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I'm looking for a replacement "master" resistor for my HP3458A. This is currently a hermetically sealed S102C resistor from VPG [or whatever the production 3458A's received]. If I replace this 40K resistor with a more TCR stable one [and recalibrate], then the resistance function in the DMM will automatically be much better.
If I order a big pile of 80K resistors from Edwin [with his special extra-stability procedures added], there are sure to be a pair of them that [in parallel] will give me a very stable 40K.
Hi Ken,
I'm also searching for a better 40k resistor for my 3458A, but your statements are incorrect in several aspects.
The 40k needs to have a T.C. as low as 0.1ppm/K (currently specified ~1.3ppm/K)
This will improve the "Temperature Coefficient With ACAL" parameter ONLY! (by a factor of 10)
Anyhow, all Ohm ranges will still have a very bad T.C. of 3ppm/K, prohibiting a useful Ohm Transfer Stability specification.
To really improve the Ohm mode for T.C., you would have to replace ALL resistors which are involved in the Ohm measuring function, by low T.C. ones, i.e. BMF types R307 = 300, R308 = 3.00k, R310=10.00k, and the complete resistor array RP300, containing 30k, 60k, 600k, 6M, 10k, 0.3k, 3k. If I did not forget any, that would make 10 additional low T.C. resistors, see picture.
Additionally, because the timely drift specification is so bad, the 40k would need to be a hermetically sealed, AND oil filled type. VHPxxx types give a typical annual drift of 2ppm/6years.
PWW resistors are not suitable, as these have typical drift rates of 20ppm/year, maybe below 5ppm/year for selected ones.
This 40k resistor is part of the DCI / current ranges, which are also quite unstable, maybe also affecting the ACAL OHM function.
Therefore, I would also like to replace most of the current shunt resistors by <1ppm/K ones.
At least, R 207=40k (0.1ppm/K 2ppm/6yr.) , R206 = 500k, R208 = 4k53, R209 = 634, R210 = 90, R211 = 9, R212 = 1 Ohm, 2W.
Frank