Ever noticed the manuals of Digital Multimeters (DMM) mention the display as 3.5, 4.5, 5.5 digits, etc? What’s up with the 0.5? Why these specific numbers, you ask? Let’s explore…

The precision digits, also known as the resolution of a DMM, tell us how finely a meter can measure. For instance, a 2.5-digit DMM can display up to 199, a 3.5-digit DMM up to 1999, and a 4.5-digit DMM up to 19999. More the number of digits, more the resolution. Well, the “half digit” or 1/2 can only display a “1” or “0”, The numerator of 1/2 is the maximum digit that can be shown(1) and the Denominator is the total number of digits possible(2). Hence 1/2.
Sometimes you might even see it mentioned 4.75 or 5.75, which is 3/4. It can display 1,2,3 or 0 in which 3 is the max and takes the numerator position out of 4 possible numbers. A 3.75-digit DMM can display up to 3999, compared to a 3.5-digit DMM which can only display up to 1999.
Now, let’s talk about DMM counts. The count of a DMM is the maximum number that the DMM can display. A higher count lets you measure higher values at a higher resolution. So, a 20000-count DMM can display from 0 to 19999, offering more detail than a 2000-count DMM. It’s another way to talk about precision without using the digits terminology.
Remember, more precision digits or counts don’t always imply greater accuracy, but they do enable more precise measurements. Accuracy is typically dictated by ADC, component tolerances and calibration factors. It’s expressed separately as a percentage of the range or in count terms in spec sheets. If it’s in counts, for instance, 5 counts, it indicates that the DMM’s LSB can have an error range of 5 units in either the positive or negative direction. Therefore, when selecting a DMM, consider its digit count and accuracy to ensure it suits your requirements.
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