AD5370
Rev. 0 | Page 25 of 28
POWER SUPPLY DECOUPLING
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
AD5370 is mounted should be designed so that the analog and
digital sections are separated and confined to certain areas of
the board. If the AD5370 is in a system where multiple devices
require an AGND-to-DGND connection, the connection should
be made at one point only. The star ground point should be
established as close as possible to the device. For supplies with
multiple pins (V
SS
, V
DD
, DV
CC
), it is recommended to tie these
pins together and to decouple each supply once.
The AD5370 should have ample supply decoupling of 10 μF in
parallel with 0.1 μF on each supply 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 capacitor should have
low effective series resistance (ESR) and low effective series
inductance (ESI)—such as is typical of the common ceramic types
that provide a low impedance path to ground at high frequencies—
to handle transient currents due to internal logic switching.
Digital lines running under the device should be avoided
because they can couple noise onto the device. The analog
ground plane should be allowed to run under the AD5370 to
avoid noise coupling. The power supply lines of the AD5370
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 digital signals should be shielded with digital
ground to avoid radiating noise to other parts of the board,
and they should never be run near the reference inputs. It is
essential to minimize noise on all VREF lines.
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 feedthrough through the board. A microstrip
technique is by far the best approach, but it is not always possible
with a double-sided board. In this technique, the component side
of the board is dedicated to ground plane, and signal traces are
placed on the solder side.
As is the case for all thin packages, care must be taken to avoid
flexing the package and to avoid a point load on the surface of
this package during the assembly process.
POWER SUPPLY SEQUENCING
When the supplies are connected to the AD5370, it is important
that the AGND and DGND pins be connected to the relevant
ground plane before the positive or negative supplies are applied.
In most applications, this is not an issue because the ground pins
for the power supplies are connected to the ground pins of the
AD5370 via ground planes. When the AD5370 is used in a hot
swap card, care should be taken to ensure that the ground pins
are connected to the supply grounds before the positive or
negative supply is connected. This is required to prevent currents
from flowing in directions other than toward an analog or
digital ground.
