[volt-nuts] 3458A resistance calibration

[frank.stellmach at freenet.de]

Dear Anton,
 
Vishay developped the C foil resistors in 1962, having a "negative" parabola with +/- 2ppm/K T.C.
In 1982 the K-foil was introduced, having a "positive" parabola, +/-1ppm/K T.C.
 
Both were still available, types S102C, and S102K, respectively.
 
The Z foil was introduced in 2002.
 
Therefore, when the HP3458A was designed, the K-foil resistor was used, in a hermetic package.
This was probably the original component, as found in the BOM of the CLIP (1.3ppm/K), and also manifested in the still valid specification of the 3458A, i.e. 1ppm/K, 10ppm/year.
 
 
The VHP101 was already available in the 1990ties, the T.C. probably specified to  <=0.5ppm/K at that time.
HP obviously switched to that more stable type about 10 years later.
 
Inside the actual VHP101, (since about 2005, or so), there were two chips in series connection, one C-foil, and one K-foil, so that the total T.C. is cancelled to an average T.C of 0.3ppm/K.
 
Frank
 
 
 
Dear Frank,
Could You give me more detail which resistor has C-foil and which K-foil.
Anton


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