What’s a half digit and are they all the same?

The most significant digit on a DMM’s display often doesn’t count 0 through 9.

There’s no industry standard defining a half digit in a digital multimeter (DMM). DMMs with half digits from different makers can offer different levels of performance. This article begins with a brief survey of exemplary definitions of a half-digit, and other fractional digits. It then briefly reviews other important and related digital multimeter (DMM) performance metrics including counts, accuracy, resolution, and measurement speed.

accuracy, precision measurement — Figure 1. Example of a 4½ digit display. (Image: Eleshop)

In a DMM, there are two types of digits. Full digits can display all the numbers from 0 to 9. Fractional digits, including half digits, have a more limited set of numbers that can be displayed. Fractional digits are the most significant digits (MSDs) on a meter. For example, a 4½ digit meter, sometimes referred to as a 4.5-digit DMM, has four full digits that can display all the numbers from 0 to 9 and an MSD that can display a more limited range of numbers. So, a 4½ digit meter can display five digits (Figure 1).

Half digits and other fractions

Initially, half digits referred to displays that add a “1” as the MSD. In the term “½ digit” the 1 refers to the maximum value of the MSD and the 2 refers to the number of values possible for the MSD, in this case, 0 and 1. Using that definition, a 4½ digit DMM can display a maximum value of 19999. Today, depending on the vendor and context, the term “half digit” can be more nuanced.

DMMs are also available with other fractional digits. For example, some displays have an MSD that can display the numbers from 0 to 3. They are sometimes referred to as ¾ digits because they can display four values from 1 to 3 with the 3 indicating the maximum value. So, in a 3¾ digit DMM, the first three digits are full digits that can display all numbers from 0 to 9 and the MSD, the fourth digit, can display the numbers from 0 to 3. The maximum reading would be 3999.

Better than digits

Digits and fractional digits are a good way to define DMM display performance, but counts can be better. It depends on the DMM maker. Some DMM makers define counts as the maximum value that can be displayed. In other cases, counts are equated with DMM resolution and higher counts correspond with higher resolution.

3½ digit DMMs are available with a wide range of counts like 2000, 3200 and 4000 (Figure 2). For example, a 2000 count 3½ digit DMM can’t measure down to a tenth of a volt when measuring 200 V or more. Some 3½ digit DMMs are available with counts of 6000 and can have a maximum display of 5999. 4½ digit DMMs are available with counts of 50000 and can provide even finer resolution.

digital meter displays — Figure 2. Examples of maximum values versus counts for DMM displays. (Image: Fluke)

Best DMM specification

While digits and fractional digits are good and counts are better, what’s the best way to define DMM performance? No single specification is best. Adding accuracy is one way to improve a DMM specification.

measurement accuracy and precision — Figure 3. Resolution differs from accuracy, though both affect measurements. (Image: Keysight)

Accuracy describes how close the displayed measurement is to the actual value (Figure 3). There are different ways to specify accuracy, including:

A percentage of the maximum scale range.
A percentage of the actual value.
In parts per million (ppm), where 1 ppm is the equivalent of 0.0001%.
It is sometimes specified over a time period or over a temperature range.

Measurement speed

Measurement speed can directly impact resolution. Faster measurement speeds rely on high-performance analog-to-digital conversion (ADC). Some applications benefit from high-performance (and more expensive) ADCs. In general, resolution decreases as measurement speed increases. Some DMMs have different measurement speeds for different settings. In applications where measurement speed is important, buyer (and user) beware.

Summary

Half digits are a useful concept when discussing DMMs but not all half digits are equal. Different DMM makers have different definitions of half digits and other factors like counts, accuracy, resolution, and measurement speed can impact the meaning of a half-digit and the potential meter’s usefulness. Designers interested in a more general discussion should look at “ISO 5725-1:2023(en) Accuracy (trueness and precision) of measurement methods and results.”

References
10 Steps to Digital Multimeter (DMM) Selection, Tektronix.
Digits and counts explained, Eleshop.
Introduction to Digital , Ram Meter
What are digits, accuracy, and resolution with a DMM , Keysight.
Why digital multimeter accuracy and precision matter , Fluke.