0
\$\begingroup\$

How does an optocoupler filter noise?

As an illustrative example, below three remote sensors are receiving pulses from a single source:

enter image description here

In the top scheme there is no optocoupler/optoisolator. In the second scheme there is an optocoupler(colored in green) right before each sensor.

And i actually tested this. I first connected the source pulse train to a 100m coaxial cable and at the end of the cable I hooked it up to an oscilloscope. I didn't perform noise analysis but the first thing I noticed was there random was ringing twice the pulse amplitude seen on the scope.

Then I used an optocoupler an the ringing completely disappeared.

This made me believe that the optoisolator really works good for long distances.

One thing I can see that it separates the ground of the source and the receiver. But that would I guess only take care of the 60/50Hz ground loop noise.

How is an optocoupler/isolator filtering the noise? How can we understand the logic behind?

\$\endgroup\$
  • \$\begingroup\$ Finding that sugar is a vital ingredient when making beer does not mean that sugar added to water makes a nice drink. \$\endgroup\$ – Andy aka Jun 27 '18 at 17:28
3
\$\begingroup\$

Then I used an optocoupler an the ringing completely disappeared.

You applied a load at the end of the cable and found that it reduced transmission line reflections to a reasonably low level. This shouldn't mean: -

This made me believe that the optoisolator really works good for long distances.

That would be an incorrect assumption based on your observation. When sending pulses down a cable, to avoid reflections and ringing you use a cable terminating resistor. The opto removed excess energy from the signal as it came across what would be an open circuit and the load that the opto supplied made things better.

One thing I can see that it separates the ground of the source and the receiver. But that would I guess only take care of the 60/50Hz ground loop noise.

That is a pretty good reason for using an opto coupler - it also prevents destructive earth fault currents passing through sensitive measurement apparatus. It is an effective counter-measure to common-mode noise or interference.

\$\endgroup\$
  • \$\begingroup\$ Thanks but in the case of 5V pulse train, how would a ground loop have any effect to the received pulse signal? \$\endgroup\$ – floppy380 Jun 27 '18 at 18:39
  • \$\begingroup\$ It all depends on what current is flowing through that ground loop. \$\endgroup\$ – Andy aka Jun 27 '18 at 19:48
0
\$\begingroup\$

The analysis is simple.

The isolation in the optocoupler is very high resistance but ~ 25 pF coupling capacitance. The input ESR of the diode is low (Zt or ESR = 10 ) + current limiting R . The Common Mode impedance across the interface is thus very high 100M?//25pF and this reduces output voltage/current by the load impedance.

The stray mutual inductance of a long cable depends on stray wires nearby carrying currents with cable capacitance causing a ripple effect of induced DC noise pulses and/or AC “hum” due this high mutual coupling impedance. When the paired input wires are not balanced or shielded or have low mutual impedance to say earth ground, then they pick up noise voltage. Thus EM shielding and filtering is often used as well as Opto isolation.

\$\endgroup\$
  • \$\begingroup\$ Should I use a twisted pair or coaxial cable if the length is long? (to send the pulse train up til to the opto) \$\endgroup\$ – floppy380 Jun 27 '18 at 18:28
  • \$\begingroup\$ STP or UTP depends on noise levels nearby . STP is better if driver impedance is suitable. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '18 at 18:30
0
\$\begingroup\$

One thing not mentioned: isolation for ground loop prevention In circuits with many loads and components, sometimes the current transfer from the ground and generate an error for analog and digital input/output.

I saw many errors in the circuit without isolation for long cable and high power circuit.

Isolation helps us analyzing each circuit separately. In the complicated circuit, when all parts have one ground many problems such as ground loop, coupling with transformer and etc may arise which analyzing errors are hard.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.