Are there any manufacturers of precision resistors other ...
I'm sure there's plenty of unknowns for me as a novice. I am trying to put together something with best tolerance / temp co to use as a reference, there's wattage considerations but beyond that not sure what I need to be looking for. I guess if I can characterise the components with a known standard / reference for a longer period of time then I can use cheaper resistors, but that's going to cost me a fair bit of change here.
What bdunham7 is getting at is that the Vishay Precision Group is quite good at playing specmanship, presenting specifications in such a way that they make the part look better than it is. I'm not saying they are not good resistors. But I'd ignore any "typical" figure in the data sheet. For example, a typical TC of 0.2 +/ 2 ppm/K, I would look at the 2 ppm/K figure and not at the bold 0.05 ppm/K figure at the top of the data sheet that might be available if you pay top dollar for selected resistors. I would also not count on this "typical" parabolic curve they claim for Z-foil. See
So take VPG datasheets with a bucket of salt, especially regarding temperature coefficient.
Also, I wouldn't focus so much on initial accuracy. For precision applications usually the most important is stability over time, temperature, humidity, load, etc. You can improve the initial accuracy by measuring them or paying someone (e.g. the manufacturer) to do that. Being extremely close to the nominal value rarely matters for a standard. But you can't do much about stability over time. And you can't improve the temperature stability except by either stabilizing the temperature or trying to find resistors with an opposite temperature coefficient to correct the tempco. See
The T.C. measurements on precision resistors might also give you an idea of other resistor brands, but don't expect < 1 ppm/K tempcos (from VPG or any other) or 50 ppm tolerance (which shouldn't be a priority at all as I just described). If you want < 1 ppm/K, prepare to have to buy a bunch of resistors and test them yourself. Some examples are TE UPW 25/50 resistors, General Resistance Econistors (sold by Rhopoint) and Edwin Pettis (Ultrohm). He is sometimes active on this forum used to make individual resistors to order for very reasonable prices, but I don't know how active he still is and what delivery times are like.
But Vishay has certainly been quite successful in gobbling up competitors.
What bdunham7 is getting at is that the Vishay Precision Group is quite good at playing specmanship, presenting specifications in such a way that they make the part look better than it is. I'm not saying they are not good resistors. But I'd ignore any "typical" figure in the data sheet. For example, a typical TC of 0.2 +/ 2 ppm/K, I would look at the 2 ppm/K figure and not at the bold 0.05 ppm/K figure at the top of the data sheet that might be available if you pay top dollar for selected resistors. I would also not count on this "typical" parabolic curve they claim for Z-foil. See TiN's website for a wealth of data on this, and on this forum especially Andreas posted a lot of data in T.C. measurements on precision resistors . Also note this this post by Dr. Frank on this subject.So take VPG datasheets with a bucket of salt, especially regarding temperature coefficient.Also, I wouldn't focus so much on initial accuracy. For precision applications usually the most important is stability over time, temperature, humidity, load, etc. You can improve the initial accuracy by measuring them or paying someone (e.g. the manufacturer) to do that. Being extremely close to the nominal value rarely matters for a standard. But you can't do much about stability over time. And you can't improve the temperature stability except by either stabilizing the temperature or trying to find resistors with an opposite temperature coefficient to correct the tempco. See how the Fluke 742A uses this to achieve a low tempco . What good is 50 ppm initial accuracy if within a year it drifted 100 ppm?The T.C. measurements on precision resistors might also give you an idea of other resistor brands, but don't expect < 1 ppm/K tempcos (from VPG or any other) or 50 ppm tolerance (which shouldn't be a priority at all as I just described). If you want < 1 ppm/K, prepare to have to buy a bunch of resistors and test them yourself. Some examples are TE UPW 25/50 resistors, General Resistance Econistors (sold by Rhopoint) and Edwin Pettis (Ultrohm). He is sometimes active on this forum used to make individual resistors to order for very reasonable prices, but I don't know how active he still is and what delivery times are like.But Vishay has certainly been quite successful in gobbling up competitors.
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