

because these couple noise onto the die. The analog ground

plane should be allowed to run under the AD7654 to avoid

noise coupling. Fast switching signals like CNVST or clocks

should be shielded with digital ground to avoid radiating noise

to other sections of the board, and should never run near

analog signal paths. Crossover of digital and analog signals

should be avoided. Traces on different but close layers of the

board should run at right angles to each other. This reduces the

effect of crosstalk through the board.

The power supply lines to the AD7654 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 supply’s impedance presented to the

AD7654 and to reduce the magnitude of the supply spikes.

Decoupling ceramic capacitors, typically 100 nF, should be

placed on each power supply pin—AVDD, DVDD, and

OVDD—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 AD7654 can be a separate supply or

can come from the analog supply AVDD or the digital interface

supply OVDD. When the system digital supply is noisy or when

fast switching digital signals are present, if no separate supply is

available, the user should connect DVDD to AVDD through an

RC filter (see Figure 18) and the system supply to OVDD and

the remaining digital circuitry. When DVDD is powered from

the system supply, it is useful to insert a bead to further reduce

high frequency spikes.

The AD7654 has five different ground pins: INGND, REFGND,

AGND, DGND, and OGND. INGND is used to sense the

analog input signal. 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.

EVALUATING THE AD7654 PERFORMANCE

A recommended layout for the AD7654 is outlined in the

documentation of the evaluation board for the

EVAL-AD7654CB. 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.

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