AD7911/AD7921
Rev. A | Page 24 of 28
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7911/AD7921
should be designed such 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 separated easily.
A minimum etch technique is generally best for ground planes,
because it gives the best shielding. Digital and analog ground
planes should be joined at only one place. If the AD7911/
AD7921 is in a system where multiple devices require an
AGND-to-DGND connection, the connection should still be
made at one point only, a star ground point that should be
established as close as possible to the AD7911/AD7921.
Avoid running digital lines under the device, because these
couple noise onto the die. The analog ground plane should be
allowed to run under the AD7911/AD7921 to avoid noise
coupling. The power supply lines to the AD7911/AD7921
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 like clocks should be shielded with
digital ground to avoid radiating noise to other sections of the
board, and clock signals should never be 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 to reduce the effects of feedthrough through the board. A
microstrip technique is by far 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 very important. The analog supply
should be decoupled with 10 μF tantalum in parallel with 0.1 μF
capacitors to AGND. To achieve the best performance from
these decoupling components, the user should endeavor to keep
the distance between the decoupling capacitor and the V
DD
and
GND pins to a minimum with short track lengths connecting
the respective pins.
