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Things to Consider When Selecting a Multimeter

True RMS versus average-

responding DMMs

Multimeter input impedance

Basically, there are two types of multimeters available: “average-responding”

and “true RMS” (root mean square). True RMS is the measurement of AC

voltage or current that reflects the amount of power dissipated by a resistive

load driven by the equivalent DC value. This power is proportional to the square

of the measured true RMS voltage, independent of wave shape. An average-

responding AC multimeter is calibrated to read the same as a true RMS meter

for sine wave inputs only. For other waveform shapes, an average-responding

meter will exhibit substantial errors. Average-responding meters normally work

well for linear loads measurement such as standard induction motors, resis-

tance heating, and incandescent lights, but if non-linear loads such as electronic

controls are present, errors occur that could make the reading be lower than

expected. Always remember to consider what kind of applications you are

measuring for before selecting a handheld DMM.

The input impedance of the DMM is very high compared to the impedance of

the circuit being measured. This design places very little load on the circuit

under test to prevent effecting the circuit’s operation and causing the measured

reading to be inaccurate. Typically handheld multimeters have input impedance

that is greater than 1 MΩ, and the input impedance varies depending on the

DMM’s design. It is especially important to select a DMM with high impedance

for applications requiring the measurement of sensitive electronics or control

circuits to ensure the accuracy of the reading.

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Agilent Technologies U1270 Series Especificações Página 4