Difference between average-responding and true RMS multimeters
An average-responding multimeter measures AC signals by computing the average value and then applying a form factor to display an RMS-equivalent value. A true RMS multimeter measures AC signals using an RMS algorithm. For a pure sine wave, both average-responding and true RMS meters can measure accurately. For distorted waveforms or typical non-sinusoidal waveforms such as square, triangular, or sawtooth waves, only a true RMS instrument will provide an accurate measurement.
What is a low-pass filter?
A low-pass filter is a circuit that allows low-frequency signals to pass while attenuating higher-frequency signals. A built-in low-pass filter will typically block signals above 1 kHz and attenuate signals between 100 Hz and 1 kHz, enabling measurement of the fundamental component from variable-frequency drives or other inverters.
What is a multimeter AC bandwidth?
AC bandwidth is the AC frequency range over which a digital multimeter can measure AC values within its specified accuracy. It is not a frequency measurement function but indicates the instrument's AC frequency response. If the measured frequency exceeds the meter's AC bandwidth, the meter cannot correctly measure the AC value. Therefore, wider AC bandwidth is important for higher-performance digital multimeters.
How does the REL function remove lead resistance effects?
When measuring resistance with a multimeter, the test leads introduce some series resistance. For precise measurement you could first measure the lead resistance and subtract it manually. The REL relative measurement function automates this: it stores the lead resistance as a zero reference and then subtracts that value from subsequent measurements, yielding a more accurate resistance reading.
Why can't my multimeter zero at the millivolt range?
Digital multimeters have very high input impedance, usually in the megaohm range, so they are sensitive to small induced voltages from ambient electromagnetic fields when the probes are open. In low-voltage ranges such as DC millivolts (mV DC) or AC millivolts (mV AC), these induced voltages can produce a nonzero display, which is normal. If the probes are shorted to form a loop, the reading will gradually approach zero.
