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

[Gene Stopp]

I once found a tempco that just didn't work in the feeback loop of an
op-amp. It was a simple inverting summer configuration, but it was totally
non-linear. I did not try to troubleshoot at the time, I just assumed that
it had something to do with the inductive characteristics of the tempco
which was probaby wirewound. It may have been that something other than a
TL082 would have worked properly, but at the time I didn't have the luxury
of troubleshooting. So I chose the divider method and this is why the ASM-1
VCO used this design (so that it's independent of tempco inductive
characteristics). I have an ASM-1 on a MIDI-CV converter in my modular mess
and I must say that they are by far the most stable and accurate of all the
VCO's in the setup. This probably has more to do with Terry Michael's core
design than any of my doing :)

However I have 3 old EN-46 VCO's in the setup too, and those use Q81's in
the feedback loops of 741's driving AD821 matched pairs, and those have
always worked great.

Anyway I've found the divider method to be at least useable, even if it's
not ideal. Any comments on why certain tempcos in certain op-amp feedback
loops may cause non-linear behavior?

Best Regards,

- Gene


-----Original Message-----
From: Ian Fritz [mailto:ijfritz at earthlink.net]
Sent: Monday, May 27, 2002 5:33 PM
To: René Schmitz; Tim Ressel; Sowa Roman; SynthDIYmailinglist
Subject: RE: [sdiy] Temperature stable lin-exp converter with a CA3086
or CA3046


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
> >>
> >>__________________________________________________
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> >
>--
>uzs159 at uni-bonn.de
>http://www.uni-bonn.de/~uzs159
>
>