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.

Image of a typical DMM used for measuring current, voltage, resistance and many other electronic and electrical parameters

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

  • DMM accuracy
  • DMM resolution

Digital multimeter accuracy

The accuracy of the digital multimeter is effectively the uncertainty surrounding the measurement. It is the amount by which the displayed reading can differ from the actual input.

There are a number of ways in which the digital multimeter accuracy may be expressed:

  • DMM Accuracy = ±(ppm of reading + ppm of range)
  • DMM Accuracy = (% Reading) + (% Range)
  • DMM Accuracy = (% Reading) + Offset

Here ppm refers to parts per million.

The way the accuracy is expressed depends upon the exact format for the instrument and also the preferences of the manufacturer. This sometimes makes comparing instruments from different manufacturers more difficult.

To give an example of how this may be calculated for a particular instrument. If 5 volt reading is being made and the specification for the DMM states that for the conditions within the laboratory, the reading will be ± 25ppm, and the 10 volt range is being used for which the accuracy is ± 8ppm. Then:

Accuracy = ±(25ppm in 5 Volts + 8ppm in 10 volts)

Accuracy = ±(5 x 25/1000000 + 10 x 8/1000000)

Accuracy = ± 205µV

Thus the indicated reading should be within 205µV of the actual value.

Effect of temperature on DMM accuracy

As with many other electronic items, temperature can have a significant effect on the measurement accuracy of a DMM.

Many precision or high accuracy digital multimeters have a temperature coefficient contained within the specification.

Although the way in which they may be expressed can vary occasionally the most common way to express them is as ±(ppm of reading + ppm of range)/°C.

DMM accuracy for AC

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 multimeter resolution

The resolution of a digital multimeter traditionally was 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.

For many new digital multimeters the traditional format for quoting the resolution of digits of display may not be appropriate. This is particularly the case for virtual instruments where the display is software controlled and therefore not a limiting factor. Instead the limiting factor is the analogue to digital converter, ADC.

For these instruments the resolution is often expressed in bits. For example a 14 bit ADC would give 2^14 distinct values, i.e. 16384 values.

It is possible to relate the digits of resolution to the number of the least significant bit.

Digits of resolution = log (Number of LSB)

Where the log is log to the base 10.

This means that for a digital multimeter with a 14 bit ADC the least significant bit is 16384.

Digits of resolution = log (16384)

Digits of resolution = 4.2

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%.

By Ian Poole

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