What's the purpose of using an analog optocoupler in this circuit? Why not a digital one?
An industrial battery charger. Normal operating voltage to monitor ranges from 120 V (Battery low) to 180 V (battery fully charged)
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Where did you get this circuit from? What is its stated purpose? – CL. Aug 24 '21 at 12:05
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1Have a look: https://www.codrey.com/learn/analog-optocouplers-quick-starter/ – Mike Aug 24 '21 at 12:12
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Opto-couplers are non-linear. This is true of the LED and of the photo-receptor - whether photo-diode or photo-transistor.
Figure 1. Close-up of the opto-isolator.
When an analog signal requires opto-isolation a common trick is to use a device with one LED and two matched photo-diodes arranged to receive the same amount of irradiance from the LED.
The idea here is that the bottom left photo-diode is inserted into the transmitter's feedback loop. This causes the transmitter op-amp output to adjust the current through U4's LED (top) so that the required current is fed back from the left photo-diode. The lower right photo-diode will then have the same current which will be fed into U6. Inverting op-amp amplifier's input current all ends up in the feedback element (because the inputs have extremely high impedance) and the result is that both U5 and U6 have "identical" currents running in their inputs and feedback loops so the U6 output voltage = V3.
Figure 2. Note that on the input side R17, 100 kΩ, determines the input current from V3 to the virtual ground and out to the feedback photo-diode. Meanwhile on the output side the photo-diode's output current (which is the same as that on the input side) flows through R18 which is also 100 kΩ. The result is that the output voltage matches the input voltage.
The result is a reasonably good analog signal transmission with most of the non-linearities ironed out.
Transistor
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It allows an analog signal to be isolated between the input and output. A digital isolator can't do that for an analog signal.
Justme
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There are no analog or digital optocouplers. The one you have it has two same photodetectors, so the opamp U5 tries to reduce the non-linearity of the LED->detector transfer function.
Marko Buršič
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Several companies make analog and digital optocouplers that vary significantly in their internal construction, performance, and intended applications. In your case, it is there for ground isolation. Can't say much more than that without a complete schematic.
AnalogKid
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