Data Sheet AD7654
Rev. D | Page 25 of 27
APPLICATION HINTS
LAYOUT
The AD7654 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 AD7654 should be
designed so 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. Digital and analog
ground planes should be joined in only one place, preferably
underneath the AD7654, or as close as possible to the AD7654.
If the AD7654 is in a system where multiple devices require
analog-to-digital ground connections, the connection should
still be made at only a star ground point established as close as
possible to the AD7654.
Running digital lines under the device should be avoided
because these couple noise onto the die. The analog ground
plane should be allowed to run under the AD7654 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 reduces the
effect of crosstalk through the board.
The power supply lines to the AD7654 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
AD7654 and to reduce the magnitude of the supply spikes.
Decoupling ceramic capacitors, typically 100 nF, should be
placed on each power supply pin—AVDD, DVDD, and
OVDD—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 AD7654 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, if no separate supply is
available, the user should connect DVDD to AVDD through an
RC filter (see Figure 19) and the system supply to 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 AD7654 has five different ground pins: INGND, REFGND,
AGND, DGND, and OGND. INGND is used to sense the
analog input signal. 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.
