AD8021 datasheet(24/28 Pages) AD

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AD7685

Rev. C | Page 24 of 28

APPLICATION HINTS

LAYOUT

The printed circuit board (PCB) that houses the AD7685

should be designed so that the analog and digital sections are

separated and confined to certain areas of the board. The

pinout of the AD7685 with all its analog signals on the left side

and all its digital signals on the right side eases this task.

Avoid running digital lines under the device because these

couple noise onto the die, unless a ground plane under the

AD7685 is used as a shield. Fast switching signals, such as CNV

or clocks, should never run near analog signal paths. Crossover

of digital and analog signals should be avoided

At least one ground plane should be used. It could be common

or split between the digital and analog section. In the latter case,

the planes should be joined underneath the AD7685.

The AD7685 voltage reference input REF has a dynamic input

impedance and should be decoupled with minimal parasitic

inductances. This is done by placing the reference decoupling

ceramic capacitor close to, and ideally right up against, the REF

and GND pins and connected with wide, low impedance traces.

Finally, the power supplies VDD and VIO should be decoupled

with ceramic capacitors, typically 100 nF, placed close to the

AD7685 and connected using short and wide traces to provide

low impedance paths and to reduce the effect of glitches on the

power supply lines.

An example layout following these rules is shown in Figure 46

and Figure 47.

EVALUATING THE PERFORMANCE OF THE AD7685

Other recommended layouts for the AD7690 are outlined in

the documentation of the evaluation board (EVAL-AD7685CB).

The evaluation board package includes a fully assembled and

tested evaluation board, documentation, and software for

controlling the board from a PC via the universal evaluation

control board (EVAL-CONTROL BRD3).

Figure 46. Example of Layout of the AD7685 (Top Layer)

Figure 47. Example of Layout of the AD7685 (Bottom Layer)

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