POWER SUPPLY BYPASSING AND GROUNDING
When accuracy is important in a circuit, it is helpful to carefully
consider the power supply and ground return layout on the
board. The printed circuit board containing the AD5620/
AD5640/AD5660 should have separate analog and digital
sections, each having its own area of the board. If the AD5620/
AD5640/AD5660 are in a system where other devices require
an AGND-to-DGND connection, the connection should be
made at one point only. This ground point should be as close as
possible to the AD5620/AD5640/AD5660.
The power supply to the AD5620/AD5640/AD5660 should be
bypassed with 10 µF and 0.1 µF capacitors. The capacitors
should be as close as physically possible to the device, with the
0.1 µF capacitor ideally right up against the device. The 10 µF
capacitors are the tantalum bead type. It is important that the
0.1 µF capacitor has a low effective series resistance (ESR) and
low effective series inductance (ESI), such as is typical of
common ceramic types of capacitors. This 0.1 µF capacitor
provides a low impedance path to ground for high frequencies
caused by transient currents due to internal logic switching.
The power supply line itself should have as large a trace as
possible to provide a low impedance path and reduce glitch
effects on the supply line. Clocks and other components with
fast switching digital signals should be shielded from other
parts of the board by digital ground. Avoid crossover of digital
and analog signals if possible. When traces cross on opposite
sides of the board, ensure that they run at right angles to each
other to reduce feedthrough effects on the board. The best
board layout technique is the microstrip technique, where the
component side of the board is dedicated to the ground plane
only and the signal traces are placed on the solder side.
However, this is not always possible with a 2-layer board.
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