Agilent Technologies 34980A Service Manual download pdf (Page 51)

Specifications 2

34980A Service Guide 39

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 

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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Agilent Technologies 34980A Service Manual Page 51