Hello Andreas, I ordered from Teltow, and got very good support from them, and especially from the Field Design Engineer of Vishay PG Heilbronn, who gave me additionally many useful hints and a fruitful discussion. He could not give me a hint, how to get such "typical" / perfect parts from Z-foil resistors, as those obviously can not be produced on order. Instead, a combination of a C- and a K foil resistor is recommended, i.e. VHP 101/103, which give +/- 10 ppm over 15..45°C, i.e. these resistors really are defined by a box method specification. But thats also not better than 0.3ppm/K. Well, the specifications and all the publications around the Vishay Z-foils in fact do not use the box method of specification. Especially the R over T diagram implies a differential TCR, although, it is in fact "usual" to use the chord slope specification, and this is explained only in some other publications, search for my citation. I wanted to build something stable like the SR104 on lower budget and much more simple, but the Z-foil resistors won't do the job. That's a pity. Anyhow, I will get a feedback, what happened to the rest of the batch. A total of 27 VHP202 resistors were produced and measured, just to fulfill my order... Frank

Hello Frank, >He could not give me a hint, how to get such "typical" / perfect parts from >Z-foil resistors, as those obviously can not be produced on order. >Instead, a combination of a C- and a K foil resistor is recommended, -i.e. >VHP 101/103, which give +/- 10 ppm over 15..45°C, i.e. these resistors >really are defined by a box method specification. But thats also not better >than 0.3ppm/K. I recently talked with the FAE from Teltow. He gave me info about the history of Vishay. Formerly the only field of operation was in strain gages. (today most of the turn around is still made in strain gages at precision group). The idea was to use the temperature compensation scheme of strain gages for precision resistors. They attach (cement) a metal foil to a ceramic substrate. So the tempco of the metal foil will be compensated by the expansion coefficient of the substrate. The following thoughts are only my guesses: - I would not be surprised if the "typical part" was a 350 Ohms resistor since it could be part of a typical strain gage bridge. (usually 350 Ohms sometimes 120 Ohms or 1000 Ohms). - The tempco may be affected by metal foil thickness. (for higher resistances they will need a thinner foil). - I do not think they change substrate thickness when they use a other foil thickness to compensate the different forces So it would be interesting if through different resistor values (foil thickness) there are systematic changes in tempco. I have now here the magic values of a LTZ1000 reference as Z201. That would be a point of start for a experiment. But unfortunately I have not the measurement equipment for a 4 wire measurement over temperature. By the way: did you specify operation from 0..60 degrees with your order. Or is this no option when placing a order? I am wondering why they measure/specify at -55 and +125 degrees for a metrology part. With best regards Andreas