What are digits, accuracy, and resolution with a DMM
Since the advent of digital multimeters (DMM), all the measurement displays have digits. It is always a prominent part of the digital multimeter’s specification. It is often displayed on the front panel of a handheld or a benchtop multimeter.
Figure 1. Bench DMM with digits displayed on the top front panel (circled in red)
The display digits refer to the level of resolution that the DMM can measure. Resolution is the level of detail that is quantifiable on a DMM. The higher the number of DMM display digits, the higher the resolution of the DMM. It is common to see handheld DMMs with display digits of 3 ½ digits and 4 ½ digits. Bench DMMs tend to have display digits of 5 ½ digits, 6 ½ digits, 7 ½ digits, and even 8 ½ digits.
Handheld DMMs are normally used for simple troubleshooting and preventive maintenance work that does not require high measurement resolution. Most benchtops have 5 ½ digits and 6 ½ digits that are good enough for many applications. The 7 ½ digits and 8 ½ digits benchtop DMMs are categorized as high-performance instruments that are used for high precision applications and as reference standards for metrology labs.
What do digits and resolution mean in a DMM?
Figure 1 shows the DMM has 7 ½ digits. Why is it important to understand the half digit and what does the total number of digits represent? Well yes, it is certainly important as the number of digits is almost always the first spec you will notice about a DMM.
Table 2 shows you how the DMM display digits correspond to the resolution counts. If you have a 3 and a half digit DMM, the half digit is the most significant digit.
The half digit can only be a 0 or 1. So a 3 and a half digit DMM offers plus or minus 2,000 counts. A 4 and a half digit DMM offers plus or minus 20,000 counts and so on. The number of digits directly translate to the number of counts. Digits and counts give us an idea of the resolution of a DMM. They are not directly related to accuracy. It is a common misconception that digits and counts are the accuracies of a DMM.
How are accuracy and resolution related in DMM measurements?
Accuracy and resolution are one of the key specifications to consider when choosing the right digital multimeter (DMM) for your measurements. Figure 3 shows how accuracy relates to the resolution on a measurement scale. Accuracy is a measure of how good these numbers are, or how much you can trust them. Resolution is the level of detail that is measurable or the number of significant digits on a digital multimeter.
Figure 3. Accuracy and resolution on a measurement scale
Let’s look at Figure 4 below, the last digit of the DMM display, the number “8”. As we are on the 1 V range, the number “8” represents 800 nV. So, on the 1 V range, we have a resolution of 100 nV. However, the question is, is the actual value really 800 nV or how close is 800 nV from the actual value? The closeness of 800 nV from the actual value is what we call accuracy. We can say that a 6 ½ digit DMM with accuracy is no better or worse than a 5 ½ digit DMM with good accuracy, even though we get more resolution from the 6 ½ digit DMM.
Figure 4. A DMM display showing the last digit as the finest resolution
How measurement speed affects resolution?
Let’s take a look at how measurement speed affects resolution of DMM. Speed is an important factor for you if you want to optimize test time. Speed is how fast an analog to digital converter (ADC) captures samples. Let’s take Keysight’s 34470A 7 ½ digit DMM to explain how measurement speed affects resolution. Ideally, you may want the highest resolution and highest measurement speed possible, but Figure 5 shows the trade-off relationship between measurement speed and the DMM’s resolution.
Therefore, resolution decreases when measurement speed increases. You may want to check your DMM’s spec sheet to determine the resolution across all speeds, and that the highest required speed of the DMM, meets your resolution requirements.
Figure 5. Example of a DMM’s tradeoff relationship between measurement speed and the DMM’s resolution
The next time you see a 6 ½ digit DMM, you can immediately identify it to an actual measurement range of +/- 1999999 or 2,000,000 resolution counts. The ½ digit refers to the most significant digit but can only be either a 0 or a 1.
You have also learned that accuracy is how good the measurements are — for example, how close the measured value is to the true value. Resolution is the level of detail that is measurable or the number of significant digits on a digital multimeter.
You have also learned how accuracy is related to resolution and how measurement speed affects resolution.
To learn more about Keysight’s True volt series DMMs, please go to www.keysight.com/find/truevolt.
To learn more before you purchase your next DMM, read 10 Things You Must Know Before Buying Your Next Benchtop Digital Multimeter.