Column: Optical isolation


Te operational amplifier LM358P-1A,


with bipolar supply voltages, provides a high input impedance and is connected as a buffer amplifier (voltage follower). Diodes D3 and D4 ensure that when −12.00V ≤ VIN


< −10.00V, VOUT = 0.00V.


Te operational amplifier LM358P-2A, with a +6.26V supply voltage, acts as a voltage limiter and is connected as a buffer amplifier. Its output is connected to the input of PUPIA1, limited by LM358P-1A. Here, PUPIA1 consists of:


1. Input section: R4, R5, C4, LM358P-3A; 2. HCNR201 high-linearity analogue optocoupler.; 3. Output section: P1, R6, C5, LM358P-4A. In this design, the circuit’s input is


powered by +5.00V, +6.26V, −12V and −12V. Likewise, the circuit’s output is powered by +12V isolated from the input. Te output of the PUPIA1 is connected to the non-inverting input terminal of the buffer amplifier LM358P-4B, obtaining VOUT


We assume VIN is = −12V to +12V. When −12.00V ≤ VIN


< −10.00V, VOUT


When −10.00V ≤ VIN (VIN


+12V, VOUT ≤ +10.00V, VOUT 5.07V, due to LM358P-2A. See VOUT


½R2 and +5V = +5.00V. To calibrate the setup, set VIN


and adjust P1 to bring VOUT


= 0.00V. =


+10V) / 4. When +10.01V ≤ VIN will be a value from 5.01V to


≤ vs VIN in


Figure 3, with some examples in Table 1. For proper operation make R2 = R3, R1 =


to +10.00V to +5.00V.


Module 2 Figures 5, 6 and 7 show the optically- isolated analogue input module 2 with its connections to the ADC port of a 5V microcontroller. As before, this circuit is also PUPIA1-based, with a high-linearity analogue optocoupler (HCNR201) providing photovoltaic isolation. Tis design is used to level-shiſt the


.


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


R7, D5 (10V zener diode) and C6 provide a 10.00V reference voltage from +12V. Tis 10.00V is divided with R8 and R9 to obtain +5.00V, which can source current to 20mA. Likewise, the 10.00V reference voltage


is also divided by using R10 and R11 to obtain a +6.26V reference voltage. Next, the 5V is connected to the non-inverting input of the buffer amplifier LM358P- 3B, whose output is fixed at 5V, sourcing current to 20mA. VIN


can be subjected to electric surge


or electrostatic discharge on the external terminal connections; the TVS (transient voltage suppressor) protects it. Dual series


Since R2 = R3, R1 = ½ R2,


Te transfer function is obtained from the following:


Figure 2: Function generator signals when none of the outputs are connected to a load


Figure 3: VOUT


vs. VIN


relationship for modules 1 and 2 14 July/August 2022 www.electronicsworld.co.uk


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