How can you determine which digital multimeter (DMM) is best for your application?
There are five key specs you need to consider before purchasing a digital multimeter to make sure you’re picking the right DMM for your testing needs: number of display digits, counts, range, resolution, and accuracy. Let’s look at each of these in more detail.
1. Display Digits
In general, most manufacturers specify a DMM display by digits of resolution. For example, the digits of resolution for Keysight digital multimeters goes from 3 ½ digits to 8 ½ digits.
A 5 ½ digit DMM, for example, has five full digits that display values from 0 to 9, and a fractional digit. Fractional digit is the most significant digit in the display. It is the ratio of the maximum value the digit can attain over the number of possible states. For example, a ½ digit has a maximum value of one and has two possible states (0 or 1). A ¾ digit has a maximum value of 3 with four possible states (0, 1, 2, or 3). The higher the digits of resolution, the more precise and accurate your measurements will be. It also means that a multimeter with higher digits of resolution will be more expensive.
Nowadays, manufacturers have started to specify the display in terms of “count” because “digits of resolution” often creates confusion. The count of a DMM refers to how large a number the multimeter can display before it changes the measurement ranges and how many digits it can show in total. This affects how precise a measurement the DMM can display. For example, a 4½ DMM can also be specified as a 19,999-display count or 20,000 display count multimeter.
Resolution is defined as the smallest change in an input signal that produces a change in the output signal. Resolution will be improved when the DMM’s measurement range is reduced. Ultimately, you want the multimeter to display the best resolution of your measurement reading. You can play around with the multimeter and select the measurement range that gives you the reading with optimum resolution.
Range is correlated to resolution, the resolution display on the multimeter will depend on the measurement range that you select.
To choose a digital multimeter, you need to know the minimum and maximum measurement range, and the resolution you require. This information is usually available in the DMM’s data sheet.
4.1 Autorange vs. Manual Range Multimeters
Most of the digital multimeters today offer both auto ranging measurements and manual ranging measurements.
4.1.1 Auto Ranging Measurements
For auto ranging measurements, all you have to do is select your desired measurement functions and let the multimeter automatically choose the best range. Normally, you should select auto ranging when you do not know the potential measurement reading range.
4.1.2 Manual Ranging Measurements
Manual ranging measurements are normally selected when the desired measurement value is known. Based on your measurement value, you can choose the desired measurement range. If the measurement value is unknown and you would like to use the manual range, it is recommended that you use the “step down” method. Start with the highest range and then step down to lower ranges to achieve the optimum resolution on the display.
The accuracy of a DMM is different from its display resolution. The accuracy is the maximum allowable limit of error in the readings. Normally, manufacturers display DC voltage (DCV) accuracy as a benchmark when comparing with other manufacturer specifications, as DCV has better accuracy compared to other functions. The accuracy specification is expressed as ±((% of reading + % of range).
There is a range of digital multimeters available on the market. To select the right multimeter, you need to know the target application, the measurement range, and required resolution. If you require a higher measurement accuracy, make sure to select a DMM with a higher resolution.
To learn more, download the How to Select a Handheld DMM That is Right for You application note.
For more information about Keysight’s DMMs, visit www.keysight.com/find/dmm.