Column: Optical isolation


Optically- isolated 0-5V analogue


input modules 3 and 4


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


I


n this month’s column we will cover optically-isolated 0-5V analogue input modules 3 and 4, with inputs to +24V DC.


Design 3 Figure 1 shows optically-isolated 0-5V analogue input design 3 for use with the ADC input of a 5V microcontroller; Figure 2 shows the connections. The circuit contains a Positive


Unipolar Photovoltaic Isolation Amplifier 2 (PUPIA2 – explained in a previous column), with a high-linearity analogue optocoupler, HCNR201, for photovoltaic isolation between the input and output parts of the circuit. The analogue input signal VIN


can be


subject to electric surge or electrostatic discharge on the input connections, so for protection there’s a transient voltage suppressor (TVS). Diode D1 protects the circuit from accidental reverse polarity of VIN


, and


a ferrite bead in series with the input path adds isolation and decoupling from high-frequency transient noises. External Schottky diodes protect the operational amplifier; even when internal ESD protection diodes are provided, the use of external diodes lowers noise and


10 April 2022 www.electronicsworld.co.uk


offset errors. Dual series Schottky barrier diodes D2 and D3 divert any overcurrent to the power supply or ground. The operational amplifier LM358P-


1A, with a + 6.26V supply voltage, connected as a buffer amplifier (voltage follower), acts as a voltage limiter and provides high input impedance. Its output is connected to the input of PUPIA2, which limits the amplifier’s input voltage. In this design, PUPIA2 consists of


the following circuit components: 1. The input of the circuit: R1, R2, C3, LM358P-2A.


2. HCNR201 high-linearity analogue optocoupler.


3. The output of the circuit: P1, R3, C4, LM358P-3A. In this design, the circuit’s input is


powered by +6.26V and +24V power supplies. Likewise, the output is powered by a +12V power supply that’s optically isolated from the supplies on the input.


PUPIA2’s output is connected to


the non-inverting input of the buffer amplifier LM358P-3B; its output voltage VOUT


is obtained from the output of the


LM358P-3B. It is assumed that the input voltage


range VIN 5.00V, VOUT


= 0-24V. When 0.00V ≤ VIN = VIN


. When 5.01V ≤ VIN ≤ 24V, VOUT will


be equal to a value from 5.01V to 5.07V, due to the electrical characteristics of LM358P-1A. See the relationship between VOUT


and VIN For input voltages up to 24V there’s


no damage to the circuit, which outputs voltages from 5.01V to 5.07V; see Table 1. To calibrate the setup, set VIN


to


+5.00V, and by adjusting P1 bring VOUT to +5.00V.


Design 4 Figures 5, 6 and 8 show optically-isolated 0-5V analogue input design 4, with its connections to a 5V microcontroller. As before, this circuit is also PUPIA2 based, with a high-linearity analogue


≤


shown in Figure 3.


Figure 1: Design 3


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50