AD7687BCPZ-R2 datasheet(26/27 Pages) AD

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Data Sheet

AD7687

Rev. E | Page 25 of 26

APPLICATIONS INFORMATION

LAYOUT

Providing a steady and stable reference voltage to the AD7687 is

critical for device operation. Prioritize design tasks aimed at

preventing voltage fluctuations at this node. Decouple the REF

pin, which has a dynamic input impedance, with minimal parasitic

inductances (see the Converter Operation Section). Achieve this

by placing the reference decoupling ceramic capacitor as close as

physically possible the REF and GND pins and connecting it

with wide, low impedance traces.

Limiting sources of noise on the analog signal nodes is

imperative in high precision ADC systems. The digital lines

controlling the AD7687 have the potential to radiate noise that

can couple into the analog signals; therefore, ensure that these

two types of signals are separated and confined to different

areas of boards housing the AD7687. Never allow fast switching

signals (such as CNV or clocks) to run near analog signal paths,

and avoid physical crossover of digital and analog signals. Do

not route digital lines under the AD7687 without a ground

plane providing adequate isolation between the two. To

facilitate these design tasks, the analog and digital pins are

located on separate sides of the device (see Figure 46).

Printed circuit boards (PCBs) housing the AD7687 must contain

at least one ground plane. Connecting analog and digital ground

on the board is not required; however, connecting these planes

underneath the AD7687 is recommended.

Finally, decouple the power supply pins of the AD7687 (VDD

and VIO) with ceramic capacitors (typically 100 nF) placed

close to the AD7687 and connected using short and wide traces

to provide low impedance paths and reduce the effect of glitches

on the power supply lines.

Figure 46 and Figure 47 show an example of a layout following

these rules.

Figure 46. Example of Layout of the AD7687 (Top Layer)

Figure 47. Example of Layout of the AD7687 (Bottom Layer)

EVALUATING THE PERFORMANCE OF THE AD7687

The EVAL-AD7687SDZ evaluation board documentation

outlines other recommended layouts for the AD7687. 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-SDP-CB1Z.

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