Digital Multimeter DMM Accuracy

- summary, tutorial or overview of the basics of digital multimeter DMM, accuracy, and how readings can be interpreted.

The accuracy of a digital multimeter can be very important in many electronics test applications. For most service or general electronics test applications, the digital multimeter accuracy will be well in excess of what is needed, but whatever the application it is necessary to understand the accuracy of the test equipment being used.

There are several constituents to what may be loosely termed the accuracy of the digital multimeter. Two of the major components are given below:

Digital multimeter resolution

The resolution of a digital multimeter is normally specified in terms of the number of digits displayed. Typically this will be a number consisting of an integer and a half, e.g. 3 ½ digits. By convention a half digit can display either a zero or 1. Thus a four and a half digit meter could display up to 19999. Occasionally a three quarters digit may be used instead of the half. When this is seen, it indicates that the DMM additional numeric can display a number higher than one, but less than nine. Often the range is extended to 399, 3999, etc. It is worth remembering that increased levels of resolution do not come without penalties. Longer settling times are required for the far right digits to reach their final value. Thus the time between readings is longer.

Digital multimeter accuracy

The accuracy of the meter is different to the display resolution. This represents the uncertainty of the reading due to inaccuracies in the DMM. The determination of the accuracy, itself has several constituents that may be all combined into the one "accuracy" figure. It will be found that the level of AC accuracy for the digital multimeter is normally less than that for DC measurements. The AC measurements will also be optimised for 50-60 Hz and this means that other frequencies may have poorer degree of accuracy. As with DC accuracy specifications, a number of counts (often greater than for DC) will be added to the accuracy percentage. Also, for waveforms other than a pure sine wave, additional inaccuracy will be encountered when measured with an average responding DMM. Even a true RMS responding DMM will have some accuracy limitations for waveforms with high peak amplitude components if measured near full scale.

Digital multimeters are generally able to provide very high degrees of accuracy; certainly they are far more accurate than their analogue counterparts which are normally only to guarantee accuracy levels of between 3 and 5%. A typical handheld or portable DMM should be able to provide accuracy levels of around 0.5% and many bench digital multimeters will quote figures of around 0.01%.

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