Analog Electronics for Measuring Systems

Davide Bucci

• Publisher: Wiley-ISTE
• Year: 2017
• ISBN:
  9781119405160
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

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.

1
Fundamentals of Sensing and Signal Conditioning

1.1. Introduction

The first element in a classic electronic measuring system is the sensor. Its role is to translate the physical quantity to be measured (called measurand) in an electrical quantity of some kind. Clearly, the goal is to obtain knowledge about the physical quantity: this translation should be done in a well-known and reliable way. Sensors are based on a wide range of principles and (our point of view being from the electronics side) we follow the classification proposed in [ASC 03]. In other words, in this chapter we will categorize sensors depending on the electrical quantity at their output: voltage, current, charge, resistance and reactance. This categorization is not the only applicable one, but it allows us to treat signal conditioning at the same time as sensors.

1.2. Voltage generating sensors

1.2.1. General description

Several physical phenomena involve the presence of a voltage between two conductors in a specific piece of equipment. That voltage can be related to a particular physical variable. They can thus be exploited to build sensors that can be seen as voltage sources and whose voltage depends on the measurand m. Very often, the electrical representation of a sensor might be a Thévenin-type equivalent circuit including a series impedance, as shown in Figure 1.1.

Figure 1.1. Thévenin representation of a voltage generating sensor

The open-circuit voltage given by the sensor is e(m) and its relation with m, the measurand, must be known and must not change considerably in time. The internal impedance of the sensor is represented by Z_c and determines the voltage drop between e(m) and V_s when a load is attached, if the current I_s is negligible.

We will proceed by detailing some examples of such sensors, that we judge are the most representative.

1.2.2. Examples

1.2.2.1. Thermocouples

A thermocouple is a temperature sensor based on the Seebeck effect. This principle, shown in Figure 1.2, is when two junctions between different metallic conductors are kept at different temperatures and a voltage difference can be measured [ASH 76]. This voltage is approximatively proportional to the temperature difference between the two junctions. This effect is a consequence of heat transport in conductors and the Seebeck coefficient is the volume property of each one. The Seebeck voltage, thus, is not generated in the junctions themselves, but on the whole length of the conductor: it is always present, even in a homogeneous circuit, but it only is observable with different conductors spliced together.

Figure 1.2. The Seebeck effect in two junctions of different conductors (A and B)

In the thermocouple’s case, we conventionally call “hot junction” and “cold junction” the connections between the two conductors and the temperature measurement is intrinsically differential. If one needs an absolute measurement, the cold junction should be kept at a constant and controlled temperature (for example employing water/ice for 0°C), or a cold junction compensation circuit may be used.

From a practical point of view, buckets containing ice and water have long ago been replaced by compensation circuits, which are more compact, much easier to run and less expensive. The idea ...

Table of contents

• Cover
• Table of Contents
• Title
• Copyright
• Introduction
• 1 Fundamentals of Sensing and Signal Conditioning
• 2 Amplification and Amplifiers
• 3 Elements of Active Filter Synthesis
• 4 Analog to Digital Converters
• 5 Introduction to Noise Analysis in Low Frequency Circuits
• Appendix: Legal Notes
• Bibliography
• Index
• End User License Agreement