AD7801

–13–

REV. 0

Power Supply Bypassing and Grounding

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board on which the

AD7801 is mounted should be designed so that the analog and

digital sections are separated and confined to certain areas of the

board. If the AD7801 is in a system where multiple devices

require an AGND to DGND connection, the connection should

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

established as closely as possible to the AD7801. The AD7801

should have ample supply bypassing of 10

µF in parallel with

0.1

µF located as close to the package as possible, ideally right

up against the device. The 10

µF capacitors are the tantalum

bead type. The 0.1

µF capacitors should have low Effective

Series Resistance (ESR) and Effective Series Inductance (ESI),

such as the common ceramic types, which provide a low

impedance path to ground at high frequencies to handle

transient currents due to internal logic switching.

The power supply lines of the AD7801 should use as large a

trace as possible to provide low impedance paths and reduce the

effects of glitches on the supply line. Fast switching signals like

clocks should be shielded with digital ground to avoid radiating

noise to other parts of the board and should never be run near

reference inputs. Avoid crossover of digital and analog signals.

Traces on opposite sides of the board should run at right angles

to each other. This reduces the effect of feedthrough through

the board. A microstrip technique is by far the best, but not

always possible with a double-sided board. In this technique, the

component side of the board is dedicated to the ground plane

while signal traces are placed on the solder side.