AD7655AST datasheet(25/28 Pages) AD

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AD7655

Rev. B | Page 25 of 28

APPLICATION HINTS

LAYOUT

The AD7655 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 AD7655 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 AD7655, or as close as possible to the AD7655.

If the AD7655 is in a system where multiple devices require

analog-to-digital ground connections, the connection should

still be made at one point only, a star ground point that should

be established as close as possible to the AD7655.

Avoid unning digital lines under the device because these

couple noise onto the die. The analog ground plane should be

allowed to run under the AD7655 to avoid noise coupling. Fast

switching signals such as 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 AD7655 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 impedance presented to the

AD7655 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 AD7655 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 17) 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 AD7655 has five 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 AD7655 PERFORMANCE

A recommended layout for the AD7655 is outlined in the

EVAL-AD7655CB 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.

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