AD7652 datasheet(22/23 Pages) AD

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REV. PrA

PRELIMINARY TECHNICAL DATA

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AD7667

at right angles to each other. This will reduce the effect of

feedthrough through the board.

The power supplies lines to the AD7667 should use as

large trace as possible to provide low impedance paths and

reduce the effect of glitches on the power supplies lines.

Good decoupling is also important to lower the supplies

impedance presented to the AD7667 and reduce the magni-

tude of the supply spikes. Decoupling ceramic capacitors,

typically 100 nF, should be placed on each power supplies

pins 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 in

the vicinity of the ADC to further reduce low frequency

ripple.

The DVDD supply of the AD7667 can be either a separate

supply or come from the analog supply AVDD or the digi-

tal interface supply OVDD. When the system digital supply

is noisy, or fast switching digital signals are present, it is

recommended that if no separate supply available, connect

the DVDD digital supply to the analog supply, AVDD,

through an RC filter as shown in Figure 5, and connect

the system supply to the interface digital supply, OVDD,

and the remaining digital circuitry. When DVDD is powered

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

ther reduce high-frequency spikes.

The AD7667 has five different ground pins: INGND, REF-

GND, AGND, DGND, and OGND. INGND is used to

sense the analog input signal. REFGND senses the refer-

ence voltage and should be a low impedance return to the

reference because it carries pulsed currents. AGND is the

ground to which most internal ADC analog signals are

referenced. This ground 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 AD7667 Performance

A recommended layout for the AD7667 is outlined in the

evaluation board for the AD7667. 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 Board.

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