Datasheet

Data Sheet AD7631
Rev. B | Page 31 of 32
APPLICATION INFORMATION
LAYOUT GUIDELINES
While the AD7631 has very good immunity to noise on the
power supplies, exercise care with the grounding layout. To
facilitate the use of ground planes that can be easily separated,
design the printed circuit board that houses the AD7631 so that
the analog and digital sections are separated and confined to
certain areas of the board. Digital and analog ground planes
should be joined in only one place, preferably underneath the
AD7631, or as close as possible to the AD7631. If the AD7631 is
in a system where multiple devices require analog-to-digital
ground connections, the connections should still be made at
one point only, a star ground point, established as close as
possible to the AD7631.
To prevent coupling noise onto the die, avoid radiating noise,
and reduce feedthrough:

Do not run digital lines under the device.

Do run the analog ground plane under the AD7631.

Do shield fast switching signals, such as
CNVST
or clocks,
with digital ground to avoid radiating noise to other sections
of the board and never run them near analog signal paths.

Avoid crossover of digital and analog signals.

Run traces on different but close layers of the board, at right
angles to each other, to reduce the effect of feedthrough through
the board.
The power supply lines to the AD7631 should use as large a
trace as possible to provide low impedance paths and reduce the
effect of glitches on the power supply lines. Good decoupling is
also important to lower the impedance of the supplies presented
to the AD7631 and to reduce the magnitude of the supply
spikes. Decoupled ceramic capacitors, typically 100 nF, should
be placed on each of the power supplies pins, AVDD, DVDD,
OVDD, VCC, and VEE. The capacitors should be placed close
to, and ideally right up against, these pins and their corresponding
ground pins. Additionally, low ESR 10 µF capacitors should be
located near the ADC to further reduce low frequency ripple.
The DVDD supply of the AD7631 can be either a separate
supply or come from the analog supply, AVDD, or from the
digital interface supply, OVDD. When the system digital supply
is noisy, or fast switching digital signals are present and no
separate supply is available, it is recommended to connect the
DVDD digital supply to the analog supply AVDD through an
RC filter, and to connect the system supply to the interface
digital supply OVDD and the remaining digital circuitry. See
Figure 27 for an example of this configuration. When DVDD is
powered from the system supply, it is useful to insert a bead to
further reduce high frequency spikes.
The AD7631 has four different ground pins: REFGND, AGND,
DGND, and OGND.

REFGND senses the reference voltage and, because it carries
pulsed currents, should be a low impedance return to the
reference.

AGND is the ground to which most internal ADC analog
signals are referenced; it must be connected with the least
resistance to the analog ground plane.

DGND must be tied to the analog or digital ground plane
depending on the configuration.

OGND is connected to the digital system ground.
The layout of the decoupling of the reference voltage is important.
To minimize parasitic inductances, place the decoupling capacitor
close to the ADC and connect it with short, thick traces.
EVALUATING PERFORMANCE
A recommended layout for the AD7631 is outlined in the
EVAL-AD7631EDZ evaluation board documentation. The
evaluation board package includes a fully assembled and tested
evaluation board, documentation, and software for controlling
the board from a PC via the EVAL-CED1Z.