[sdiy] Temperature stable lin-exp converter with a CA3086 or CA3046

[patchell]

    The way I see the issue of where to put the Tempco resistor is basically, six
of one, half dozen of the other.

    If you put the tempco in the divider, the compensation is not exact, but, the
offset voltage of the opamp is a smaller portion of the output signal.

    If you put the tempco in the feedback of the opamp (and since it is difficult
to get large values because of the fact that the tempcos are usually wire wound
using copper wire), the offset voltage makes up a large portion of the signal,
creating potential for more errors...although, this problem can be solved by using
a premium quality opamp.

Ian Fritz wrote:

> Hi Rene --
>
> A good reason for putting the tempco in the feedback path.  I guess the
> voltage divider scheme is more popular, but I have never quite understood why.
>
>    Ian
>
> At 01:37 PM 5/27/2002, René Schmitz wrote:
> >Hi all!
> >
> >The maths for this can be found in the expo tutorial at my website.
> >Infact there is an approximation involved for a divider which has the
> >tempco in its lower leg. So in some sense that not truely a perfect
> >compensation, however one can argue that the error is vanishingly small.
> >(I.e. smaller than the other imperfections.) When you put the tempco in
> >the feedback loop of an opamp the compensation would be exact.
> >
> >Cheers,
> >  René
> >
> >At 12:31 27.05.02 -0600, Ian Fritz wrote:
> > >Sorry, but Hal is wrong on this (As is John Simonton, who makes the same
> > >incorrect statement).
> > >
> > >The temperature dependence of the converter goes as exp(qV/kT).
> > >
> > >V is proportional to the tempco resistance. If the tempco resistance is
> > >proportional to absolute T, then the converter is properly compensated.
> > >
> > >This is very simple physics.
> > >
> > >What seems to confuse people is that this tempco resistance R = AT gives a
> > >tempco of (1/R)(dR/dT) = 1/T. This indeed changes with temperature, but
> > >nevertheless is what is needed to provide correct compensation.
> > >
> > >Hal Chamberlin is not a physicist.
> > >
> > >   Ian
> > >
> > >
> > >At 12:02 PM 5/27/2002, Tim Ressel wrote:
> > >>I must disagree:
> > >>
> > >>--- Ian Fritz <ijfritz at earthlink.net> wrote:
> > >> > A standard tempco resistor (with resistance
> > >> > proportional to absolute
> > >> > temerature over the operating range) will in fact
> > >> > give *exact* compensation
> > >> > (cancels the 1/kT factor in the exponent). The idea
> > >> > that compensation is
> > >> > correct at one temperature only is a common
> > >> > misconception, unfortunately
> > >> > promulgated by some people who should know better.
> > >>
> > >>I quote from Chamberlin:
> > >>
> > >>"Note that the compensation is exact only at 27C
> > >>because the exponential converter temperature goes as
> > >>1/T rather than as KT, which the resistor provides."
> > >>
> > >>This was the line that convinced me to go with AN299.
> > >>I don't go against Hal Chamberlin, no matter how much
> > >>its been discussed here.
> > >>
> > >>--TR
> > >>
> > >>__________________________________________________
> > >>Do You Yahoo!?
> > >>Yahoo! - Official partner of 2002 FIFA World Cup
> > >>http://fifaworldcup.yahoo.com
> > >
> > >
> >--
> >uzs159 at uni-bonn.de
> >http://www.uni-bonn.de/~uzs159
> >
> >

--
 -Jim
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