AD7678
Rev. A | Page 25 of 28
APPLICATION HINTS
LAYOUT
The AD7678 has very good immunity to noise on the power
supplies. However, care should still be taken with regard to
grounding layout.
The printed circuit board that houses the AD7678 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. Digital and
analog ground planes should be joined in only one place,
preferably underneath the AD7678, or at least as close to the
AD7678 as possible. If the AD7678 is in a system where
multiple devices require analog-to-digital ground connections,
the connection should still be made at one point only, a star
ground point that should be established as close to the AD7678
as possible.
The user should avoid running digital lines under the device,
because these will couple noise onto the die. The analog ground
plane should be allowed to run under the AD7678 to avoid
noise coupling. Fast switching signals like
CNVST
or clocks
should be shielded with digital ground to avoid radiating noise
to other sections of the board, and should never run near
analog signal paths. Crossover of digital and analog signals
should be avoided. Traces on different but close layers of the
board should run at right angles to each other. This will reduce
the effect of feedthrough through the board. The power supply
lines to the AD7678 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 supply’s impedance presented to the AD7678 and
to reduce the magnitude of the supply spikes. Decoupling
ceramic capacitors, typically 100 nF, should be placed close to
and ideally right up against each power supply pin (AVDD,
DVDD, and OVDD) 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 AD7678 can be a separate supply or
can come from the analog supply, AVDD, or the digital
interface supply, OVDD. When the system digital supply is
noisy or when fast switching digital signals are present, and if
no separate supply is available, the user should connect the
DVDD digital supply to the analog supply AVDD through an
RC filter (see Figure 21), and connect the system supply to the
interface digital supply OVDD and the remaining digital
circuitry. When DVDD is powered from the system supply, it is
useful to insert a bead to further reduce high frequency spikes.
The AD7678 has four different ground pins: REFGND, AGND,
DGND, and OGND. REFGND senses the reference voltage and
should be a low impedance return to the reference because it
carries pulsed currents. AGND is the ground to which most
internal ADC analog signals are referenced. This ground 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. The decoupling capacitor should be close to the
ADC and should be connected with short and large traces to
minimize parasitic inductances.
EVALUATING THE AD7678’S PERFORMANCE
The evaluation board for the AD7678 allows a quick means to
measure both dc (histograms and time domain) and ac (time
and frequency domain) performances of the converter. The
EVAL-AD7678CBZ is an evaluation board package that includes
a fully assembled and tested evaluation board, documentation,
and software. The accompanying software requires the use of a
capture board that must be ordered seperately from the evalua-
tion board (see the Ordering Guide for information). The
evaluation board can also be used in a standalone configuration
and does not use the software when in this mode. Refer to the
EVAL-AD76XXEDZ and EVAL-AD76XXCBZ data sheets
available from www.analog.com for evaluation board details.
Two types of data capture boards can be used with the EVAL-
AD7678CBZ:
USB based (EVAL-CED1Z recommended)
Parallel port based (EVAL-CONTROL BRD3Z not
recommended because many newer PCs do not include
parallel ports any longer)
The recommended board layout for the AD7678 is outlined in
the evaluation board data sheet.
