EVAL-AD7902SDZ datasheet(27/29 Pages) AD

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AD7902

Data Sheet

Rev. B | Page 26 of 28

LAYOUT

Design the printed circuit board (PCB) of the AD7902 such that

the analog and digital sections are separated and confined to

certain areas of the board. The pinout of the AD7902, with its

analog signals on the left side and 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 AD7902 is used

as a shield. Do not run fast switching signals, such as CNVx or

clocks, near analog signal paths. Avoid crossover of digital and

analog signals. To avoid signal fidelity issues, take care to ensure

monotonicity of digital edges in the PCB layout.

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

between the digital and analog sections. In the latter case, join

the planes underneath the AD7902.

The AD7902 voltage reference inputs, REF1 and REF2, have a

dynamic input impedance. Decouple these reference inputs with

minimal parasitic inductances by placing the reference decoupling

ceramic capacitor in close proximity to (ideally, right up

against) the REFx and GND pins and then connecting them

with wide, low impedance traces.

Finally, decouple the power supplies, VDDx and VIOx, with

ceramic capacitors, typically 100 nF. Place them in close proximity

to the AD7902 and connect them using short, wide traces to

provide low impedance paths and to reduce the effect of glitches

on the power supply lines.

See Figure 53 for an example of layout following these rules.

EVALUATING PERFORMANCE OF THE AD7902

Other recommended layouts for the AD7902 are outlined in the

EVAL-AD7902SDZ User Guide. The package for the evaluation

board (EVAL-AD7902SDZ) includes a fully assembled and

tested evaluation board, user guide, and software for controlling

the board from a PC via the EVAL-SDP-CB1Z.

Figure 53. Example Layout of the AD7902 (Top Layer)

REF1

VDD1

IN1+

IN1–

GND

REF2

VDD2

IN2+

IN2–

GND

VIO1

SDI1

SCK1

SDO1

CNV1

VIO2

SDI2

SCK2

SDO2

CNV2

GND

REF

GND

VDD

GND

VIO

GND

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