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From: grayt@spock (Tom Gray)
Newsgroups: sci.electronics
Subject: Re: Need analog opto-isolator or equiv.
Message-ID: <4582@halligan>
Date: 25 Sep 90 09:52:11 GMT
References: <222@nachos.SSESCO.com>
Reply-To: grayt@halligan (Tom Gray)
Organization: Mitel. Kanata (Ontario). Canada.
Lines: 65

Newsgroups: sci.electronics
Subject: Re: Need analog opto-isolator or equiv.
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References: <222@nachos.SSESCO.com>
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In article <222@nachos.SSESCO.com> elmquist@nachos.SSESCO.com (Chris Elmquist) writes:
>Does anyone have a nifty circuit that will replicate a 0v to 5v DC
>signal across an opto-isolated path..??  I need to read a meter
>in a hostile environment with an A/D in a PC.  I don't want the
>PC to get near this meter... this meter will be read, maybe, 10 or
>15 times a second so frequency response is not critical...
>
>I've thought about serial A/D and D/A chips...  but I haven't
>found any that use the same data format over the serial link...
>maybe I missed something.
>
>I've also though about a voltage->frequency ... frequency->voltage
>converter and this seems workable...  maybe someone has a circuit
>like this.
>
>Any ideas would be appreciated.
>
>Thanks--  Chris
>elmquist@nachos.ssesco.com


 The most straight forward way to do this would would be to linearize an
opto-coupler. You can do this by using two opto-couplers. One opto-coupler
is used to create a feedback loop around an op amp. The op amp is
used to create a constant current source in the feedback path - a typical
cook book op amp circuit.

The input porion of the opto-coupler is driven by a current which you wish
to repilcate across some isolating barrier. It is connected to the current sourc
source and to ground. The ungounded input of the opto is connected to the
inverting input of an op amp. The output of the op amp is connectd
to ground through the output section of the opto-coupler.
A feedback lop is set up with the optical connection of the opto-coupler
and the circuit is stable.

Now the interesting part occurs. Connect the light emiiting portion
of another opto in series with the output of the op amp and
the light emiitoing portion of the first opto. The feedback of
the op amp is making sure that the current though the light emitting
prtion of the first opto is usfficent to produce the input current
in its light receiving portion. Similarly the same current is
flowing through the light emitting portion of the second opto
as that of the first. Thus the current in the light receiving
prtion of the second opto will be the same as the current in the
 light receivng portion of the first opto. However this is the 
input current which is supposed to be transferred across
the barrier.

The op amp feed back calculates the voltages required to
create a liner current transfer mechanism from two
opto's. As long as the transfer characteristics of
the two opto's are amtched, there will be a linear
tranfer characteristic across the barrier.