Measurement of Low-Frequency AC Signals with a Digital Multimeter
Most modern multimeters can measure AC signals with frequencies as low as 20 Hz. However, some applications require measuring signals at even lower frequencies. To perform such measurements, you need to select an appropriate multimeter and configure it properly. Here are some examples:
The Agilent 34410A and 34411A multimeters use digital sampling technology and can perform true-RMS measurements down to 3 Hz. They digitally increase the settling time to 2.5 seconds with a slow filter. For optimal measurements, note the following:
Setting the correct AC filter is critical. The filter is used to smooth the output of the true-RMS converter.For frequencies below 20 Hz, the correct setting is LOW.With the LOW filter selected, a 2.5-second delay is inserted to ensure stable multimeter readings.Set the low filter using the following command:VOLTage:AC:BANDwidth MIN
If you know the maximum level of the signal under test, use manual ranging to speed up measurements.Long settling times for low-frequency measurements can significantly slow down auto-ranging.We recommend using manual range.
The 34401A uses a DC-blocking capacitor to prevent the AC RMS converter from measuring DC signals,allowing the multimeter to measure the AC component on its best range.When measuring sources with high output impedance, allow sufficient time for the DC-blocking capacitor to stabilize.Settling time is not affected by the frequency of the AC signal, but can be influenced by any changes in the DC component.
