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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: opto isolators
References: <3DC9BE19.1980C7C0@webaccess.net> <email@example.com> <3DCA598F.58EDB5F4@webaccess.net> <firstname.lastname@example.org>
Date: Thu, 07 Nov 2002 13:40:55 GMT
NNTP-Posting-Date: Thu, 07 Nov 2002 05:40:55 PST
Geraldo Sazias wrote:
> "Chuck Simmons" wrote in message
> > Geraldo Sazias wrote:
> > >
> > > "Chuck Simmons" wrote in message
> > > news:3DC9BE19.1980C7C0@webaccess.net...
> > > > Dan wrote:
> > > > >
> > > > > Do opto isolators work as digital or analogue? IOW, are they used as
> > > > > switches or can they measure incremental changes, like 0-5v? I can't
> > > seem to
> > > > > find what I need in the data sheets I've found searching the web.
> > > > >
> > > > > Dan
> > > >
> > > > Depending on type, they can be used analog. The best for analog use a
> > > > photodiode instead of a phototransistor. The diode type are
> > > > for analog applications. Most of these also have a transistor for use
> > > > a buffer but it can be ignored for high linearity applications. I used
> > > > such optoisolators to drive the high side tube circuit in a 0 to
> > > > volt opamp having a totem pole output circuit. Remaining nonlinearity
> > > > any was removed by closing the loop around the opamp.
> > > >
> > > > In the 70s, Burr-Brown Research made various isolated amplifier and
> > > > power supply products. The amplifier products were split between
> > > > feedback types (transformer coupled) and optocoupled types. These
> > > > had very excellent linearity.
> > > >
> > >
> > > Some of the specs I've seen led me to believe that the linearity of a
> > > optocoupler is far less than can be achieved with transformers. Is this
> > > true?
> > This seems to be true. Burr-Brown used two optocouplers in their
> > optocoupler isolation amplifiers. The second was for local feedback. The
> > flyback coupled devices did mot need this double structure. The flyback
> > devices were more expensive anyway, however, in spite of a unique method
> > of winding the transformer on the ceramic substrate using an automatic
> > wire bonding machine.
> Is the linearity inferiour, even with feedback?
The Burr-Brown devices with optocouplers did not have a built in
isolated power supply so they did not have the flyback transformer. The
flyback isolation types used the power supply built in for both power
and signal isolation. The signal and the feedback were accomplished on
the flyback by regulating the amplitude. It was a really quite unique
method at the time. I don't recall the circuits as this was more than 20
years ago. I was a test engineer at that time and had more interest in
how to test the puppies rather than worrying about what was inside them.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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