AD7938 Analog Devices, AD7938 Datasheet - Page 31

• Manufacturer Part Number: AD7938
• Description: 8-Channel, 1.5 MSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 1.5MSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,5V p-p,Uni (Vref),Uni (Vref) x 2,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

Data Sheet
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7938/AD7939
should be designed so that 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 easily separated.
Generally, a minimum etch technique is best for ground planes
since it gives the best shielding. Digital and analog ground
planes should be joined in only one place, and the connection
should be a star ground point established as close to the ground
pins on the AD7938/AD7939 as possible. Avoid running digital
lines under the device as this couples noise onto the die. The
analog ground plane should be allowed to run under the
AD7938/AD7939 to avoid noise coupling. The power supply
lines to the AD7938/AD7939 should use as large a trace as
possible to provide low impedance paths and reduce the effects
of glitches on the power supply line.
Fast switching signals, such as clocks, should be shielded with
digital ground to avoid radiating noise to other sections of the
board, and clock signals should never run near the analog
inputs. Avoid crossover of digital and analog signals. Traces on
opposite sides of the board should run at right angles to each
other. This reduces the effects of feedthrough through the
board. A microstrip technique is by far the best 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
0.1 µF capacitors to GND. To achieve the best performance
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 effective series inductance (ESI), such as
the common ceramic types or surface-mount types, which
provide a low impedance path to ground at high frequencies to
handle transient currents due to internal logic switching.
Rev. C | Page 31 of 36
PCB DESIGN GUIDELINES FOR CHIP SCALE
PACKAGE
The lands on the chip scale package (CP-32-2) are rectangular.
The printed circuit board pad for these should be 0.1 mm
longer than the package land length and 0.05 mm wider than
the package land width. The land should be centered on the
pad. This ensures that the solder joint size is maximized. The
bottom of the chip scale package has a thermal pad. The
thermal pad on the printed circuit board should be at least as
large as this exposed pad. On the printed circuit board, there
should be a clearance of at least 0.25 mm between the thermal
pad and the inner edges of the pad pattern. This ensures that
shorting is avoided. Thermal vias can be used on the printed
circuit board thermal pad to improve thermal performance of
the package. If vias are used, they should be incorporated 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 printed circuit board thermal pad to AGND.
EVALUATING AD7938/AD7939 PERFORMANCE
The recommended layout for the AD7938/AD7939 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. The evaluation board
controller can be used in conjunction with the AD7938/AD7939
evaluation board, as well as many other Analog Devices evaluation
boards ending in the CB designator, to demonstrate and
evaluate the ac and dc performance of the AD7938/AD7939.
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the
AD7938/AD7939. The software and documentation are on the
CD that ships with the evaluation board.
AD7938/AD7939