AC signals are typically characterized by their root-mean-square (rms) amplitude, which is a measure of their total energy. To compute the rms value of a waveform, take the square root of the mean value of the square of the signal level. Some DMMs, including the NI 4065, do this nonlinear signal processing in the analog domain. However, the NI 4070/4071/4072 and the NI 4080/4081/4082 use digital signal processing to compute the rms value from digitized samples of the AC waveform. By computing rms values (a traditionally analog problem) in the digital domain, the DMM speeds up AC measurements in two ways, as follows:

  • Allows computation of fully-settled RMS values with only four cycles of the waveform
  • Automatically rejects the DC component of the signal, making possible an AC Volts DC Coupled mode, which completely bypasses the slow-setting input coupling capacitor required on traditional DMMs

The result is quiet, accurate, and fast-settling AC readings.

The rms algorithm used by the DMM requires at least 4 cycles of the waveform to obtain a quiet reading. For example, it requires a measurement aperture of 4 ms to accurately measure a 1 kHz sine wave. The measurement aperture also needs to be long enough to obtain the requested resolution. For example, although a 40 µs aperture is long enough to measure the rms value of a 100 kHz sine wave, it is not long enough to obtain readings with 6½ digit accuracy. Thus, the period of the waveform being measured and the desired resolution both affect the necessary aperture. NI-DMM selects the shortest aperture to satisfy both requirements.

For aperture times for the device, refer to the Measurement Defaults table for your device.

The period of the measured waveform, rather than the period of its lowest-frequency component, determines the required minimum aperture, as shown in Figures A, B, and C. Figure A shows a 1 kHz sine wave. This signal can be measured in 4 ms because it is 4 cycles of the waveform. Figure B shows a 1.1 kHz sine wave. It can also be measured in 4 ms because it is slightly more than 4 cycles of the waveform. Figure C shows a signal that is the sum of the signals in Figures A and B. Although the minimum frequency component of this signal is 1 kHz, 4 msec is not long enough to measure the rms value of the whole signal. In this case, the signal has a period of 10 msec, so it requires a 40 msec aperture for an accurate measurement.

Figure 1. A
Figure 2. B
Figure 3. C