Fluke 2625A, 2620A user manual True RMS Measurements, Effects of Internal Noise in AC Measurements

2620A, 2625A

Users Manual

Table 5-1. Ohms Test Voltage

Range

300.00 Ω

3.0000 kΩ

30.000 kΩ

300.00 kΩ

3.0000 MΩ

10.000 MΩ

∙4-Terminal Configuration

Typical Full Scale Voltage

0.22

0.25

0.29

0.68

2.25

2.72

In 4-terminal configuration, the instrument uses a second pair of leads to automatically eliminate measurement-lead and internal-relay resistance errors.

With measurement lead and internal relay resistances eliminated, this configuration yields the most accurate readings.

Four-terminal measurements are allowed on channels 1 through 10 only. However, two pairs of high (H) and low (L) terminals are needed for this type of measurement. The first pair is provided by the selected channel (any of channels 1 through 10). The second pair is provided by the channel 10 numbers higher. For example, channels 2 and 12 (or 7 and 17) could provide the two pairs of terminals. This second channel provides the necessary two additional terminals, and is therefore not available for any other use until the first channel is changed to a function other than 4-terminal OHMS or RTD.

A 4-terminal configuration is illustrated in Figure 5-2.

True RMS Measurements

The instrument measures the true rms value of ac voltages. In physical terms, the rms (root-mean-square) value of a waveform is the equivalent dc value that causes the same amount of heat to be dissipated in a resistor. True rms measurement greatly simplifies the analysis of complex ac signals. Since the rms value is the dc equivalent of the original waveform, it provides a reliable basis for comparing dissimilar waveforms.

Effects of Internal Noise in AC Measurements

With the input shorted and the instrument set for ac volts (VAC) measurement, internal amplifier noise causes a typical display reading of approximately 0.50 mV. Since the instrument is a true rms responding measurement device, this noise contributes minimally to the reading at the specified floor 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 as:

total rms digits = 0.502 +15.002 = 15.008

The display will read 15.01. At the 28.00 mV input level on the 300.00 mV range in the slow rate, the display will read 28.00 with no observable error.

Waveform Comparison (True RMS vs Average Responding)

Figure 5-3 illustrates the relationship between ac and dc components for common waveforms and compares readings for true-rms measurements (such as with the instrument) and average-responding measurements. For example, consider the first waveform, a 1.41421V (zero-to-peak) sine wave. Both the instrument and rms-calibrated

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