How Do Digital Multimeters Work?
Last Updated on July 14, 2021 by Gabriel Goddy
You might have wondered how digital multimeters work. Are they really different from the analog models? You’ll find out the answers to your questions in this guide if you are patient enough to read through.
What is a digital multimeter?
A digital multimeter is a test tool used to measure two or more electrical values—principally voltage (volts), current (amps), and resistance (ohms). It is a standard diagnostic tool for technicians in the electrical/electronic industries.
Digital multimeters long ago replaced needle-based analog meters due to their ability to measure with greater accuracy, reliability, and increased impedance. Fluke introduced its first digital multimeter in 1977.
Digital multimeters combine the testing capabilities of single-task meters—the voltmeter (for measuring volts), ammeter (amps), and ohmmeter (ohms). Often, they include several additional specialized features or advanced options. Technicians with specific needs, therefore, can seek out a model targeted to meet their needs.
Digital multimeters (DMM), once called voltmeters, have been described as the tape measure for engineers in the new millennium. With electronics and electrical circuits included in almost every consumer and industrial product, DMMs are a must for designing, testing, and troubleshooting. Here’s a look at the features and capabilities of DMMs, as well as how to use them safely.
The face of a digital multimeter typically includes four components:
- Display: Where measurement readouts can be viewed.
- Buttons: For selecting various functions; the options vary by model.
- Dial (or rotary switch): For selecting primary measurement values (volts, amps, ohms).
- Input jacks: Where test leads are inserted.
How a DMM works – fundamentals
When looking at how a digital multimeter works, it is necessary to understand the core technologies that are generally used.
For the DMM, one of the key processes involved in this is that of the analog to digital conversion.
There are many forms of analog to digital converter, ADC. However, the one that is most widely used in digital multimeters, DMMs is known as the successive approximation register or SAR.
Some SAR ADCs may only have resolution levels of 12 bits, but those used in test equipment including DMMs generally have 16 bits or possibly more dependent upon the application.
Typically for DMMs resolution levels of 16 bits are generally used, with speeds of 100k samples per second. These levels of speed are more than adequate for most DMM applications, where high levels of speed are not normally required. Typically for most bench or general test instruments, measurements only need to be taken at a maximum rate of a few a second, possibly ten a second.
As the name implies, the successive approximation register ADC operates by successively homing in on the value of the incoming voltage.
The first stage of the process is for the sample and hold circuit to sample the voltage at the input of the DMM and then to hold it steady.
With a steady input voltage, the registration starts at half its full-scale value. This would typically require the most significant bit, MSB set to “1” and all the remaining ones set to “0”. Assuming that the input voltage could be anywhere in the range, the mid-range means that the ADC is set in the middle of the range and this provides a faster settling time. As it only has to move a maximum of the full scale rather than possibly 100%.
To see how it works take the simple example of a 4-bit SAR. Its output will start at 1000. If the voltage is less than half the maximum capability the comparator output will be low and that will force the register to a level of 0100. If the voltage is above this, the register will move to 0110, and so forth until it homes in on the nearest value.
It can be seen that SAR converters, need one approximating cycle for each output bit, i.e. an n-bit ADC will require n cycles.
DMMs are tools for measuring volts (V), ohms (Ω), and amperes (A). Most DMMs have other features and characteristics, but measuring those three variables is the foundation for all electrical measurements. You should also know the different ways DMMs display their measurements.
Resolution: for instance, refers to how fine a DMM’s measurement is. By knowing the meter’s resolution, you can determine if it can see a small change in the measured signal. For example, if the DMM has a resolution of 1 mV on the 4 V range, it can see a change of 1 mV (1/1,000 of a volt) while reading a 1 V signal.
You wouldn’t buy a ruler marked in one-inch (or one-centimeter) segments if you had to measure down to a quarter-inch (or one millimeter). A thermometer that measures only in whole degrees isn’t much use when your normal temperature is 98.6 °F. You need a thermometer with a one-tenth-degree resolution.
Accuracy: is the largest allowable error under specific operating conditions. In other words, it is an indication of how close the DMM’s measurement is to the actual value of the signal. Accuracy for DMMs is usually expressed as a percent of the reading. Accuracy of one percent of reading means that for a displayed reading of 100 V, the actual value of the voltage could be anywhere between 99 V and 101 V.
Digital and analog displays: For high accuracy and resolution, digital displays show three or more digits for each measurement. The analog needle display is less accurate and has lower effective resolution because you have to estimate values between the lines. A bar graph shows signal changes and trends just like an analog needle but is more durable and less prone to damage.
Analog meter specifications are determined by the error at full scale, not at the displayed reading. Typical accuracy for an analog meter is ± 2 % or ± 3 % of full scale. At one-tenth of full scale, these become 20 % or 30% of reading. Typical basic accuracy for a DMM is between ± (0.7 %) and ± (0.1 %) of reading, or better.
Saving and sharing results: As equipment has become more complex and more powerful, so have the DMMs. Wireless test tools can send results to each other and to smartphones, where you can share data, images, and notes with colleagues. Wireless DMMs, other related test tools, and smartphone apps (such as Fluke Connect) let engineers make the best decisions faster than ever before, saving time and increasing their productivity.
Types of Digital Multimeter
Digital types of multimeter are available in three types.
- Fluke Multimeter
The fluke digital multimeter can be designed with various collaboration functions. Generally, it includes a large display and this instrument is used to measure the voltage as well as electrical resistance. Some kinds of devices are available with advanced features to measure humidity, duty cycle, pressure, frequency temperature, etc. The fluke multimeter is one of the most frequently and famous instruments.
This kind of multimeter is mainly used for calibration efforts and used to calibrate currents, volts & other electrical units.
The fluke multimeters are protected against the transient voltage. It is a small portable device used to measure voltage, current, and test diodes. The multimeter has multi selectors to select the desired function. The fluke MM automatically ranges to select most measurements. This means the magnitude of the signal does not have to be known or determined to take an accurate reading, it directly moved to the appropriate port for the desired measurement. The fuse is protected to prevent damage if connected to the wrong port.
- Clamp Digital Multimeter
The clamp digital multimeter is used to measure the electricity flow. As the name suggests, this multimeter includes the feature namely clamp which measures the amps whenever the probes measure the volts. The adjustment of power utilization otherwise watts can be done through multiplying the reading of voltage with the amps. This multimeter also includes an additional feature that is different kinds of settings. The appropriate feature is used while measuring.
This kind of multimeter includes fixed tools for measuring the current flow. This device extremely changes from the fluke type because, in the fluke multimeter, it utilizes a clamp to measure the flow of current. So, this instrument is usually recommended for professionals only.
- Autoranging Multimeter
The auto-ranging multimeter is a simple multimeter to utilize even though it is similarly the most costly of all kinds of digital multimeters. This multimeter includes a knob in the center and has less position. So it doesn’t switch automatically to measure. This instrument is applicable in simple projects. For beginners as well as electricians at home, this instrument is highly recommended. Generally, it measures a single component at a time.
Advantages and Disadvantages of Digital Multimeters
The advantages of a digital multimeter include the following.
- It gives an automatic o/p display.
- The measurement results of the meter can record and store in memory and synchronizes through a PC
- It includes auto polarity functions
- The meter reading accuracy cannot depend on the charging of the battery
- It ensures accuracy
- Resistance toward mechanical damage.
- Zero adjustments cannot be required
- Measurement accuracy is high
- Measuring ranges can be selected through manual or automatically
The disadvantages of digital multimeter include the following
- As compared to analog, it is expensive
- This multimeter does not work properly through measurement fluctuations. It can be tricky to discover one for your exact needs.
Generally, multimeters are used to measure resistance, inductance, frequency & conductance. Some of the meters are used for measuring temperature and duty cycle. But advanced meters are used with some features to measure decibels, microamps, RPM, etc.