Analog Devices AD9830 Instruction Manual page 13

AD9830
Grounding and Layout
The printed circuit board that houses the AD9830 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 which can be separated easily. A mini-
mum etch technique is generally best for ground planes as it
gives the best shielding. Digital and analog ground planes
should only be joined in one place. If the AD9830 is the only
device requiring an AGND to DGND connection, then the
ground planes should be connected at the AGND and DGND
pins of the AD9830. If the AD9830 is in a system where mul-
tiple devices require AGND to DGND connections, the con-
nection should be made at one point only, a star ground point
that should be established as close as possible to the AD9830.
Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed
to run under the AD9830 to avoid noise coupling. The power
supply lines to the AD9830 should use as large a track as is pos-
sible 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. Avoid crossover of digital
and analog signals. Traces on opposite sides of the board
should run at right angles to each other. This will reduce the ef-
fects 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 other
side.
Good decoupling is important. The analog and digital supplies
to the AD9830 are independent and separately pinned out to
minimize coupling between analog and digital sections of the
device. All analog and digital supplies should be decoupled to
AGND and DGND respectively with 0.1 µF ceramic capacitors
in parallel with 10 µF tantalum capacitors. To achieve the best
from the decoupling capacitors, they should be placed as close
as possible to the device, ideally right up against the device. In
systems where a common supply is used to drive both the AVDD
and DVDD of the AD9830, it is recommended that the system's
AVDD supply be used. This supply should have the recom-
mended analog supply decoupling between the AVDD pins of
the AD9830 and AGND and the recommended digital supply
decoupling capacitors between the DVDD pins and DGND.
REV. B –13–