E1C07 09/14/2010
14:43:48 Page 260
Chapter 7
Sampling, Digital Devices,
and Data Acquisition
7.1 INTRODUCTION
Integrating analog electrical transducers with digital devices is cost-effective and commonplace.
Digital microprocessors are central to most controllers and data-acquisitions systems today. There
are many advantages to digital data acquisition and control, including the efficient handling and
rapid processing of large amounts of data and varying degrees of artificial intelligence. But there are
fundamental differences between analog and digital systems that impose some limitations and
liabilities upon the engineer. As pointed out in Chapter 2, the most important difference is that
analog signals are continuous in both amplitude and time, whereas digital signals are discrete
(noncontinuous) in both amplitude and time. It is not immediately obvious how a digital signal can
be used to represent the continuous behavior of a process variable.
This chapter begins with an introduction to the fundamentals of sampling, the process by which
continuous signals are made discrete. The major pitfalls are explored. Criteria are presented that
circumvent the loss or the misinterpretation of signal information while undergoing the sampling
process. We show how a discrete series of data can actually contain all of the information available
in a continuous signal, or at least provide a very good approximation.
The discussion moves on to the devices most often involved in analog and digital systems. Analog
devices interface with digital devices through an analog-to-digital (A/D) converter. The reverse
process of a digital device interfacing with an analog device occurs through a digital-to-analog (D/A)
converter. A digital device interfaces with another digital device through a digital input–output (I/O)
port. These interfaces are the major components of computer-based data-acquisition systems.
Necessary components and the basic layout of these systems are introduced, and standard methods
for communication between digital devices are presented. Digital image acquisition and processing
are introduced because of their increasing importance in a wide variety of applications ranging from
quality assurance inspection to high-speed imaging.
Upon completion of this chapter, the reader will be able to
describe analog, discrete time, and digital signals,
properly choose a sample rate for data acquisition to eliminate aliasing,
clearly describe the functioning of A/D and D/A converters,
define and calculate quantization errors,
260
14:43:48 Page 260
Chapter 7
Sampling, Digital Devices,
and Data Acquisition
7.1 INTRODUCTION
Integrating analog electrical transducers with digital devices is cost-effective and commonplace.
Digital microprocessors are central to most controllers and data-acquisitions systems today. There
are many advantages to digital data acquisition and control, including the efficient handling and
rapid processing of large amounts of data and varying degrees of artificial intelligence. But there are
fundamental differences between analog and digital systems that impose some limitations and
liabilities upon the engineer. As pointed out in Chapter 2, the most important difference is that
analog signals are continuous in both amplitude and time, whereas digital signals are discrete
(noncontinuous) in both amplitude and time. It is not immediately obvious how a digital signal can
be used to represent the continuous behavior of a process variable.
This chapter begins with an introduction to the fundamentals of sampling, the process by which
continuous signals are made discrete. The major pitfalls are explored. Criteria are presented that
circumvent the loss or the misinterpretation of signal information while undergoing the sampling
process. We show how a discrete series of data can actually contain all of the information available
in a continuous signal, or at least provide a very good approximation.
The discussion moves on to the devices most often involved in analog and digital systems. Analog
devices interface with digital devices through an analog-to-digital (A/D) converter. The reverse
process of a digital device interfacing with an analog device occurs through a digital-to-analog (D/A)
converter. A digital device interfaces with another digital device through a digital input–output (I/O)
port. These interfaces are the major components of computer-based data-acquisition systems.
Necessary components and the basic layout of these systems are introduced, and standard methods
for communication between digital devices are presented. Digital image acquisition and processing
are introduced because of their increasing importance in a wide variety of applications ranging from
quality assurance inspection to high-speed imaging.
Upon completion of this chapter, the reader will be able to
describe analog, discrete time, and digital signals,
properly choose a sample rate for data acquisition to eliminate aliasing,
clearly describe the functioning of A/D and D/A converters,
define and calculate quantization errors,
260