MPXM2102AS datasheet(524/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1646

Noise Considerations

for Integrated Pressure Sensors

Prepared by Ador Reodique, Sensor and Systems Applications Engineering and Warren Schultz, Field Engineering

INTRODUCTION

Motorola Integrated Pressure Sensors (IPS) have trimmed

outputs, built–in temperature compensation and an amplified

single–ended output which make them compatible with

Analog to Digital converters (A/D’s) on low cost micro–control-

lers. Although 8–bit A/D’s are most common, higher resolution

A/D’s are becoming increasingly available. With these higher

resolution A/D’s, the noise that is inherent to piezo–resistive

bridges becomes a design consideration.

The two dominant types of noise in a piezo–resistive inte-

grated pressure sensor are shot (white) noise and 1/f (flicker

noise). Shot noise is the result of non–uniform flow of carriers

across a junction and is independent of temperature. The

second, 1/f, results from crystal defects and also due to wafer

processing. This noise is proportional to the inverse of fre-

quency and is more dominant at lower frequencies3.

Noise can also come from external circuits. In a sensor sys-

tem, power supply, grounding and PCB layout is important and

needs special consideration.

The following discussion presents simple techniques for

mitigating these noise signals, and achieving excellent results

with high resolution A/D converters.

EFFECTS OF NOISE IN SENSOR SYSTEM

The transducer bridge produces a very small differential

voltage in the millivolt range. The on–chip differential amplifier

amplifies, level shifts and translates this voltage to a single–

ended output of typically 0.2 volts to 4.7 volts. Although the

transducer has a mechanical response of about 500 Hz, its

noise output extends from 500 Hz to 1 MHz. This noise is

amplified and shows up at the output as depicted in Figure 1.

There is enough noise here to affect 1 count on an 8 bit A/D,

and 4 or 5 counts on a 10 bit A/D. It is therefore important to

consider filtering. Filtering options are discussed as follows.

Figure 1. MPX5006 Raw Output

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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