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

[Ian Fritz]

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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