HP 34401A User Manual page 226

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Chapter 8 Specifications
Interpreting Multimeter Specifications
Resolution
Resolution is the numeric ratio of the maximum displayed value divided
by the minimum displayed value on a selected range. Resolution is
often expressed in percent, parts-per-million (ppm), counts, or bits.
1 2
For example, a 6
-digit multimeter with 20% overrange capability can
display a measurement with up to 1,200,000 counts of resolution.
This corresponds to about 0.0001% (1 ppm) of full scale, or 21 bits
including the sign bit. All four specifications are equivalent.
Accuracy
Accuracy is a measure of the "exactness" to which the multimeter's
measurement uncertainty can be determined relative to the calibration
reference used. Absolute accuracy includes the multimeter's relative
accuracy specification plus the known error of the calibration reference
relative to national standards (such as the U.S. National Institute of
Standards and Technology). To be meaningful, the accuracy specifications
must be accompanied with the conditions under which they are valid.
These conditions should include temperature, humidity, and time.
There is no standard convention among multimeter manufacturers for
the confidence limits at which specifications are set. The table below
shows the probability of non-conformance for each specification with the
given assumptions.
Specification
Probability
Criteria
of Failure
Mean 2 sigma
4.5%
Mean 3 sigma
0.3%
Mean 4 sigma
0.006%
Variations in performance from reading to reading, and instrument to
instrument, decrease for increasing number of sigma for a given
specification. This means that you can achieve greater actual measurement
precision for a specific accuracy specification number. The HP 34401A is
designed and tested to meet performance better than mean 4 sigma of
the published accuracy specifications.
8
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