Fluke 2620A, 2625A user manual Making Mixed Measurements

Additional Considerations 5

Making Mixed Measurements

average-responding measurement devices display the correct rms reading of 1.0000V (the dc component equals 0). However, consider the 2V (peak-to-peak) square wave.

Both types of measurement correctly display the dc component (0V), but the instrument also correctly measures the ac component (1.0000V). The average-responding device measures 1.111V, which amounts to an 11% error.

Average-responding measurement devices have been in use for a long time; you may have accumulated test or reference data based on such instruments. The conversion factors in Figure 5-3 can aid in converting between the two measurement methods.

Making Mixed Measurements

With multiple channels, Hydra Series II allows mixing of measurement types. But the possibility of the ac signal on one channel affecting measurements on other channels must be considered. This effect is known as cross talk.

The ac volts signal could be either a voltage that is to be measured on another channel (which is known as a normal mode signal) or an ac voltage signal on another channel that is present between the channel inputs and earth ground (which is known as a common mode signal). A common mode signal could occur, for example, if an unshielded thermocouple were to be used to measure the metal case temperature of an ac power line diode.

AC voltage cross talk can affect the various measurement types differently. It can cause other ac voltage measurements to read too high. DC and resistance (OHMS) measurements could either shift or read noisy. Frequency measurements could be noisy, or, in the extreme case, the cross talk frequency could actually become the measured signal.

Cross talk can occur at numerous places: in the device or process being monitored, in the wiring to the Hydra Series II instrument, or within the Hydra Series II instrument itself.

Fortunately, precautions can be taken to provide error-free measurements by minimizing the effects of cross talk. Use the following guidelines to minimize cross talk between ac volts signal inputs and other sensitive channels:

∙Keep any input wiring carrying ac volts signals physically separate from other sensitive channel’s input wiring.

∙Avoid connecting inputs with ac volts signals adjacent to sensitive channel inputs. (Leave unconnected channels between the inputs, if possible).

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