[sdiy] Tempco adjuster idea

[Ian Fritz]

Hi Rene --

>>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.

So maybe I could save one amp.  I'll think about that method some more.

>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.

Yes, and the tempco of carbon film increases with the R value, also.  But I 
could measure one from a batch and assume the rest are the same.

But what would you recommend for a negative tempco component?  Thermistors 
are really awfully nonlinear.

>>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.

Yep.  :-)  But I've done this by soldering in different values until I get 
the right one, so I don't think it's any worse.

>>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.

All true. But if the residual drift after fixing the scale factor is nearly 
linear it can be measured and a single compensator can be used to take care 
of all these sources at once.  This is what I did with my previous 
VCO's.  I used a combination of metal film and carbon film resistors to 
make the oscillator trip point slightly temperature dependent.  As you say, 
measuring the drift accurately is much harder than actually fixing it.

>>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.

Yep.  But this isn't an impossible task.  Good laboratory instrumentation 
(Keithley, HP, etc.) routinely makes it into the 50 ppm/K regime, and so 
can we!

   Ian