AN1646 Freescale Semiconductor / Motorola, AN1646 Datasheet

AN1646

Manufacturer Part Number

AN1646

Description

AN1646 Noise Considerations for Integrated Pressure Sensors

Manufacturer

Freescale Semiconductor / Motorola

Datasheet

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SEMICONDUCTOR APPLICATION NOTE

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

outputs, built–in temperature compensation and an amplified

single–ended output which make them compatible with Ana-

log to Digital converters (A/D’s) on low cost micro–controllers.

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.

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 sec-

ond, 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 frequencies

REV 2

Motorola, Inc. 2001

Motorola Sensor Device Data

Motorola Integrated Pressure Sensors (IPS) have trimmed

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

INTRODUCTION

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

Figure 1. Raw Output

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

needs special consideration.

mitigating these noise signals, and achieving excellent results

with high resolution A/D converters.

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 am-

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

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

consider filtering. Filtering options are discussed as follows.

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

The following discussion presents simple techniques for

The transducer bridge produces a very small differential

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

EFFECTS OF NOISE IN SENSOR SYSTEM

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AN1646 Summary of contents

Page 1

... Figure 1. There is enough noise here to affect 1 count bit A/D, and counts bit A/ therefore important consider filtering. Filtering options are discussed as follows. Figure 1. Raw Output Go to: www.freescale.com Order this document by AN1646/D 1 ...

Page 2

Freescale Semiconductor, Inc. NOISE FILTERING TECHNIQUES AND CONSIDERATIONS For mitigating the effects of this sensor noise, two general approaches are effective, low pass filtering with hardware, and low pass filtering with software. When filtering with hard- Figure 2. Integrated Pressure ...

Page 3

Freescale Semiconductor, Inc. Figure 4. Output After Low Pass Filtering Figure 5. Output with 10 Averaged Samples For More Information On This Product, Motorola Sensor Device Data Go to: www.freescale.com 3 ...

Page 4

Freescale Semiconductor, Inc. Figure 6. Output with 64 Averaged Samples Figure 7. Filtered Sensor Output and Averaged Over 10 Samples For More Information On This Product to: www.freescale.com Motorola Sensor Device Data ...

Page 5

Freescale Semiconductor, Inc. POWER SUPPLY Since the sensor output is ratiometric with the supply volt- age, any variation in supply voltage will also proportionally ap- pear at the output of the sensor. The integrated pressure sensor is designed, characterized and ...

Page 6

Freescale Semiconductor, Inc. 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 ...

Page 7

... ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; Silicon Harbour Centre, 2 Dai King Street, Tai Po Industrial Estate, Tai Po, N.T., Hong Kong. 852–26668334 Technical Information Center: 1–800–521–6274 HOME PAGE: http://www.motorola.com/semiconductors/ For More Information On This Product, Motorola Sensor Device Data are registered trademarks of Motorola, Inc. Motorola, Inc Equal Go to: www.freescale.com 7 AN1646/D ...

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