Chapter 9 Specifications

Interpreting Internal DMM SpecificationsInterpreting 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 multimeter. The number of digits is equal to the maximum number of “9’s” the multimeter can measure or display. This indicates the number of full digits. Most multimeters have the ability to overrange and add a partial or “1⁄2” 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 61⁄2-digit measurement with 20% overrange capability.

Sensitivity

Sensitivity is the minimum level that the internal DMM can detect for a given measurement. Sensitivity defines the ability of the internal DMM 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 μV. To be able to respond to an adjustment this small, this measurement would require a multimeter with a sensitivity of at least 1 μV. You could use a 61⁄2-digit multimeter if it has a 1 Vdc or smaller range. You could also use a 41⁄2-digit multimeter 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.

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