optocoupler in analog signal path?

ollebula

Joined May 12, 2004
6
Hello.

I have to make my small signal path (souce delivers 1mV ac) able of withstanding 1kV and therefor need some isolating device. The easiest and cheapest way of dealing with this must be an optocoupler. But since they're not really linear my question is how much they will distort my signal? Especially since the signal is very small. Will I introduce a lot of noise by amplifying the signal before the coupler and damp it afterwards?

Please have a look at my schematic and comment it!

olof

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ollebula

Joined May 12, 2004
6
damn ... I forgot the TVS diods (or some similar device). They're supposed to lift the left part of the circuit to the potential of the transient voltage and make the op survive.

olof

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bipin

Joined Mar 21, 2004
80
Hi Olof,
If your input is a pure AC signal then your opto will couple only the negative half of your signal. I think if you superimpose(Spelling?) the input with a negative voltage then you can have the entire signal transmitted. you can block the dc at the final end. and there are optos available which holds linearity for a wide driving current range.(check out SHARP make)
Bye the way why did you use same power earth for both the sides? isnt it spoiling the isolation?
bipin

beenthere

Joined Apr 20, 2004
15,819
HI,

Your problem with the optocoupler is its lack of linerarity. I made a control circuit some time ago to run a variable power supply floating at about 10 kv. I used a voltage to frequency converter to generate a pulse train, so the optocoupler only had to pass the frequency. The other end used a frequency to voltage converter to restore the control voltage, but floating on the 10 kv potential.

Something like this will likely work for your application, but the low signal level will make it a challenge. Consider amping up the signal level for the v to f converter, and proportionally reducing it back at the application side.

ollebula

Joined May 12, 2004
6
Same earth will definetly spoil the isolation....

I've found optocouplers that claim to have the same linearity as a 13 bit A/D (don't really get what thats means - An A/D can be unlinear as well!). They point out audio signal path as a typical application. If thats true a shouldn't need to do a v-f - transform, should I?

LOC201 - linear optocouplers
www.clair.com