Data Acquisition Handbook

1 A RefeRence foR DAQ AnD AnAlog & DigitAl SignAl conDitioning A/D Conversion Accelerometers Amplification AttenuationCold-Junction Compensation D/A Conversion Digital I/O InterfacingDisplacement & Position Sensing Electrical Measurements FilteringFrequency Measurement Fundamental Signal ConditioningInstrumentation Amplifiers Isolation Multiplexing Noise ReductionRMS Measurements RTDs Sensors Strain Gages ThermistorsThermocouples Transducer Electronic data Sheets Transducers VibrationHandbookdataacquisitionThird EdiTionA RefeRence foR DAQ AnD AnAlog & DigitAl SignAl conDitioning handbookdaTaacquisiTioniiCopyright Notice 2004-2012 by Measurement Computing CorporationAll rights reserved. No part of this book may be reproduced in any form, or by any means, without the permission of Measurement Computing 2004-2012 in the United States of AmericaMeasurement Computing Computing OF CONTENTS.

2 IiiPREFACE ..ixChAPTER 1 INTROduCTION TO data Acquisition ANd SIgNAL CONdITIONINg ..1 Signals, Sensors, and Signal Conditioning ..1 data Acquisition 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 : Effective Number of The ENOB Test ..10 AdC Output Averaging Benefits ..11 Improved Accuracy ..11 More Stable Readings ..11 ChAPTER 3 MuLTIPLExINg ANd SAMPLINg Economy of Multiplexing ..13 Sampled- data Systems ..13 Solid-State Switches vs . Relays ..13 Speed ..14 Sequence vs . Software-Selectable Ranges ..14iiiTable of ContentsMeasurement Computing Computing OF CONTENTSF undamental Concepts.

3 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 ..24AC voltage ..24 Low AC Voltage ..24 Outlet Level AC Voltage ..24 High Voltage ..24 Effective, Peak, Average, and RMS Voltage ..25 Current ..26A Fundamental Problem ..26 Insertion Loss ..27 Common-Mode Limitations ..27 Shunts ..27 Current Transformers ..28 Hall-Effect Sensors .. Measurements ..30 Voltage Divider Method ..31 Constant Current/Voltage Drop ..31 Small Deviations ..31 Wheatstone Principle ..32 Anderson Loop Alternative ..32 Single-Ended and differential Principle ..34 When to Make Single-Ended Measurements.

4 34 When to Make Differential Measurements ..34 High Common-Mode Voltage Measurements ..36 Kelvin Computing Computing Acquisition HandbookBasic Principle ..37 Examples of Kelvin-Type Measurements ..37 ChAPTER 5 FuNdAMENTAL SIgNAL ..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 ..44A Fundamental Problem ..44 High Common-Mode Amplifiers ..44 Integrated Instrumentation Amplifiers ..47 Programmable-Gain Instrumentation Amplifiers .. Filters ..48 High-Pass Filters ..49 Passive vs . Active Filters ..49 Switched-Capacitance Filters .. Dividers ..51 Buffered Voltage Dividers ..52 Balanced Differential Dividers ..52 High-Voltage Dividers.

5 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 .. Linearization is Needed ..60 Software Linearization ..61 Hardware Linearization ..62 Circuit to Instrumentation Circuits ..63 Overload Protection ..64 ESD Protection ..64 Measurement Computing Computing OF CONTENTSChAPTER 6 TEMPERATuRE Basics ..65 The Gradient Nature of Thermocouples ..65 Cold-Junction Compensation ..66 Type Mixing ..68 Thermocouple Measurement Environments ..70 Additional Concerns ..71 RTd Measurements ..73 Basics of Resistance Temperature Detectors ..73 Measurement Approaches 2, 3, and 4-Wire Configurations ..74 Thermistor of Thermistors ..78 Linearization.

6 78 Measurement Approaches Temperature Measurement ..80 ChAPTER 7 STRAIN MEASuREMENTS ..81 Strain Basics ..81 Strain Measurement 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 Transducers ..95 ..96 ..97 LVDT: Linear Variable Differential Transformers Magnetostrictive Position Sensor ..Linear Encoders ..98 Rotational Absolute Encoders ..99 Measurement Computing Computing Acquisition HandbookIncremental Encoders ..99 Quadrature Encoders.

7 100 Errors ..100 ChAPTER 10 NOISE REduCTION ANd grounding for Safety ..103 Grounding for Robust Instrumentation ..104ground of Ground Loops ..105 How Ground Loops Are Created ..105 How to Eliminate Ground Loops ..105 Crosstalk in data Acquisition Crosstalk ..106 Dynamic Crosstalk ..106 Shielded Wiring ..106 Benefits ..106 Proper Installation and Use of Shields ..107 Isolation and Floating data Acquisition Systems ..108 Isolation ..108 Circuit Protection ..108 Rejection of High Common-Mode Voltage ..108 Fundamental Application Mistakes ..110 Isolation Transformers ..111 Analog Isolators ..112 Wireless Techniques ..112 Noise Averaging ..113 Analog Filtering ..113 Passive vs . Active Filters ..114 Differential Voltage Measurements ..114 ChAPTER 11 dIgITAL ANd PuLSE-TRAIN CONdITIONINg ..117digital I/O Interfacing.

8 117 Digital Signals ..117 High Current and Voltage Digital I/O ..117digital Inputs ..118 Asynchronous Digital-Input Readings ..118 Synchronous Digital-Input Readings ..119 Externally Triggered Digital-Input Readings ..119 Measurement Computing Computing OF CONTENTS digital Isolation ..119 Pulse Train Signal Conversion ..120 Pulse-Train Integration ..120 Digital-Pulse Counting ..121 Frequency Measurement by Gated-Pulse Counting ..123 Timing Applications ..123 ChAPTER 12 TRANSduCER ELECTRONIC data ShEETS ..125 Smart 1: NCAP Information Model ..127 Part 2: Transducer to Microprocessor Protocol ..127 Part 3: Digital Communications and Format ..127 Part 4: Mixed-Mode Communications Protocol ..127 Part 5: Wireless Communications Protocol ..127 TEdS Composition .. Computing (508) 946-5100 third edition of this Handbook has been totally revised to include new chapters on Electrical Measurements, Vibration and Sound, Displacement and Position Sensing, and Transducer Electronic data Sheets (TEDS).

9 It also includes several new subjects and expands on selected items including Fundamental Signal Conditioning. All chapters have been enhanced to address more practical applications than theoretical measurement issues. They cover a major topic with sufficient detail to help readers under-stand the basic principles of sensor operation and the need for careful system interconnec-tions. The Handbook also discusses key issues concerning the data Acquisition system s multiplexing and signal conditioning circuits, and analog-to-digital converters. These three functions establish the overall accuracy, reso-lution, speed, and sensitivity of data acquisi-tion systems and determine how well the systems perform. data Acquisition systems measure, store, display, and analyze information collected from a variety of devices. Most measurements require a transducer or a sensor, a device that converts a measurable physical quantity into an electrical signal.

10 Examples include temper-ature, strain, acceleration, pressure, vibration, and sound. Yet others are humidity, flow, level, velocity, charge, pH, and chemical composition. ixPreface to the Third EditionSensors come in numerous shapes, sizes, and specifications. They connect between the measured physical device and the signal condi-tioner s input. Most sensors are purchased off-the-shelf, but in some cases, they are custom made specifically for a particular measurement requirement. Regardless of input, however, the output signal is usually a voltage, current, charge, or resistance and all can be conditioned and handled equally well. Manufacturers frequently provide specifications, application notes, and principles of operation for their specific sensor to help users apply the device in the most efficient way. Signal conditioners accept sensor output signals and convert them into a form that the data Acquisition system can manipulate.