Understanding Digital Multimeter (DMM) specifications

[Barley_Li]

Digital Multimeters (DMM) are one of the most common of all electrical and electronics test instruments.
Each DMM may be slightly different in its specifications and features. Understanding the terms in DMM specifications can help us to maximize our measurement efficiency.

1. Accuracy
   This is the largest allowable error that your DMM occurs under specific operating conditions. In most cases, accuracy is expressed as a percentage and indicates how close the displayed measurement is to the actual (standard) value of the signal measured. Accuracy requires a comparison to an accepted industry standard.
   Some applications, such as calibration of automotive, medical aviation or specialized industrial equipment, may require higher accuracy. A reading of 100.0 V on a DMM with an accuracy of ±2% can range from 98.0 V to 102.0 V. This may be fine for some applications, but unacceptable for sensitive electronic equipment.

2. Resolution
   This is the smallest increment of change a DMM can detect and display.
   For examples, If a digital multimeter (DMM) has a resolution of 1 mV on the 3 V range, it is possible to see a change of 1 mV while reading 1 V. The user could see voltage changes as small as 0.001 value.

3. Range
   DMM’s range and resolution are related. Many DMMs provide an autorange function to select the correct range in measurement. This provides both a meaningful reading and the best resolution of a measurement.
   If the measurement is higher than the set range, most DMM will display OL (overload). The most accurate measurement is obtained at the lowest possible range setting without overloading the multimeter.

4. Precision
   Precision refers to a digital multimeter’s ability to provide the same measurement repeatedly under the same measurement condition. In some cases, if measurements are repeatable, a DMM’s precision is more important to determine an error pattern and make compensation for it.

5. Display Digits
   This is used to describe a DMM’s resolution.
   For example, a 3½-digit DMM can display three full digits and a half digit. The three full digits display a number from 0 to 9. The half digit, considered the most significant digit, displays a 1 or remains blank. A 4½-digit DMM can display four full digits and a half, indicative of higher resolution.

6. Display Counts
   Display Counts tell you what a DMM will display before it changes to the next range.
   For example, a DMM has 50000 counts. This means that its range changes when it hits 50000 on display. To illustrate, a 50000 count meter can read 49.999 V on the display, but when it tries to display 50 V, it will only read 050.00 V, or simply 50.00 V instead. The multimeter moves to the next range, which generally also means one digit of resolution will be lost.

For more DMM technical information, please check out the following TechForum threads:

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