Caddock defines a Voltage Coefficient and a separate TC.

Yes, I know. For other precision resistors, (low voltage ones, I have to admit), like the BMF from AE, they instead define the T.C. and additionally the dissipation / temperature rise, i.e. 0.14°C/mW, but no voltage coefficient.

In my case here the effect of the voltage coefficient is greater than the effect of self heating.

Did you measure that, or do you assume that only, from the datasheet?

Therefore, I assume it is a "real" voltage coefficient. The voltage coefficient is defined as 0,02ppm/V max. I think this would be the worst case. Caddocks specifications are usually very conservative.

I bought some matched pairs from caddock a while ago for another project and the application engineer calculates the effect of the voltage coefficient in the same way.

Well, you estimated this p.d. effect, and it was lower than this assumed pure voltage coefficient effect. So my assumption could be true as well, as Caddock may really specify very conservatively, so that the quadratic over linear effects are generously included.

The quadratic p.d. effect is well understood, as the mechanism can be easily calculated.

But I'd really like to learn about the physics of the linear voltage coefficient, as I did not find any explanation yet.

Frank

P.S.: I just downloaded their datasheet..

http://www.caddock.com/Online_catalog/Mrktg_Lit/TypeUSF.pdfCaddock spends a whole paragraph about a 'power coefficent', describing in detail the p.d. effect.

The boo-boo is, that this parameter later on is NOT specified at all!

They only specify the 'voltage coefficient' of 0.02ppm/V, without explaining what this parameters means, in contradiction to the 'power coefficient'

They also missed to specify the heat transfer resistance, which is necessary for calculation of the p.d. effect, or the 'power coefficient'.

Where did you get these 40K/W from?

That's the point in Caddocks specifications, that always disturbed me.

They are not better than Vishay, concerning fat lettered promotion w/o content:

"

**Zero Nominal TC**"

"

**Extremely Low Power Coefficient**"