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AD7609

Data Sheet

Rev. A | Page 32 of 36

LAYOUT GUIDELINES

The printed circuit board that houses the AD7609 should be

designed so that the analog and digital sections are separated

and confined to different areas of the board.

Use at least one ground plane. It can be common or split

between the digital and analog sections. In the case of the split

plane, the digital and analog ground planes should be joined in

only one place, preferably as close as possible to the AD7609.

If the AD7609 is in a system where multiple devices require

analog-to-digital ground connections, the connection should

still be made at only one point, a star ground point, which

should be established as close as possible to the AD7609. Good

connections should be made to the ground plane. Avoid sharing

one connection for multiple ground pins. Individual vias or

multiple vias to the ground plane should be used for each

ground pin.

Avoid running digital lines under the devices because doing

so couples noise onto the die. Allow the analog ground plane

to run under the AD7609 to avoid noise coupling. Shield fast-

switching signals like CONVST A, CONVST B, or clocks with

digital ground to avoid radiating noise to other sections of the

board, and they should never run near analog signal paths.

Avoid crossover of digital and analog signals. Run traces on

layers in close proximity on the board at right angles to each

other to reduce the effect of feedthrough through the board.

The power supply lines to the AVCC and VDRIVE pins on the

AD7609 should use as large a trace as possible to provide low

impedance paths and reduce the effect of glitches on the power

supply lines. Where possible, use supply planes. Good connec-

tions should be made between the AD7609 supply pins and the

power tracks on the board; this should involve the use of a single

via or multiple vias for each supply pin.

Good decoupling is also important to lower the supply imped-

ance presented to the AD7609 and to reduce the magnitude of

the supply spikes. Place the decoupling capacitors close to,

ideally right up against, these pins and their corresponding

ground pins. Place the decoupling capacitors for the REFIN/

REFOUT pin and the REFCAPA and REFCAPB pins as close as

possible to their respective AD7609 pins. Where possible, they

should be placed on the same side of the board as the AD7609

device. Figure 60 shows the recommended decoupling on the

top layer of the AD7609 board. Figure 61 shows bottom layer

decoupling. Bottom layer decoupling is for the four AVCC pins

and the VDRIVE pin.

Figure 60. Top Layer Decoupling REFIN/REFOUT, REFCAPA, REFCAPB, and

REGCAP Pins

Figure 61. Bottom Layer Decoupling

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