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Motorola Sensor Device Data
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For similar reasons it is desirable to run sensor output sig-
nals and their return traces as close to each other as pos-
sible. Minimizing this loop area will minimize the amount of
external noise that is picked up by making electrical connec-
tions to the sensor.
DIGITAL LAYOUT
The primary layout issue with digital circuits is ground
partitioning. A good place to start is with the architecture that
is shown in Figure 9. This architecture has several key attrib-
utes. Analog ground and digital ground are both separate
and distinct from each other, and come together at only one
point. For analog ground it is preferable to make the one point
as close as possible to the analog to digital converter’s
ground reference (VREFL). The power source ground con-
nection should be as close as possible to the microcontrol-
ler’s power supply return (VSS). Note also that the path from
VREFL to VSS is isolated from the rest of digital ground until
it approaches VSS.
DIGITAL GROUND/GROUND PLANE
POWER
GROUND
SENSOR/ANALOG
GROUND
VREFL
VSS
Figure 9. Ground Partitioning
In addition to grounding, the digital portion of a system
benefits from attention to avoiding 90 degree angles, since
there are generally a lot of high speed signals on the digital
portion of the board. Routing with 45 degree angles or curves
minimizes unwanted reflections, which increases noise immu-
nity. Single traces are easy, two forty five degree angles or a
curve easily accomplish a 90 degree turn. It is just as important
to avoid 90 degree angles in T connections. Figure 10 illus-
trates correct versus incorrect routing for both cases.
Figure 10. 90 Degree Angles
SINGLE TRACE
AVOID
GOOD PRACTICE
T–CONNECTION
AVOID
GOOD PRACTICE
CONCLUSION
Piezo–resistive pressure sensors produce small amounts
of noise that can easily be filtered out with several methods.
These methods are low pass filtering with an RC filter, averag-
ing or a combination of both which can be implemented with
minimal hardware cost.
In a mixed sensor system, noise can be further reduced by
following recommended power supply, grounding and layout
techniques.
REFERENCES
[1] AN1626 Noise Management in Motor Drives, Warren
Schultz, Motorola, Inc.
[2] Noise Reduction Techniques In Electronic Systems 2nd
Edition, Henry W. Ott, John Wiley & Sons.
[3] Noise: Comparing Integrated Pressure Sensors and Op
Amps, Ira Basket, Motorola Sensor Products Division
internal paper.
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Freescale Semiconductor, Inc.
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