Column: Optical isolation
the capacitive and magneto-galvanic approaches raise this to 60kHz, with some modern high-speed analogue optocouplers managing several MHz. Based on the previous project
reported in Electronics World, here we present a set of low-cost and easy-to-use optically-isolated analogue input and output modules for use with CPUs and PLCs. If the ADC is part of the CPU module
(assumed in this project), which is desirable in embedded applications, it is very common for the on-chip ADC to be unipolar (0-5V). This means that the ADC can only sample positive voltage levels, as opposed to a bipolar ADC that can sample signals that take both positive and negative values over time. Therefore, a special type of inverting op-amp (operational amplifier) circuit called a level shifter is required in such cases. For example in the case of designing 0-5V - +5V to 0-5V analogue input signal converter and -10V - +10V to 0-5V analogue input signal converter. Similarly, a level shifter is also
necessary when designing a 0-5V to -5V - +5V analogue output signal converter and a 0-5V to -10V - +10V analogue output signal converter. Another specific signal-conditioning
circuit required is a buffer amplifier, which is an opamp-type circuit used to transfer a voltage from a circuit with high output impedance to one with low input impedance.
The modules There are twelve optically-isolated analogue input modules developed in this project, as shown in Table 1. There are also seven optically-isolated analogue output modules developed, as shown in Table 2. It can be seen that there are four variants of the 0-5V optically-isolated analogue input module, because each can withstand a different maximum voltage value, and, also, the power supply requirement of each module is different. The remaining optically-isolated analogue input modules come with two versions, each requiring a different set of power
LM358P op-amps are used in this
project in the design of level shifters from bipolar to unipolar signals,
level shifters from unipolar to bipolar signals, buffer amplifiers (voltage followers) and non-inverting summing amplifiers
supplies. Optically-isolated analogue outputs, 0-5V to -5V - +5V signal converters and 0-5V to -10V - +10V signal converters also have two versions with a different set of power supplies, all of which will be considered in this series, too. The modules developed in this project
are specifically designed around the low- cost LM358P op-amp manufactured by Texas Instruments. Therefore, any variant of the LM358P, such as LM358, LM358A, LM358N, etc., or LM358P op-amps manufactured by other companies may not function in all modules as reported here. LM358P op-amps are used in this
project in the design of level shifters from bipolar to unipolar signals, level shifters from unipolar to bipolar signals, buffer amplifiers (voltage followers) and non- inverting summing amplifiers. A top view of the LM358P PDIP
package is shown in Figure 2. Some of the LM358P’s specifications are as follows:
Figure 2: Top view of the LM358P PDIP package
www.electronicsworld.co.uk December 2021/January 2022 17
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