This is a conceptual question about opto isolators couplers and their use in pulse or digital signals.
In a single ended transmitter receiver system for digital signals if the noise is at the signal line(hot), then low pass filter is used to mitigate the noise as in Figure 1 below:
In such system if there is EMI or a noise that would also bounce the ground RC filter will not help since the noise is also at the ground line as shown in red loop in Figure 2 above.
Are opto isolators used in such cases? Is the idea to break the loop and prevent the EMI to induce a looping current?
Im trying to understand why or when the opto isolators should be used and how does it prevent the interfernce caused noise.
I'm not quite sure I understand your question. Are you talking about common-mode noise? If so, the optoisolator can actually be thought of as a differential input device, and thus, common mode noise will be cancelled.
I didn't draw an opto isolator. In Figure 2 Im imagining EMI is hitting through radiation or conduction to both lines.
Yes I think EMI is always common mode interference.
I was wondering why the inference does not occur after the optoisolator.
I'm not quite sure I understand your question. Are you talking about common-mode noise? If so, the optoisolator can actually be thought of as a differential input device, and thus, common mode noise will be cancelled.
Yes I meant EMI common mode interference. For example recently I had such issue:
I was looking for a possible way to eliminate such spike only at node X which is caused by EMI.
The thing is in such cases I also need a another power supply at the other side of the opto.
And the opto's output wires will be long like 100cm. Since I cannot put the supply very local to the output. Should I use STP cables after the opto in that case?
Im sorry maybe this is not related to this question but Im a bit desperate to find a hint about these.
Isolation does a number of things. One is that it lets you split a large system into isolated pieces and keep the noise generated in each piece to itself. Second it lets your system be immune to that noise.
I didn't draw an opto isolator. In Figure 2 Im imagining EMI is hitting through radiation or conduction to both lines.
Yes I think EMI is always common mode interference.
I was wondering why the inference does not occur after the optoisolator.