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AD7278BUJZ-REEL7 Datasheet(PDF) 25 Page - Analog Devices

Part No. AD7278BUJZ-REEL7
Description  3 MSPS, 12-/10-/8-Bit ADCs in 6-Lead TSOT
Download  28 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7278BUJZ-REEL7 Datasheet(HTML) 25 Page - Analog Devices

 
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AD7276/AD7277/AD7278
Rev. C | Page 25 of 28
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7276/AD7277/
AD7278 should be designed so that the analog and digital
sections are separated and confined to certain areas of the
board. This design facilitates using ground planes that can easily
be separated.
To provide optimum shielding for ground planes, a minimum
etch technique is generally best. All AGND pins of the AD7276/
AD7277/AD7278 should be sunk into the AGND plane. Digital
and analog ground planes should be joined in one place only. If
the AD7276/AD7277/AD7278 are in a system where multiple
devices require an AGND-to-DGND connection, the connection
should still be made at only one point, a star ground point
established as close as possible to the ground pin on the
AD7276/AD7277/AD7278.
Avoid running digital lines under the device because this
couples noise onto the die. However, the analog ground plane
should be allowed to run under the AD7276/AD7277/AD7278
to avoid noise coupling. The power supply lines to the AD7276/
AD7277/AD7278 should use as large a trace as possible to provide
low impedance paths and reduce the effects of glitches on the
power supply line.
To avoid radiating noise to other sections of the board,
components with fast-switching signals, such as clocks, should
be shielded with digital ground, and they should never be run
near the analog inputs. Avoid crossover of digital and analog
signals. To reduce the effects of feedthrough within the board,
traces on opposite sides of the board should run at right angles
to each other. A microstrip technique is by far the best method,
but it is not always possible to use this approach with a double-
sided board. In this technique, the component side of the board
is dedicated to ground planes, and signals are placed on the
solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 μF ceramic capacitors in parallel with
0.1 μF capacitors to GND. To achieve the best results from these
decoupling components, they must be placed as close as possible
to the device, ideally right up against the device. The 0.1 μF
capacitors should have low effective series resistance (ESR) and
low effective series inductance (ESI), such as is typical of common
ceramic or surface-mount types of capacitors. Capacitors with
low ESR and low ESI provide a low impedance path to ground
at high frequencies, which allow them to handle transient
currents due to internal logic switching.
EVALUATING PERFORMANCE
The recommended layout for the AD7276/AD7277/AD7278 is
outlined in the evaluation board documentation. The evaluation
board package includes a fully assembled and tested evaluation
board, documentation, and software for controlling the board
from the PC via the evaluation board controller. To demonstrate/
evaluate the ac and dc performance of the AD7276/AD7277,
the evaluation board controller can be used in conjunction with
the AD7276/AD7277 evaluation board, as well as with many
other Analog Devices evaluation boards ending in the CB
designator,
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the AD7276/
AD7277. The software and documentation are on a CD shipped
with the evaluation board.


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