But after my measurements and detailed discussion with the German application engineer,
I do not trust their typical values any more, but their maximum values only.
It can be tricky business to measure this stuff properly, and perhaps it's possible you could have gotten a bad batch?
Unless you have measured multiple series from multiple production runs over multiple years, such a broad claim is hard to justify I think.
But I can certainly understand being once bitten, twice shy.
Dave,
You are right, such low T.C. measurements were very delicate.
But my experimental setup was very stable and skilled in physical aspects.
I was able to measure the "real" physically defined T.C. = dR/dT, compared to the averaged butterfly definition of Vishay.
All five resistors had a perfect linear T.C. curve around 25°C; no sign of parabolic behaviour.
Obviously, the batch I received did not show that ideal behaviour.
I claimed at Vishay, and their answer was exactly, that those typical parameters are not what they guarantee, only the maximum values.
Especially the parabolic shape of T.C., which is required for those fabulous 0.05ppm/K, was not confirmed/guaranteed by Vishay, in an official letter to me.
But this parabolic shape shows up only, if the thermal expansions of the ceramic carrier and of the conducting metal foil can be controlled, so that they inversely match in magnitude exactly.
The special Z type magic consists of compensating all 1st order temperature drifts, so that the 2nd order, quadratic terms remain.
There were several scientific papers stressing the characteristical parabolic shape, from Dr. Zandman, see here:
http://www.vishaypg.com/docs/60108/VFR_TN108.pdfFor me as a physicist, this feature of exact matching would the core of their technology.
If they negate that feature, they do not really control that technology, and one has to account for worst case parameters only.
In a four-eye-talk, I also asked the very competent application engineer, how to order Z type parts with a guaranteed parabolic T.C. shape and with those extremely low T.C.s.
His answer was, that it is not possible for Vishay to deliver such parts.
The only way is to order VHP101 parts, which consist of a pair of the older S- and K-type resistors in series connection, having inverse T.C.s and thereby compensating the overall T.C to near zero.
Their guaranteed mean T.C. of 10ppm over 30°C is also an order of magnitude higher than the 0.05ppm/K for the propagated single Z type.
That way, Vishay PG and their Z foil technology lost their charm for me totally.
But I gained a deep insight in this technology.
At last, I am not willing to pay that fortune (80€/EA for VHP202Z) expecting T.C. << 1ppm/K.
Wirewounds (3ppm/K) are as good as the Z foils, for one-tenth of their price.
Only the hermetically sealed, oil filled parts have an advantage, i.e. concerning annual stability.
Frank