[volt-nuts] HP 3458A DC current accuracy

[Bob Smither]

On 07/11/2012 09:01 PM, Mike S wrote:
> On 7/11/2012 5:15 AM, Frank Stellmach wrote:
>> This is the worst realized electrical unit, i.e. the 'mise en pratique'
>> is difficult to an error level of about 1e-7 only.
> 
> On 7/11/2012 6:50 PM, Bob Smither wrote:
>> I may be off here, but I doubt that thermal (Johnson) noise would limit the
>> precision of current readings.
>> A one ohm resistor in a 1 Hz bandwidth would be 0.13 nV or 0.13 ppm of the 1 mV
>> reading.
> 
> It seems you just showed that it does (limit the precision).

Hmmm - I guess at 8.5 digits theoretically yes.  But the OP noted:

"But the 24h 100uA DC current accuracy is quoted as 10ppm Reading + 6ppm Range,
giving a maximum uncertainty of 16ppm when measuring 100uA. Why the big
discrepancy?"

so a 0.13 ppm error is pretty small compared to the stated DCA accuracy of the
3458A.  My point was it seems like whatever is degrading the reading to 16 ppm
is not the thermal noise.

-- 
Bob Smither, PhD                                   Circuit Concepts, Inc.
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            If a person has integrity, nothing else matters.
        If a person doesn't have integrity, nothing else matters.
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Smither at C-C-I.Com  http://www.C-C-I.Com  281-331-2744(office)  -4616(fax)
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