AN1646 Freescale Semiconductor / Motorola, AN1646 Datasheet - Page 6

• Manufacturer Part Number: AN1646
• Description: AN1646 Noise Considerations for Integrated Pressure Sensors
• Manufacturer: Freescale Semiconductor / Motorola

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.
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 (V
nection should be as close as possible to the microcontrol-
ler’s power supply return (V
V
it approaches V
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-
REFL
For similar reasons it is desirable to run sensor output sig-
The primary layout issue with digital circuits is ground
In addition to grounding, the digital portion of a system
6
to V
SENSOR/ANALOG
DIGITAL GROUND/GROUND PLANE
SS
GROUND
is isolated from the rest of digital ground until
Figure 9. Ground Partitioning
SS
.
DIGITAL LAYOUT
REFL
). The power source ground con-
SS
). Note also that the path from
Freescale Semiconductor, Inc.
For More Information On This Product,
GROUND
POWER
Go to: www.freescale.com
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.
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.
following recommended power supply, grounding and layout
techniques.
[1] AN1626 Noise Management in Motor Drives, Warren
[2] Noise Reduction Techniques In Electronic Systems 2nd
[3] Noise: Comparing Integrated Pressure Sensors and Op
Piezo–resistive pressure sensors produce small amounts
In a mixed sensor system, noise can be further reduced by
Schultz, Motorola, Inc.
Edition, Henry W. Ott, John Wiley & Sons.
Amps, Ira Basket, Motorola Sensor Products Division
internal paper.
AVOID
AVOID
Figure 10. 90 Degree Angles
SINGLE TRACE
T–CONNECTION
REFERENCES
CONCLUSION
Motorola Sensor Device Data
GOOD PRACTICE
GOOD PRACTICE