Many instrumentation engineers and scientists often deal with analog electronic issues when approaching delicate measurements. Even if off-the-shelf measuring solutions exist, comprehension of the analog behavior of the measuring system is often a necessity.
This book provides a concise introduction to the main elements of a low frequency analog acquisition chain. It aims to be sufficiently general to provide an introduction, yet specific enough to guide the reader through some classical problems that may be encountered in the subject. Topics include sensors, conditioning circuits, differential and instrumentation amplifiers, active filters (mainly for anti-aliasing purposes) and analog to digital converters. A chapter is devoted to an introduction to noise and electronic compatibility.
This work is intended for people with a general background in electronics and signal processing, who are looking for an introduction to classical electronic solutions employed in measuring instruments involving low frequency analog signal processing.
A measuring system is a coordinated ensemble of different devices allowing a measurement operation through their interaction.
Thanks to its intrinsic flexibility, electronics is a powerful tool available to measurement science. This book is therefore dedicated to the exploration of several recurrent problems in this context, for what concerns the analog part of the measurement chain. We try to follow the usual analog signal path through a general acquisition chain and we describe the elements most frequently found there, with a level of generality sufficient to be useful in different domains (physics, biology.).
Figure I.1 shows the most traditional and general organization of a complete acquisition system from the sensor to the data storage system. Every measurement operation starts with a goal, which is the determination of a quantity (temperature, gas pressure, electric signals from heart beating, etc.). This quantity is called the measurand.
The sensor has the role of translating the measurand into an electrical quantity. If needed, the latter is in turn transformed into a voltage by a signal conditioning system. Chapter 1 describes the most used classes of sensors along with some classic conditioning strategies.
Figure I.1. Functional organization of a very general acquisition system. Shaded elements are treated in this book, in the chapters indicated
The output voltage is then amplified and filtered to obtain amplitudes that are easy to manipulate and reduce as much as possible the noise, which is inevitably present along with the useful signal. Amplifiers (especially differential ones) and filters are, respectively, described in Chapters 2 and 3. In fact, filtering the analog signal has a paramount importance in those situations where a risk of signal aliasing appears. Filters employed in this context usually have a low-pass response and are called anti-aliasing filters. The overall quality of a measurement chain depends (even critically in some cases) on the quality of such a filter.
Digital electronics offers a huge range of very advanced signal-processing capabilities. It is very easy, today, to acquire a signal with an analog to digital converter in order to further process it or for storage purposes. The interface between analog and digital worlds is assured by a sample and hold circuit, working in tandem with an analog to digital converter. Those two devices can be shared among different separate acquisition channels because of a multiplexer. This is described in Chapter 4.
Noise is the companion of every analog circuit and the main performance limiting factor. Understanding its origins and behavior is, therefore, a key factor to design high-performance systems. We briefly introduce noise analysis in low-frequency circuits in Chapter 5.
Finally, a control system monitors every element of the measurement system, and usually a computer manipulates acquired data for storage or visualization. We will not discuss these elements in this book.
In this book, we discuss the analog elements described above to a certain degree of detail: sensors, amplifiers and filters, for low-frequency acquisition systems. We insist that the overall quality of measurements is determined individually by each element through its interaction in the chain. For this reason, when possible, we present some examples, inspired by application notes and literature.
This book is addressed to readers with a background in electronic circuits who want to begin to have an idea of the usual problems that arise when designing low-frequency analog circuits that treat the signal coming from a sensor. To limit the overall size of the book, we decided to concentrate on solutions based on discrete devices and integrated circuits (i.e. the specific problems associated with the design of analog integrated circuits will not be addressed). The main prerequisites are:
- - AC and DC analysis of circuits, transfer functions and basics of operational amplifiers;
- - concepts of power, calculation and interpretation of the root mean square value of a voltage of a current;
- - being able to subdivide a complex circuit in more elementary blocks;
- - know the most frequently used electronics devices and understand data sheets and technical literature dedicated to real devices;
- - basic concepts of signal processing (Fourier transform, sampling Nyquist-Shannon theorem, filtering);
- - basic probability and statistical tools (probability density functions, expected values, statistical independence, etc.).
Those prerequisites are addressed in undergraduate electronics courses in most engineering faculties as well as books [MAL 15].
I.3. Scope of the book
When writing a book about engineering, it is somewhat difficult to find the good trade-off between abstraction and practical craftsmanship that together constitute the core of a field such as electronics. We choose to employ maths when necessary (for example while discussing filter synthesis in Chapter 3 or for the noise analysis in Chapter 5), yet we tried to keep the mathematical developments close to the engineering problems and the real-world intuition.
On the other hand, when possible, we present extract from data sheets and technical literature. It should be clear, however, that we do not want to endorse a particular producer or a particular model. We just selected those components that, for a reason or another, may appear to be rather significative of a certain class of devices.
The relation between electric circuits and measurement techniques started very early in the 19th Century and still continues today. This means that:
- - an incredibly huge number of solutions are already known for the most disparate measurement situations;
- - ready-made low-cost integrated circuits and modules are available, accomplishing wonderfully complex measurement tasks.
Having said that, reading a small book about electronic measurement techniques may seem a futile exercise. Something has to be considered though. First of all, knowing how things work helps when a ready-made solution fails to accomplish its duty. In fact, a culture about analog electronic circuits is useful today more than ever, and culture is no black magic.
Moreover, after all, someone has to do the hard stuff since ready-made solutions do not build themselves alone.
Of course, we live in a society where access to information is widespread and inexpensive. There are of course excellent textbooks in public libraries, but it is also easy and very convenient to browse for technical information on the Internet. However, one must know what to search for and must already have a solid background to critically select the most relevant and meaningful search results from the "noise floor".
Some application notes from the semiconductor industry are wonderfully written and incredibly informative. For example, it is a sheer pleasure to read Jim William's application notes from Linear Technology. They are crystal clear, full of analog wisdom, intellectually honest, informative and fun. In one word, they are terrific. Some of them are explicitly cited among the references. Other resources are simply not worth reading and contain errors or obscure and uninformative descriptions. Particularly dull are those that, instead of producing real original content, just copy/paste information found elsewhere, with minor cosmetic changes.
This book may constitute an useful starting point for deeper investigations.
I.4. Conventions for schematics and voltages
NOTE.- Color versions of the figures in this book (where applicable) have been made available at www.iste.co.uk/bucci/analog.zip.
Figures in this book have been drawn with FidoCadJ, an open source multiplatform program. The symbols employed in this book are the classical symbols for components employed in electronic engineering and should not be ambiguous.
However, a risk of confusion exists for a specific point: we indicate voltages in the figures by means of arrows, whose heads point toward the conventional positive terminal. This is the traditional convention followed, for example, in Italian and French engineering faculties. However, in other places, the opposite convention is followed: be careful if you are not used to this notation.
Finally, when we talk in general of "the voltage of a node of a circuit", the conventional negative term is implicitly supposed to be the reference node. We employ p for the Laplace variable, except when, during filter synthesis, we normalize the frequency. In this case, we indicate it with s.
This book originates from a collection of handouts written for a course in analog electronics and taught to biomedical engineering undergraduate students in GrenobleINP-Phelma. I would have never tried to transform my crude course handouts into this book without the constant encouragement of Dr. Mireille Mouis (IMEP-LAHC), whom I would like to thank very warmly. Those early handouts contained countless issues and errors, which have been pointed out by students, who also had a number of useful suggestions. My colleagues Pr. Laurent Aubard (Grenoble INP-Phelma), Pr. Franco Maddaleno and Dr. Massimo Ortolano (both from Politecnico di...