[sdiy] Tempco adjuster idea

[René Schmitz]

Hi Ian,

> Interesting idea.  I've always kept away from dividers because they 
> distort the T dependence.  But maybe I should look at the actual 
> response more closely.  Carbon film resistors have a negative tempco, so 
> maybe common components on hand could be used.

Actually this can also be played with an opamp. Use the NTC over the 
summing resistor, and the PTC in the feedback path. Two opamps at worst, 
because you need to make up for the inversion. But the idea is the same.
Carbon resistors would be an interesting variation. Though I think NTCs 
would be more reliable, as the tempco of a carbon resistor is not really 
a spec'ed parameter and thus likely to have variations.

> OTOH, my new proposal has a "knob" to dial in either a positive or 
> negative correction.

The hard thing is to make a measurement setup so that the calibration 
can be done. I mean knowing that there is a zero-tc point somewhere on 
that knob is one thing. Finding that spot is another one.

> According to my calculations, 3000 ppm/K tempco coefficient corresponds 
> to -247 ppm/K in scale-factor drift.  And the 3260 ppm/K units I just 
> bought should only give -66 ppm/K scale drift.   So you are correct, 
> these are pretty good.

66ppm is the theoretical limit for the expo scale factor alone. Other 
sources will likely outweigh that error. (TC of integration cap, 
comparator thresholds and so on...)  If one considers that typical metal 
film resistors have 100ppm or 50ppm drift alone that really puts this 
figure in perspective.

> OTOH Jim's active compensation results are incredibly good.  There is 
> some scatter in his data, but if you take the end points you get 10 
> ppm/K scale drift.  If you use a least squares fit you get 40 ppm/K.

I must admit that I haven't studied it. (As I'm not really interested in 
active compensations anymore.)

> So I'm looking into how to get into that rarified atmosphere with 
> passive compensation.

Right the air in that region is really thin.

Cheers,
  René

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