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Similar to how optocouplers are used to isolated digital signals, how can an analog signal be transmitted across an isolation barrier?

I am looking at a signal between 0 V and 5 V. I want to read the signal, and output a signal with the same voltage at the other end.

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  • \$\begingroup\$ The best way I've seen is a special version of an optoisolator that has matched elements to be used in a feedback configured opamp. They're usually termed linearized optocouplers. I suggest that you read this link; google.ca/url?sa=t&source=web&rct=j&url=https://… \$\endgroup\$
    – Reinderien
    Mar 28, 2018 at 2:38
  • \$\begingroup\$ What's the nature of the signal? One analog signal, or multiple? What are you going to do with the analog signal once it crosses the barrier? What's the overall application? What are you ultimately trying to accomplish? The approach varies based on these aspects. \$\endgroup\$ Mar 28, 2018 at 3:10
  • \$\begingroup\$ @NickAlexeev Single analog signal, goes to the feedback pin of a switch mode power supply control. The input voltage to the optocoupler comes from the bottom resistor of a voltage divider on the power supply output. \$\endgroup\$
    – A.S.
    Mar 28, 2018 at 3:16
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    \$\begingroup\$ @NickAlexeev LT3751. See related question here: How to control switch mode power supply feedback voltage through optocoupler? \$\endgroup\$
    – A.S.
    Mar 28, 2018 at 3:35
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    \$\begingroup\$ @A.S. This question sounds like a duplicate or an XY problem. No offense. Let's move our conversation to that earlier one. \$\endgroup\$ Mar 28, 2018 at 3:55

3 Answers 3

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As your question really pertains to feedback control of an SMPS ...

What gets fed back to an isolated SMPS controller is generally not the output voltage, but a signal that indicates whether the output voltage is above or below a set point. This is easy to transmit through an optocoupler.

Note that if you are isolating a SMPS that expects the actual output voltage on its feedback pin, you will need to accommodate the different gain and delay.

Look at the many application notes from Diodes Inc, Texas Instruments, or On Semiconductor for the TL431 and TLV431 for examples of how to use them as opto drivers in isolated power supplies.

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There are special analog optocouplers exactly for such purposes you are decribing.

For example see the circuit with an IL300 below.

Such optocouplers have not only one but two photodiodes that are illuminated by the same LED.
The reason for two photodiodes is to use one for a feed-back-loop on the transmitter side that takes care that the photocurrent \$I_{P1}\$ is proportional to the input voltage \$V_{in}\$. The second photodiode on the receiver side will have a photocurrent \$I_{P2}\$ that is proportional to the first one because it is illuminated by the same LED. The photo current on the receiver side is transformed into a voltage signal \$V_{out}\$ by a TIA.
This way an analog voltage signal can be transfered with high linearity across a galvanic isolation.

enter image description here

There are also other methods, e.g. using a

  • voltage to PWM converter or a
  • voltage to frequency converter

and

  • a common optocoupler or
  • a transformer

to transfer the digital signal to the isolated side and convert it back to a voltage signal.

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Have a look at isolated amplifiers. Texas Instruments offers for example AMC1311. You need to scale input with a resistor divider to match the input voltage range. You can scale the output using an opamp in difference amplifier topology.

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