Data Acquisition Handbook V

Data Acquisition Handbook V

d ata a c q u i s i t i o n Handbook A RefeRence foR DAQ AnD AnAlog & DigitAl SignAl conDitioning A/D Conversion • Accelerometers • A mplification • Attenuation Cold-Junction Compensation • D/A Conversion • Digital I/O Interfacing Displacement & P o s i t i o n S e n s i n g • Electrical Measurements • F i l t e r i n g Frequency Measurement • Fundamental Signal Conditioning Instrumentation Amplifiers • Isolation • Multiplexing • Noise Reduction RMS Measurements • R T D s • S e n s o r s • Strain Gages • Thermistors Thermocouples • Transducer Electronic Data Sheets • Transducers • V i b r a t i o n d aTa a c q u i s i T i o n handbook A RefeRence foR DAQ AnD AnAlog & DigitAl SignAl conDitioning T h i r d E d i T i o n ii Copyright Notice © 2004-2012 by Measurement Computing Corporation All rights reserved. No part of this book may be reproduced in any form, or by any means, without the permission of Measurement Computing Corporation. Published 2004-2012 in the United States of America iii Table of Contents TABLE OF CONTENTS. iii PREFACE . ix ChAPTER 1 INTROduCTION TO DatA ACquISITION ANd SIgNAL CONdITIONINg ......1 Signals, Sensors, and Signal Conditioning ...........................1 data Acquisition Systems . 1 PC-Based data Acquisition Equipment ..............................2 ChAPTER 2 ANALOg TO dIgITAL CONvERSION . .3 AdC Types ....................................................3 Successive-Approximation ADCs . 3 Voltage-to-Frequency ADCs . 4 Integrating ADCs: Dual Slope . 4 Sigma-Delta ADCs . 5 Accuracy and Resolution .........................................6 Accuracy vs . Resolution . 6 ADC Accuracy vs . System Accuracy . 8 AdC Noise histograms . 9 ENOB: Effective Number of Bits . .9 The ENOB Test . 10 AdC Output Averaging Benefits ..................................11 Improved Accuracy . 11 More Stable Readings . 11 ChAPTER 3 MuLTIPLExINg ANd SAMPLINg ThEORY . .13 The Economy of Multiplexing ....................................13 Sampled-Data Systems . 13 Solid-State Switches vs . Relays . 13 Speed . 14 Sequence vs . Software-Selectable Ranges . 14 Measurement Computing (508) 946-5100 [email protected] mccdaq.com Measurement Computing (508) 946-5100 [email protected] mccdaq.com iv TABLE OF CONTENTS Fundamental Concepts .........................................15 Sampling Rates . 15 Source Impedance . 16 Sample-and-Hold ADCs . 16 Nyquist Theorem . 17 Aliasing and Fourier Transforms . 17 Settling Time . 21 ChAPTER 4 ELECTRICAL MEASuREMENTS . .23 Safety Concerns ...............................................23 Making valid Measurements .....................................23 dC voltage ...................................................23 Instrumentation Level DC Voltage . 23 High DC Voltage . 23 Low DC Voltage . 24 AC voltage ...................................................24 Low AC Voltage . 24 Outlet Level AC Voltage . 24 High Voltage . 24 Effective, Peak, Average, and RMS Voltage . 25 Current ......................................................26 A Fundamental Problem . 26 Insertion Loss . 27 Common-Mode Limitations . 27 Shunts . 27 Current Transformers . 28 Hall-Effect Sensors . 29 Resistance . .30 Basic Measurements . 30 Voltage Divider Method . 31 Constant Current/Voltage Drop . 31 Small Deviations . 31 Wheatstone Bridges . .32 Basic Principle . 32 Anderson Loop Alternative . 32 Single-Ended and differential Measurements . .34 Basic Principle . 34 When to Make Single-Ended Measurements . 34 When to Make Differential Measurements . 34 High Common-Mode Voltage Measurements . 36 Kelvin Connections . 37 Measurement Computing (508) 946-5100 [email protected] mccdaq.com Measurement Computing (508) 946-5100 [email protected] mccdaq.com Data Acquisition Handbook v Basic Principle . 37 Examples of Kelvin-Type Measurements . 37 ChAPTER 5 FuNdAMENTAL SIgNAL CONdITIONINg . .39 Amplification .................................................39 Data Acquisition Front Ends . 39 Operational Amplifiers . 40 Inverting Amplifier Stages . 41 Non-Inverting Amplifier Stages . 42 Differential Amplifiers . 42 Programmable-Gain Amplifiers . 43 Instrumentation Amplifiers ......................................44 A Fundamental Problem . 44 High Common-Mode Amplifiers . 44 Integrated Instrumentation Amplifiers . 47 Programmable-Gain Instrumentation Amplifiers . 47 Filtering . .47 Low-Pass Filters . 48 High-Pass Filters . 49 Passive vs . Active Filters . 49 Switched-Capacitance Filters . 50 Attenuation . 51 Voltage Dividers . 51 Buffered Voltage Dividers . 52 Balanced Differential Dividers . 52 High-Voltage Dividers . 52 Compensated Voltage Dividers and Probes . 52 Isolation .....................................................53 When Isolation is Required . 53 Isolation Amplifiers . 53 Analog Isolation Modules . 54 Digital Method of Isolation . 56 Inherently Isolated Sensors . 57 Linearization . .60 Why Linearization is Needed . 60 Software Linearization . 61 Hardware Linearization . 62 Circuit Protection . .63 Hazards to Instrumentation Circuits . 63 Overload Protection . 64 ESD Protection . 64 Measurement Computing (508) 946-5100 [email protected] mccdaq.com Measurement Computing (508) 946-5100 [email protected] mccdaq.com vi TABLE OF CONTENTS ChAPTER 6 TEMPERATuRE MEASuREMENT . .....................................65 Thermocouple Basics ...........................................65 The Gradient Nature of Thermocouples . 65 Cold-Junction Compensation . 66 Type Mixing . 68 Thermocouple Measurement Pitfalls . 70 Noisy Environments . 70 Additional Concerns . 71 RTd Measurements ............................................73 Basics of Resistance Temperature Detectors . 73 Measurement Approaches – 2, 3, and 4-Wire Configurations . 74 Thermistor Measurements . 78 Basics of Thermistors . 78 Linearization . 78 Measurement Approaches – Temperature Measurement . 80 ChAPTER 7 STRAIN MEASuREMENTS . 81 Strain Basics ..................................................81 Strain Measurement Configurations . 81 Wheatstone Bridge . 81 Full-Bridge Circuits . 82 Half-Bridge Circuits . 83 Quarter-Bridge Circuits . 83 Calibration Approaches . 87 Transducers and Load Cells . 87 Strain Diaphragm Pressure Gages . 87 ChAPTER 8 vIBRATION ANd SOuNd ...........................................89 Sensors for vibration and Sound Measurements .....................89 Strain-Gage Accelerometers . 89 Piezoelectric Transducers . 90 Low-Impedance Transducers . 91 ChAPTER 9 dISPLACEMENT ANd POSITION SENSINg .............................95 Linear displacement . .95 String-Potentiometer Transducers . 95 LVDT: Linear Variable Differential Transformers . 96 Magnetostrictive Position Sensor . 97 Linear Encoders . 98 Rotational Position . 98 Absolute Encoders . 99 Measurement Computing (508) 946-5100 [email protected] mccdaq.com Measurement Computing (508) 946-5100 [email protected] mccdaq.com Data Acquisition Handbook vii Incremental Encoders . 99 Quadrature Encoders . 100 Errors . 100 ChAPTER 10 NOISE REduCTION ANd ISOLATION . 103 Controlling Noise . 103 The grounding Conflict . .103 Grounding.

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