AD7940 datasheet(23/28 Pages) AD

23 / 28 page

AD7686

Rev. B | Page 23 of 28

APPLICATION HINTS

LAYOUT

The printed circuit board (PCB) that houses the AD7686

should be designed so that the analog and digital sections are

separated and confined to certain areas of the board. The

pinout of the AD7686, 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 doing so

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

AD7686 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 sections. In the latter

case, the planes should be joined underneath the devices.

The AD7686 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 connecting it with wide, low impedance

traces.

Finally, the AD7686 power supplies VDD and VIO should be

decoupled with ceramic capacitors (typically 100 nF) placed

close to the AD7686 and connected using short and wide traces.

This provides low impedance paths and reduces the effect of

glitches on the power supply lines. Examples of layouts that

follow these rules are shown in Figure 45 and Figure 46.

EVALUATING PERFORMANCE

Other recommended layouts for the AD7686 are outlined in

the documentation of the evaluation board (EVAL-AD7686CB).

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 45. Example of Layout of the AD7686 (Top Layer)

Figure 46. Example of Layout of the AD7686 (Bottom Layer)

AD7940 датащи(PDF) 23 Page - Analog Devices