Column: Optical isolation
Optically- isolated analogue ınput/output modules
By Professor Murat Uzam, Department of Electrical and Electronics Engineering, Yozgat Bozok University, Turkey
P
rogrammable logic controllers (PLCs) are computer-based digital systems with wide application areas, from simple traffic control
to complex electrical power grids, and from environmental control systems to process control. PLCs typically consist of a power
supply, central processing unit (CPU) and digital and analogue inputs and outputs. In this series of articles we will focus on the hardware design tasks related to the analogue inputs and outputs. Among the commonly-used input current and voltage ranges are 4-20mA, 0-5V, 0-10V, -5V to +5V, and -1V to +10V. Similarly, widely-used analogue output current and voltage ranges include 4-20mA (in industrial process control systems), 0-5V, 0-10V, -5V to +5V, and -10V to +10V for actuator controllers. Analogue input signals require
analogue-to-digital converters (ADCs), and, on the opposite side, the output signals require digital-to-analogue converters (DACs) to be processed by the CPU or PLC. In this case, PLC hardware design tasks focus on the input and output signal conditioning. In high-precision setups, for each analogue input signal there must be
an input signal conditioning module containing an ADC (12-bit or 16- bit) with all the necessary amplifiers, isolaters, and more. This signal chain is reversed in analogue output signals, with the applicable DAC (12-bit or 16-bit), together with the necessary amplifiers, buffers, isolators, and more, frequently required by industrial applications demanding very high accuracy. Naturally, these types of signal-conditioning modules are rather expensive.
The setup Previously, a project published in Electronics World, in issues 1985 to 1997, presented 13 analogue input and seven analogue output modules. The project’s aim was to develop low-cost and easy-to-use analogue input signal- conditioning modules for use with CPUs and PLCs. We assumed the CPUs to be 5V microcontrollers. In practice in industrial applications,
to provide electrical isolation between the analogue input and output signals and the CPU, as well as to provide protection from surges and spikes, isolation is necessary. Commonly-used isolation methods are a transformer, and capacitive or optical coupling. Transformer coupling tends to restrict the bandwidth to below 10kHz, whereas
Figure 1: Block diagram of the optically-isolated analogue input and output modules and their connection to a 5V microcontroller through its ADC and DAC channels
16 December 2021/January 2022
www.electronicsworld.co.uk
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