Data Acquisition Handbook

PREFACE

This edition of the Data Acquisition Handbook has been revised to include chapters on Electrical Measurements, Vibration and Sound, Displacement and Position Sensing, and Transducer Electronic Data Sheets (TEDS). 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 understand the basic principles of sensor operation and the need for careful system interconnections. 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, resolution, speed, and sensitivity of data acquisition 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. Examples include temperature, strain, acceleration, pressure, vibration, and sound. Yet others are humidity, flow, level, velocity, charge, pH, and chemical composition.

Sensors come in numerous shapes, sizes, and specifications. They connect between the measured physical device and the signal conditioner’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. Signal conditioners typically amplify, filter, isolate, and linearize these signals. They also convert current to voltage and voltage to frequency, provide other functions such as simultaneous sample and hold (SS&H), and supply a bias voltage or signal excitation for certain transducers. They may come with single-ended inputs or differential inputs for improving signal-to-noise ratios. The output of the signal conditioner, in turn, connects to the input of an analog-to-digital converter (ADC) embedded within the data acquisition system. Finally, the ADC converts the conditioned analog signal to a digital signal that can be transferred out of the data acquisition system to a computer for processing, graphing, and storing.

Introduction to Data Acquisition and Signal Conditioning
Chapter 1 discusses signals, sensors, and signal-conditioning techniques and how they relate to data acquisition system fundamentals. It also covers personal computers and how laptop or notebook computers work with data acquisition systems.

Analog-to-Digital Conversion
Chapter 2 discusses four basic ADC types, including their accuracy and resolution. Also covered are topics such as ADC output averaging, discrete sampling, input and source impedance, and differential voltage measurements. Yet others include simultaneous sample and hold methods, selectable input ranges, aliasing, digital filtering, and Fourier Transforms.

Multiplexing
Chapter 3 covers the fundamental principles of multiplexing and their benefits and economies.

Electrical Measurements
Chapter 4, Chapter 6, Chapter 7, Chapter 8, and Chapter 9 discuss basic electrical measurements, the characteristics of various sensors, how to use sensors to measure the most common types of electrical and physical quantities, and the signals that they represent. Topics include voltage, current, resistance, charge, temperature, strain, position, velocity, acceleration, and sound. The sensors that produce them include thermocouples, RTDs, thermistors, strain gages, accelerometers, and linear and rotational displacement sensors.

Fundamental Signal Conditioning
Chapters 5 and Chapter 11 discuss the most widely used techniques for analog, digital, and pulse-train signal conditioning. They comprise operational, differential, and high-gain amplifiers for filtering, attenuation, isolation, linearization, and circuit protection for analog signals. They also cover topics on digital I/O interfacing, frequency measurements, and pattern generation for digital signals.

Noise Reduction and Isolation
Chapter 10 is dedicated to simplifying the somewhat difficult topic of electrical noise interference, using the best shielding and grounding techniques, and identifying the major sources of crosstalk. It also discusses how to select the proper amplifiers and sensors, as well as how to use certain isolation and wiring techniques to minimize or eliminate significant noise in data-acquisition systems.

Transducer Electronic Data Sheets (TEDS)
Chapter 12 covers Transducer Electronic Data Sheets. TEDS is a class of so-called smart sensors that contain an onboard memory chip. The chip stores information regarding transducer calibration, manufacturer information, and many other data.

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