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AD7264BCPZ Datasheet(PDF) 29 Page - Analog Devices

Part No. AD7264BCPZ
Description  1 MSPS, 14-Bit, Simultaneous Sampling SAR ADC
Download  30 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7264BCPZ Datasheet(HTML) 29 Page - Analog Devices

 
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AD7264
Data Sheet
Rev. D | Page 28 of 29
APPLICATIONS INFORMATION
GROUNDING AND LAYOUT
The analog and digital supplies to the AD7264 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The printed circuit
board (PCB) that houses the AD7264 should be designed so
that the analog and digital sections are separated and confined
to certain areas of the board. This design facilitates the use of
ground planes that can be easily separated.
To provide optimum shielding for ground planes, a minimum
etch technique is generally best. All five AGND pins of the AD7264
should be sunk in the AGND plane. Digital and analog ground
planes should be joined in only one place. If the AD7264 is 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, that should be established as close as possible
to the ground pins on the AD7264.
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 AD7264 to avoid noise coupling. The
power supply lines to the AD7264 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, fast
switching signals, such as clocks, should be shielded with digital
ground, and clock signals should never 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 the best method but is not always possible with a
double-sided board. In this technique, the component side of
the board is dedicated to ground planes, while signals are placed
on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 μF tantalum capacitors in parallel with
100 nF 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 the
common ceramic types or surface-mount types. These low ESR
and low ESI capacitors provide a low impedance path to ground
at high frequencies to handle transient currents due to internal
logic switching.
PCB DESIGN GUIDELINES FOR LFCSP
The lands on the chip scale package (CP-48-1) are rectangular.
The PCB pad for these should be 0.1 mm longer than the package
land length, and 0.05 mm wider than the package land width,
leaving a portion of the pad exposed. To ensure that the solder
joint size is maximized, the land should be centered on the pad.
The bottom of the chip scale package has a thermal pad. The
thermal pad on the PCB should be at least as large as the exposed
pad. On the PCB, there should be a clearance of at least 0.25 mm
between the thermal pad and the inner edges of the pad pattern
to ensure that shorting is avoided.
To improve thermal performance of the package, use thermal vias
on the PCB, incorporating them in the thermal pad at 1.2 mm
pitch grid. The via diameter should be between 0.3 mm and
0.33 mm, and the via barrel should be plated with 1 oz copper
to plug the via. The user should connect the PCB thermal pad
to AGND.


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