[volt-nuts] HP 3458A DC current accuracy (part2)
frank.stellmach at freenet.de
Mon Jul 9 19:26:50 UTC 2012
but it does not explain completely the mediocre specification of
resistance and current ranges, that's true.
Especially the 1µA range, measured directly by the internal 40kOhm
standard resistor (TC 1.3ppm/K, and "precise" to 2ppm within 24h )
should be on the same level of accuracy in the specification, which is
not the case at all.
Also, HP has not specified a transfer accuracy for the reistance ranges,
although the instrument is capable for stable measurements if the
environmental temperature is kept constant... Flukes version /
modification, the 3458HFL specifies resistance transfer accuracy.
Well, to understand the instrument further, you may download the
schematics (CLIP), and you will see, that the 100µA shunt resistor has a
T.C. of 10ppm/k only. Therefore, it is not that stable over temperature
(and time), so the limits have to be wider.
HP could have done much better, either with better resistors for all
ranges, but also with the specification, or better said, with the
validation of DC resistance and current.
Perhaps the DCI specifications had been estimated very generously,
saving time, instead of trying to validate to the lowest possible
My understanding of the instrument is, that its main purpose was for
military use (because of the broad env. temp. range) and for DCV and ACV
The other modes were an add on, and less attention was paid for them,
and it was not the intention to make it a "metrological" instrument in
Other 8.5 digits instruments beat the 3458A by far on those ranges..
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