Optocoupler Placement

This might help. I didn't read the whole thing. OPTOCOUPLERS:

Most of the time it does not matter where an optocoupler should be placed. But you give no information if your situation dictates where an optocoupler should be placed or even if an optocoupler is needed. Some of the reasons to use an optocoupler are to maintain ground isolation, change in logic levels, noise immunity. At the same time sometimes an optocoupler should not be used. Optocouplers have a non-linear input impedance so high speed signals traveling across transmission line lengths (nominal 1/10 of the wave length of the highest frequency) will always produce reflections that may corrupt data.

I would place an optocoupler as close as is convenient to the site where the signal is generated, unless there is an overriding reason not to do so.

Keep in mind that the clock signals are generally a.) very sensitive, and b.) very critical. Therefore, be sure to decouple the opto from the clock with a buffer of some sort.

Good luck with the project.

QL

If you have to seperate the cirquit at optocoupling stage the wise desicion is to use the interconnecting wiring at the lowest impedance thus less succeptible to interference side which in most if not all optocouplers is it's input side so put the coupler in the driven cirquit and keep it's input impedance as low as the driving cirquit can handle (e.g by adding a resistor in parallel with coupler photodiode) 53 2E 4D 2E

thanx for the reply but kindly make your reply simple. did you mean..."optocoupler should be placed at destination end"

You want simple, I'll give it to you. You've not given us enough information to help you.

ok i try

i have designed a circuit generating a clock. this clock is to be input to a black box (im unaware of whats inside) but its an INS. until and unless i do not receive it in my lab, im unaware of its interior.

now i wish to know, whether i should add an optocoupler on my circuit or should ask the ins designer to add it to his side.

i wish to use optocoupler because teh clock is ttl based while the ins setup is operated at 28v.

is it ok now

From what you've given I would prefer the optocoupler to be at the receiving end. This way the higher voltage circuitry will not at all exist on your circuitry. If this is not practical for the "black box" fabricator I would insist that they provide you at least an interface schematic. You cannot be held responsible for connecting to a circuit you know nothing about.

One final caveat, if your driving circuitry uses true TTL signals (74XX chip set) then the maximum current sink and source will not be identical. Current sourcing will be considerably less. Many TTL compatible components today sink and source the same higher amount of current.