Fluke 45 Effects of Internal Noise in AC Measurements, Calculated AC + DC RMS Measurements

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Users Manual

By contrast, many meters use average-responding ac converters rather than true rms converters. The scale factor in these meters is adjusted so that they display the rms value for a harmonic-free sine wave. However, if a signal is not sinusoidal, average-responding meters do not display correct rms readings.

Effects of Internal Noise in AC Measurements

With the input shorted in an ac function (volts ac, current ac, or frequency) the display will read approximately 50 due to internal amplifier noise. Because the meter is a true rms responding meter, this noise has only minimal contribution to the reading at the specified floor (1500 counts in the medium rate) of each range. When the rms value of the two signals (internal noise and range floor) is calculated, the effect of the noise is shown:

Total rms digits = (502 + 1500 2 ) = 1500.8

The display will read 1501. At the down-range point (2800 display counts in the medium rate), the display will read 2800 with no observable error.

Note

Do not use the relative modifier (REL) to "zero" the meter because the relative reading is simply subtracted from the present reading.

Calculated (AC + DC) RMS Measurements

When Yand Z, or Cand Dare pressed simultaneously, the meter will alternately take a dc and an ac measurement then calculate and display the rms value:

RMS value = dc 2 + ac 2

In the dual display mode, when the volts ac and volts dc functions are selected, the 10 MΩ dc input divider is in parallel with the ac-coupled 1 MΩ ac divider.

Note

When measuring AC + DC, (or any dual display combination of AC and DC) In the fast reading rate, the Fluke 45 may show significant reading errors. This results from a lack of filtering on the DC portion of the measurement for the fast reading rate. To avoid this problem, use only the "slow" and "medium" reading rates for AC + DC or AC and DC combinations.

Waveform Comparison (True RMS vs. Average-Responding Meters)

Figure 4-3 illustrates the relationship between ac and dc components for common wave- forms, and compares readings for true rms meters and average-responding meters. For example, consider the first waveform, a 1.41421 V (zero-to-peak) sine wave. Both the Fluke 45 and rms-calibrated average-responding meters display the correct rms reading of 1.0000 V (the dc component equals 0). However, consider the 2 V (peak-to-peak) square wave. Both types of meter correctly measure the dc component (0 V), but your Fluke 45 also correctly measures the ac component (1.0000 V). The average-responding meter measures 1.111 V, which amounts to an 11 % error.

Since average-responding meters have been in use for so long, you may have accumulated test or reference data based on them. The conversion factors in Figure 4-3 should help you convert between the two measurement methods.

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