AD7951 Analog Devices, AD7951 Datasheet - Page 31

• Manufacturer Part Number: AD7951
• Manufacturer: Analog Devices

Specifications of AD7951

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: Bip 10V,Bip 5.0V,Uni 10V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

APPLICATION INFORMATION
LAYOUT GUIDELINES
While the AD7951 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 AD7951 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
AD7951, or as close as possible to the AD7951. If the AD7951 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 AD7951.
To prevent coupling noise onto the die, avoid radiating noise,
and to reduce feedthrough:
• Do not run digital lines under the device.
• Do run the analog ground plane under the AD7951.
• 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 AD7951 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 AD7951, 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. 0 | Page 31 of 32
The DVDD supply of the AD7951 can either be 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 AD7951 has four different ground pins: REFGND, AGND,
DGND, and OGND.
•
•
•
•
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 AD7951 is outlined in the
EVAL-AD7951CB 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.
AD7951