Specifications 2
34980A Service Guide 41
Interpreting Internal DMM Specifications
This section is provided to give you a better understanding of the terminology used and
will help you interpret the internal DMM’s specifications.
Number of Digits and Overrange The “number of digits” specification is the most
fundamental, and sometimes, the most confusing characteristic of a instrument. The
number of digits is equal to the maximum number of “9’s” the instrument can measure or
display. This indicates the number of full digits. Most instruments have the ability to
overrange and add a partial or “½” digit.
For example, the internal DMM can measure 9.99999 Vdc on the 10 V range. This
represents six full digits of resolution. The internal DMM can also overrange on the 10 V
range and measure up to a maximum of 12.00000 Vdc. This corresponds to a 6½-digit
measurement with 20% overrange capability.
Sensitivity Sensitivity is the minimum level that the instrument can detect for a given
measurement. Sensitivity defines the ability of the instrument to respond to small
changes in the input level. For example, suppose you are monitoring a 1 mVdc signal
and you want to adjust the level to within ±1
V. To be able to respond to an adjustment
this small, this measurement would require a instrument with a sensitivity of at least
1
V. You could use a 6½-digit instrument if it has a 1 Vdc or smaller range. You could
also use a 4½-digit instrument with a 10 mVdc range.
For ac voltage and ac current measurements, note that the smallest value that can be
measured is different from the sensitivity. For the internal DMM, these functions are
specified to measure down to 1% of the selected range. For example, the internal DMM
can measure down to 1 mV on the 100 mV range.
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. For example, a 6½-digit instrument
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.
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