Table Of Content

Signal Conditioning and PC-Based Data Acquisition Handbook IOtech's Signal Conditioning & PC-Based Data Acquisition Handbook The 128-page Signal Conditioning & PC-Based Data Acquisition Handbook is a comprehensive reference tool that helps engineers design accurate applications, avoid common pitfalls, and understand the requirements needed for proper transducer use. It introduces basic signal conditioning and data acquisition techniques and provides practical information on temperature, strain, acceleration, analog-to-digital conversion, multiplexing, general amplification, noise reduction, and digital signal conditioning. HTML Request your FREE excerpts from the book appear below, and the IOtech Catalog! full text is now available in PDF format. For a free print copy, please fill out our online request form! HTML excerpts of the Signal Conditioning & PC- Based Data Acquisition Handbook Chapter 1 (Introduction) Chapter 3 (Multiplexing) PDF version of the Signal Conditioning & PC-Based Data Acquisition Handbook Preface Chapter 1 Introduction to Data Acquisition & Signal Conditioning Chapter 2 Analog-to-Digital Conversion Chapter 3 Multiplexing Chapter 4 Temperature Measurement Chapter 5 Strain & Acceleration Chapter 6 General Amplification Chapter 7 Noise Reduction & Isolation Chapter 8 Digital & Pulse-Train Conditioning Chapter 9 Product Selection Guide (1.8 MB) Index http://www.iotech.com/prsigcon.html (1 of 2) [28/07/01 18:42:02] Signal Conditioning and PC-Based Data Acquisition Handbook Request your copy of the Signal Conditioning Handbook today! [ HOME | PRODUCTS | TECH SUPPORT | CONTACT | SEARCH/MAP | ABOUT IOTECH | SHOP ONLINE ] ® Copyright 2000, IOtech Inc. Privacy Policy http://www.iotech.com/prsigcon.html (2 of 2) [28/07/01 18:42:02] Preface Signal Conditioning Handbook Preface Any industry that performs testing or monitoring has an array of transducers specifically designed for its particular measurement requirements. The intent of this handbook is to introduce the reader to the most commonly used transducer interfaces and to provide practical information for dealing with the most frequently encountered transducers and their associated signals. Transducers are available for measuring many physical quantities, such as temperature, pressure, strain, vibration, sound, humidity, flow, level, velocity, charge, pH, and chemical composition, among others. In most cases, the transducer manufacturer provides applica- tion notes on the transducer’s use and principles of operation. The main questions to consider when selecting a transducer are: • What are the electrical characteristics (amplitude, frequency, source impedance) of the transducer’s output? • What kind of power supply/excitation is required? • What is the transducer’s specified accuracy? • Over what range of amplitude and frequency are the measurements accurate? • How is the transducer calibrated? • How can transducer accuracy and calibration be verified? • In what environment (temperature, humidity, vibration, pressure) is the transducer able to operate? It is generally unwise to view a transducer as a black box that provides a specified output for a certain input. Knowing how a transducer works is imperative to making reliable measurements. i Chapter 1 Introduction Chapter 1 Introduction 1 Chapter 1 Introduction This handbook is intended for newcomers to the field of data acquisition and signal conditioning. Emphasis is given to general discussions of ADC measurement and the signal conditioning requirements of selected transducer types. For more detailed descriptions of the signal conditioning schemes discussed, contact IOtech for Applica- tions Notes and Product Specification Sheets. Additional information on IOtech products is available in Chapter 9, a product selection guide. Most measurements begin with a transducer, a device that converts a measurable physical quality, such as temperature, strain, or acceleration, to an electrical signal. Trans- ducers are available for a wide range of measurements, and come in a variety of shapes, sizes, and specifications. This book is intended to serve as a primer for making measurements by interfacing transducers to a computer using signal conditioning. Signal conditioning converts a Transducer transducer’s signal so that an analog- to-digital converter (ADC) can measure the signal. Signal conditioning Signal can include amplification, filtering, Conditioning differential applications, isolation, simultaneous sample and hold (SS&H), current-to-voltage conversion, volt- ADC age-to-frequency conversion, linearization and more. Signal conditioning also includes excitation or bias for Computer transducers that require it. Fig. 1.01: Generic signal-conditioning scheme Figure 1.01 depicts a generic data acquisition signal-conditioning configu- ration. The transducer is connected to the input of the signal conditioning electronics. The output of the signal conditioning is connected to an ADC input. The ADC converts the analog voltage to a digital signal, which is transferred to the computer for processing, graphing, and storage. Analog-to-Digital Conversion. Chapter 2 includes a discussion of the four basic ADC types, as well as issues such as accuracy, noise reduction, and discrete sampling considerations. Topics such as input and source impedance, differential voltage measurements, simultaneous sample and hold, selectable input ranges, multiplexing, and isolation are also discussed. A section on discrete sampling considerations, which covers aliasing, windowing, fast Fourier transforms (FFTs), standard Fourier transforms, and digital filtering, is also included. 2 Chapter 1 Introduction Multiplexing. Chapter 3 includes a discussion on multiplexing, current measurements, and the associated issues such as simultaneous sample and hold, input buffering, and methods of range and gain selection. Signal Conditioning by Chapter. Signal conditioning for a wide variety of transducer types, including temperature, strain, force, torque, pressure, and acceleration, is discussed in Chapter 4 through Chapter 8. The temperature measurement section in Chapter 4 describes the principles of operation, signal conditioning, linearization, and accuracy of thermocouples, RTDs, and integrated circuits (ICs). The strain gage section in Chapter 5 discusses the Wheatstone bridge, as well as the use of strain gages in quarter-bridge, half-bridge, and full-bridge configurations. The use of strain gages in load cells is described, along with excitation and signal conditioning requirements. The piezoelectric transducers (PZTs) section in Chapter 5 describes the use of these devices in voltage and charge amplification configurations and with low- and high-impedance transducers. The pressure transducer section covers both strain-diaphragm transducers used for quasi-static pressure measurements and PZT-based pressure transducers used in dynamic measurements. Chapter 6 begins with a brief review of general amplification. This section then describes data acquisition front ends, source impedance and multiplexing, filters, and single-ended and differential measurements. The measurement of high voltages and DC and AC currents is also discussed. Chapter 7 discusses noise reduction and isolation, including specific methods of isolation, such as magnetic, optical, and capacitive. This handbook also describes digital signal conditioning in Chapter 8, including speed and timing issues. Also, covered in this chapter are frequency measurements, pulse counting, and pulse timing. The frequency of a signal can be measured using two methods: conversion to a voltage that is read by an ADC, or gated pulse counting. Both methods are described, as is the use of counters for timing applications. Chapter 9, IOtech’s Product Selection Guide, features PC-based data acquisition sys- tems, signal conditioning options, and temperature measurement instruments. 3 Chapter 1 Introduction 4 Chapter 2 Analog-to-Digital Conversion Chapter 2 Analog-to-Digital Conversion.... 5 Chapter 2 Analog-to-Digital Conversion This chapter examines general considerations for analog-to-digital converter (ADC) measurements. Discussed are the four basic ADC types, providing a general description of each while comparing their speed and resolution. Issues such as calibration, linearity, missing codes, and noise are discussed, as are their effects on ADC accuracy. This chapter also includes information on simultaneous sample and hold (SS&H) and selectable input ranges. Finally, this chapter contains a section on discrete sampling, which includes Fourier theory, aliasing, windowing, fast Fourier transforms (FFTs), standard Fourier transforms, and digital filtering. ADC Types An ADC converts an analog voltage to a digital number. The digital number represents the input voltage in discrete steps with finite resolution. ADC resolution is determined by the number of bits that represent the digital number. An n-bit ADC has a resolution of 1 part in 2n. For example, a 12-bit ADC has a resolution of 1 part in 4096 (212=4,096). Twelve-bit ADC resolution corresponds to 2.44 mV for a 10V range. Similarly, a 16-bit ADC’s resolution is 1 part in 65,536 (216=65,536), which corresponds to 0.153 mV for a 10V range. Many different types of analog-to-digital converters are available. Differing ADC types offer varying resolution, accuracy, and speed specifications. The most popular ADC types are the parallel (flash) converter, the successive approximation ADC, the voltage- to-frequency ADC, and the integrating ADC. Descriptions of each follow. Parallel (Flash) Converter Vref Vinput Encoder The parallel converter is the simplest R/2 + ADC implementation. It uses a refer- – ence voltage at the full scale of the in- R + put range and a voltage divider com- – posed of 2n + 1 resistors in series, where R Binary output n is the ADC resolution in bits. The + – value of the input voltage is deter- R mined by using a comparator at each + of the 2n reference voltages created in – R/2 the voltage divider. Figure 2.01 depicts Comparators a 2-bit parallel converter. Fig. 2.01: 2-bit parallel converter Flash converters are very fast (up to 500 MHz) because the bits are determined in parallel. This method requires a large number of comparators, thereby limiting the resolution of most parallel converters to 8 bits (256 comparators). Flash converters are commonly found in transient digitizers and digital oscilloscopes. 6 Chapter 2 Analog-to-Digital Conversion Successive Approximation ADC A successive approximation ADC em- DAC ploys a digital-to-analog converter (DAC) and a single comparator. It effectively makes a bisection or binomial search by beginning with an output of zero. It provisionally sets each bit of the DAC, beginning with the most sig- nificant bit. The search compares the Comparator Control omuetapsuutr eodf .t h Ife sDetAtCin tgo a t bhiet vtoo lotangee c baeuisnegs Vinput +– reloggisicte &rs Douigtpituatls the DAC output to rise above the input voltage, that bit is set to zero. A dia- Fig. 2.02: Successive approximation ADC gram of a successive approximation ADC is shown in Figure 2.02. Successive approximation is slower than flash conversion because the comparisons must be performed in a series, and the ADC must pause at each step to set the DAC and wait for it to settle. However, conversion rates over 200 kHz are common. Successive approximation is relatively inexpensive to implement for 12- and 16-bit resolution. Con- sequently, they are the most commonly used ADCs, and can be found in many PC-based data acquisition products. Voltage-to-Frequency ADC Figure 2.03 depicts the voltage-to-fre- Vin Voltage-to-frequency converter quency technique. Voltage-to-frequency ADCs convert an input voltage Digital to an output pulse train with a fre- pulse train quency proportional to the input voltage. Output frequency is determined Timing by counting pulses over a fixed time in- circuitry terval, and the voltage is inferred from the known relationship. Pulse Digital counter outputs Voltage-to-frequency conversion has a high degree of noise rejection, because the input signal is effectively integrated Fig. 2.03: Voltage-to-frequency ADC over the counting interval. Voltage-to- frequency conversion is commonly used to convert slow and often noisy signals. 7