AD7457
Rev. A | Page 16 of 20
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7457 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes that can be easily separated. A minimum
etch technique is generally best for ground planes, because it
gives the best shielding. Digital and analog ground planes
should be joined in only one place, and the connection should
be a star ground point established as close as possible to the
GND pin on the AD7457.
Avoid running digital lines under the device, because this
couples noise onto the die. The analog ground plane should be
allowed to run under the AD7457 to avoid noise coupling. The
power supply lines to the AD7457 should use as large a trace as
possible to provide low impedance paths and reduce the effects
of glitches on the power supply line. Fast switching signals,
such as clocks, should be shielded with digital ground to avoid
radiating noise to other sections of the board, and clock signals
should never run near the analog 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 effects
of feed through the board. A micro strip technique is the best,
but is not always possible with a double-sided board.
In this technique, the component side of the board is dedicated
to ground planes, while signals are placed on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 µF tantalum capacitors in parallel with
0.1 µF capacitors to GND. To achieve the best from these
decoupling components, place them as close as possible to
the device.
