Practically every DMM has a DC and an AC measurement function. Voltage testing is commonly used to test and verify the outputs of instruments, components, or circuits. Voltage is always measured between two points, so two probes are needed. Some DMM connectors and probes are colored; red is intended for the positive point that you want to actually take a measurement of and black is intended for the negative point that is typically a reference or ground. However, voltage is bidirectional, so if you were to switch the positive and negative points, the measured voltage would simply be inverted.
There are usually two different modes for measuring voltage: AC and DC. Typically, DC is denoted with a V with one dashed line and one solid line while AC is denoted with a V with a wave. Be sure to select the correct range and mode for your application.
Figure 1. AC voltage (left) and DC voltage (right) measurements are commonly used to test and verify outputs of instruments, components, or circuits.
There are several terms and concepts you should be familiar with when measuring AC or DC voltage.
An ideal voltmeter has an infinite input resistance so that the instrument does not draw any current from the test circuit. However, in reality, there is always some resistance that affects measurement accuracy. To minimize this problem, a DMM’s voltage measurement subsystems are often designed to have impedances in the 1s to 10s of MΩ. If you are measuring low voltages, even this resistance can be enough to add unacceptable inaccuracies to your measurement. For this reason, lower voltage ranges often have a higher impedance option such as 10 GΩ.
With some DMMs, you can select the input resistance. For most applications, it can be said the higher the impedance, the more accurate the measurement. However, there are a few cases where you might choose the lower impedance. For instance, a conduit that has many different wires inside might have coupling across the wires. Even though the wires are open and floating, the DMM still reads a voltage. The higher impedance isn’t sufficient to eliminate these ghost voltages, but a low impedance provides a path for this built-up charge and allows the DMM to correctly measure 0 V. An example of this at a lower voltage range is if you had traces close together on a circuit.
When measuring AC signals (voltage or current), the crest factor can be an important parameter when determining accuracy for a specific waveform. The crest factor is the ratio of the peak value to the rms value and is a way to describe waveform shapes. Typically, the crest factor is used for voltages, but can be used for other measurements such as current. It is technically defined as a positive real number, but most often it is specified as a ratio.
Equation 2. The crest factor is a measure of how extreme the peaks are in a waveform
A constant waveform with no peaks has a crest factor of 1 because the peak value and the rms value of the waveform are the same. For a triangle waveform, it has a crest factor of 1.732. Higher crest factors indicate sharper peaks and make it more difficult to get an accurate AC measurement.
Figure 2. The crest factor of an AC signal can affect the accuracy
An AC multimeter that measures using true rms specifies the accuracy based on a sine wave. It indicates, through the crest factor, how much distortion a sine wave can have and still be measured within the stated accuracy. It also includes any additional accuracy error for other waveforms, depending on their crest factor.
For example, if a given DMM has an AC accuracy of 0.03 percent of the reading. You are measuring a triangle waveform, so you need to look up any additional error with a crest factor of 1.732. The DMM specifies that for crest factors between 1 and 2, there is additional error of 0.05 percent of the reading. Your measurement then has an accuracy of 0.03 percent + 0.05 percent for a total of 0.08 percent of the reading. As you can see, the crest factor of a waveform can have a large affect on the accuracy of the measurement.
Most DMMs offer the ability to do a null offset. This is useful for eliminating errors caused by connections and wires when making a DC voltage or resistance measurement. First, you select the correct measurement type and range. Then connect your probes together and wait for a measurement to read. Then select the null offset button. Subsequent readings subtract the null measurement to provide a more accurate reading.
In addition to performing a null offset, another way to improve voltage and resistance measurement accuracy is by enabling a feature called auto zero. Auto zero is used to compensate for internal instrument offsets. When the feature is enabled, the DMM makes an additional measurement for every measurement you take. This additional measurement is taken between the DMM input and its ground. This value is then subtracted from the measurement taken, thus subtracting any offsets in the measurement path or ADC. Although it can be very helpful in improving the accuracy of the measurement, auto zero can increase the time it takes to perform a measurement.