AD7634 Analog Devices, AD7634 Datasheet - Page 31

• Manufacturer Part Number: AD7634
• Manufacturer: Analog Devices

Specifications of AD7634

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 670kSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 10V p-p,20 V p-p,40 V p-p
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

APPLICATION INFORMATION
LAYOUT GUIDELINES
While the AD7634 has very good immunity to noise on the
power supplies, exercise care with the grounding layout. To facili-
tate the use of ground planes that can be easily separated, design
the printed circuit board that houses the AD7634 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 AD7634, or
as close as possible to the AD7634. If the AD7634 is in a system
where multiple devices require analog-to-digital ground connect-
ions, the connections should still be made at one point only, a
star ground point, established as close as possible to the AD7634.
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 AD7634.
• Do shield fast switching signals, like CNVST or clocks, with
• Avoid crossover of digital and analog signals.
• Run traces on different but close layers of the board, at right
The power supply lines to the AD7634 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 AD7634, 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, and 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
in the vicinity of the ADC to further reduce low frequency ripple.
digital ground to avoid radiating noise to other sections of
the board, and never run them near analog signal paths.
angles to each other, to reduce the effect of feedthrough through
the board.
Rev. A | Page 31 of 32
The DVDD supply of the AD7634 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 AD7634 has four different ground pins: REFGND, AGND,
DGND, and OGND.
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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 AD7634 is outlined in the EVAL-
AD7634CBZ 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-CONTROL BRD3.
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.
AD7634