What are the technical parameters and measurement methods of a digital multimeter?
1. Resolution, word count and bits
Resolution refers to the multimeter's ability to resolve small signals when making measurements. Knowing the resolution of your multimeter, you can determine whether it can observe small changes in the signal being measured. For example, if a digital multimeter has a resolution of 1mV on the 4V range, it can observe a change of 1mV (1/1000 V) when reading 1V.
If you had to measure lengths as small as 1/4 inch (or 1 mm), you wouldn't buy a ruler with graduations as small as 1 inch (or 1 cm). If normal temperature is 98.6 degrees Fahrenheit, a thermometer that only measures whole degrees isn't of much use. You need a thermometer with a resolution of 0.1 degrees.
The words "bits" and "words" are used to describe the resolution of a multimeter. They can be grouped by the number of words or digits displayed by the digital multimeter.
A 3½-digit multimeter displays three whole digits (0 to 9) and one "half digit" (which displays only a "1" or is left blank). A 3½-digit multimeter has a display resolution of 1,999 digits. A 4½-digit multimeter has a display resolution of up to 19,999 digits. Compared with "bits", using "words" can more accurately describe the resolution of a multimeter. Today's 3½-digit multimeters may have resolutions as high as 3,200, 4,000, or 6,000 digits.
For some measurements, a 3,200-digit multimeter provides better resolution. For example, if you want to measure 200V or more, a 1,999-digit multimeter cannot measure 0.1V. A 3,200 watt multimeter can display up to 0.1V when measuring voltages up to 320V. This resolution is the same as that of a more expensive 20,000-digit multimeter until voltage exceeds 320V.
2. Accuracy
Accuracy is the maximum allowable error under specific operating conditions. In other words, accuracy indicates how close the measured value displayed by the digital multimeter is to the actual value of the signal being measured.
The accuracy of a digital multimeter is usually expressed as a percentage of the reading. An accuracy of 1% of reading means that if the displayed reading is 100V, the actual voltage could be anywhere between 99V and 101V.
Specifications may also include a bit range that is added to the basic accuracy specification. The range represents the number of words by which the rightmost digit of the displayed value may vary. Thus, the accuracy in the above example can be expressed as "±(1 %+2)". Therefore, if the display reads 100V, the actual voltage value will be between 98.8V and 101.2V.
Analog multimeter parameters are determined by the full-scale error, not by the percentage of the displayed reading. The typical accuracy of an analog multimeter is ±2% or ±3% of full scale. At 1/10 full scale, accuracy becomes 20% or 30% of reading. Typical basic accuracy of a digital multimeter is based on between ±(0.7%+1) and ±(0.1%+1) of the reading or better.
