Column: Optical isolation


Optically- isolated input modules for a −10V to +10V to 0-5V signal converter


By Professor Murat Uzam, Department of Electrical and Electronics Engineering, Yozgat Bozok University, Turkey


I


n this month’s column we will cover two optically-isolated analogue input modules for a −10V to +10V to 0-5V signal converter, with DC input voltages


varying between −12V and +12V, powered by +5.00V, +6.26V, −12V, +12V, +12V (isolated) for module 1 and −12V, +12V and +12V (isolated) for module 2.


Module 1 Figure 1 shows the −10V - +10V to 0-5V signal converter for use with the ADC input of a 5V microcontroller; its connections are shown in Figure 2. Te circuit is based on the Positive Unipolar Photovoltaic Isolation Amplifier 1 (PUPIA1 – explained in a previous month), with an HCNR201 high-linearity analogue optocoupler for photovoltaic isolation. Te circuit’s output is isolated from VIN


. Tis design is used to level-shiſt the


bipolar −10V - +10V analogue input signals to provide a unipolar 0-5V signal. Te


transfer function is obtained from:


Since R2 = R3, R1 = ½ R2,


VIN can be subjected to electric surge


or electrostatic discharge on the external terminal connections, but adding a transient voltage suppressor (TVS) gives it good protection. A ferrite bead is connected in series with the input path to add isolation and decoupling from high-frequency transient noises. External Schottky diodes protect the operational amplifier. Even when internal ESD protection diodes are present, using external diodes lowers noise and offsets errors. Dual series Schottky barrier diodes D1 and D2 divert any overcurrent to the positive or negative power supply.


Figure 1: -10V - +10V to 0-5V signal converter – module 1 www.electronicsworld.co.uk July/August 2022 13


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