AC RMS is the most useful measurement for real-world waveforms because it does not depend on the shape of the signal. Most of the time, RMS measurement is described as a measure of equivalent heating value with a relationship to the amount of power dissipated by a resistive load driven by the equivalent DC value. For example, a 1Vpk sine wave will deliver the same power to a resistive load as a 0.707Vdc signal. A true RMS reading on a signal will give you a better idea of the effect the signal will have on your circuit.
If an AC RMS reading does not make sense, do not automatically assume there is something wrong with your circuit; the trouble might be with how you made the measurement. Study this list of five considerations that can affect your AC RMS measurement below:
- Take note on the measurement scale
Most meters specify AC inputs down to 5 or 10 percent of full scale, some even lower. For maximum accuracy, you need to measure as close to full scale as you can. In some cases, you might need to override auto scaling. Make sure the peak of the signal does not overload and saturate the meter’s input circuitry.
- Settling time consideration
RMS measurements require time-averaging over multiple periods of the lowest frequency being measured. Be sure to select your digital multimeter’s appropriate low frequency filter to allow for the fundamental to be captured. The lower the AC filter frequency is, the longer the settling time, and the longer it will take to make the measurement. Consequently, if you are not concerned about low frequencies in a measurement and your DMM has selectable averaging filters, switch to a faster filter.
- AC and DC coupling
It is easy to overlook this simple issue when you are in a hurry. If your meter is AC coupled (or has selectable AC coupling), it inserts a capacitor in series with the input signal that blocks the DC component in your signal. Blocking the DC may not be desirable, depending on the signal and what you are trying to accomplish. If you are expecting to include the DC component, but the meter is AC coupled, the results can be dramatically wrong. As a side note, if you need to measure a small AC signal riding on a large DC offset but your meter doesn’t provide AC + DC directly, you can measure the AC component using AC coupling and measure the DC component separately, then square each and take the square root of the sum, sqrt(Vac^2+ Vdc^2).
- Low-level measurement errors
When measuring AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will act as an antenna, and a properly functioning digital multimeter will measure the signals received. The entire measurement path, including the power line, acts as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in series with the DMM’s input. For this reason, apply low-level AC voltages to the digital multimeter through shielded cables, and connect the shield to the input LO terminal. Connect the DMM and the AC source to the same electrical outlet whenever possible, and minimize the area of any ground loops that cannot be avoided.
- Bandwidth errors
Signals that are rich in harmonics can produce low-reading measurements if the more significant of these components are not included in the measurement. Check the instrument’s data sheet to find the bandwidth of your multimeter. Then make sure your signals do not exceed it.
Making accurate, true RMS AC measurements with modern digital multimeters is simple and straightforward. However, you need to avoid common traps and pay attention to details. To get accurate results, know your meter and its measurement capabilities.
To learn more, download the Make Better AC RMS Measurements With Your Digital Multimeter application note.