[volt-nuts] HP3457 quirks and gripes

[Greg Burnett]

Mark wrote:
Well,  actually the last digit in ANY instrument is just marketing 
department eyewash...  but can be useful for getting a feel for the trend of 
the measurements.

As far a 6.5 digit meters needing a .5 ppm reference...  that is for 
absolute accuracy,  not measurement resolution.  Look at the 3458 specs... 
8.5 digit meter,  2 ppm ref at best.  Absolute accuracy is in the 6 digit 
range...  and it is the best meter on the market (and has been for the last 
20 years).
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The LSD resolution is useful *if* the meter's short-term noise and drift are 
low enough. For example the 3458A's 10 minute spec on 10Vdc range is 0.05ppm 
of reading + 0.05ppm of range. On the same range its 24 hour spec is 0.5ppm 
of reading + 0.05ppm of range.

The 3455A is an example of the opposite situation (in which its selectable 
ppm LSD is pretty much meaningless because it's short-term noise and drift 
are too high).

Greg