MPXM2102AS datasheet(529/670 Pages) MOTOROLA

529 / 670 page

AN1646

3–383

Motorola Sensor Device Data

www.motorola.com/semiconductors

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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