From Introduction to Instrumentation, Sensors, and Process Control

4.1 Introduction

Process control electronic systems use both analog and digital circuits. The study of electronic circuits, where the input and output current and voltage amplitudes are continually varying, is known as analog electronics. In digital electronics, the voltage amplitudes are fixed at defined levels, such as 0V or 5V, which represent high and low levels, or "1"s and "0"s. This chapter deals with the analog portion of electronics.

In a process control system, transducers are normally used to convert physical process parameters into electrical signals, so that they can be amplified, conditioned, and transmitted to a remote controller for processing and eventual actuator control or direct actuator control. Measurable quantities are analog in nature; thus, sensor signals are usually analog signals. Consequently, in addition to understanding the operation of measuring and sensing devices, it is necessary to understand analog electronics, as applied to signal amplification, control circuits, and the transmission of electrical signals.

4.2 Analog Circuits

The basic building block for analog signal amplification and conditioning in most industrial control systems is the operational amplifier (op-amp). Its versatility allows it to perform many of the varied functions required in analog process control applications.

4.2.1 Operational Amplifier Introduction

Op-amps, because of their versatility and ease of use, are extensively used in industrial analog control applications. Their use can be divided into the following categories.

  1. Instrumentation amplifiers are typically used to amplify low-level dc and low frequency ac signals (in the millivolt range) from transducers. These signals can have several volts...

Products & Services
Signal Conditioners
Signal conditioners provide amplification, filtering, converting, and other processes required to make sensor output suitable for reading by computer boards.
Sample-and-Hold Amplifiers
Sample-and-hold amplifiers freeze analog voltage instantly. During this process the HOLD command is issued and analog voltage is available for an extended period.
Amplifier and Comparator Chips
Amplifier and comparator chips are board-level components for amplifying voltage, current, or power.
Analog Comparators
Analog comparators are amplifiers that compare the magnitude of voltages at two inputs. An analog comparator is an operational amplifier with negative feedback removed, and with no feedback and very high gain, the output voltage goes to one extreme to the other.
Logic Signal Switches
Logic signal switches are electronic devices which activate or deactivate a signal once an event has occurred.

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