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I found this circuit that I wanted to quickly implement. He is using an optoisolator and feeding back the output using it. I understand that the optoisolator is isolating the sensitive circuit from noise but if I didn't care about the noise, would the circuit function reasonably if I replace the optoisolator with a large resistor (say 10M)?

circuit

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  • \$\begingroup\$ Why are they using an analogue opto-isolator with feedback to communicate a square wave? If no isolation is needed and the grounds are the same then a 1 to 3kOhm resitor from 5V to 4.5V square wave points might work ok. Removing the output 33kOhm resistor should allow for larger linking resistors values if desired. \$\endgroup\$ – KalleMP Sep 20 at 16:24
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No, using a 10MΩ resistor will make the output voltage collapse: The slightest current you draw on the output will cause a big voltage drop across this 10MΩ resistor.

If you're sure the optoisolator only isolates for noise and not for safety, ground loops, etc, you can either directly connect the 5V Square Wave input signal as the output or, when the signal is too weak, replace the whole circuit by a unity gain buffer amplifier.

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  • \$\begingroup\$ What do you mean by safety? How does a optoisolator help with safety? \$\endgroup\$ – Prateek Dhanuka Sep 19 at 8:07
  • \$\begingroup\$ E.g. in power supplies, the primary side (e.g. 230Vac) is considered dangerous and the secondary side (e.g.12Vdc) is considered safe, no direct electrical connection should be made between the primary and secondary sides. Still, feedback signals are needed from the secondary side to the primary side to generate a stable output. Here, optoisolators can be used to tackle that issue. \$\endgroup\$ – Huisman Sep 19 at 8:13
  • \$\begingroup\$ Thank you for explaining that. That is certainly not the case here \$\endgroup\$ – Prateek Dhanuka Sep 19 at 8:25
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This optoisolator is a non-linear current attenuator but using a mirrored PD in the feedback loop becomes linearized like a servo.

The voltage input converted to a diode current makes it a voltage controlled current mirror with some 4500 Vrms isolators.

Thus the grounds on either side do not need to be the same and this provides excellent common-mode noise immunity with high voltage isolation and a linear response to 200kHz.

To simulate the circuit with a common ground simply requires you to read the datasheet. If it says 20mA LED current yields 200uA output that’s an attenuation of 100. The voltage depends on the input resistance above 1.2V and output collector load.

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