Chapter 8 Tutorial

Measurement Fundamentals

Low-Level AC Measurement Errors When measuring ac voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will act as an antenna and the internal DMM will measure the signals received. The entire measurement path, including the power line, act as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in series with the instrument’s input. For this reason, you should apply low-level ac voltages to the instrument through shielded cables. You should also connect the shield to the input LO terminal.

Be sure to minimize the area of any ground loops that cannot be avoided. A high-impedance source is more susceptible to noise pickup than a low-impedance source. You can reduce the high-frequency impedance of a source by placing a capacitor in parallel with the instrument’s input terminals. You may have to experiment to determine the correct capacitance value for your application.

Most extraneous noise is not correlated with the input signal. You can determine the error as shown below.

Voltage Measured = ````````````√ V 2 + Noise 2

in

Correlated noise, while rare, is especially detrimental. Correlated noise will always add directly to the input signal. Measuring a low-level signal with the same frequency as the local power line is a common situation that is prone to this error.

You should use caution when switching high-level and low-level signals on the same module. It is possible that high-level charged voltages may be discharged onto a low-level channel. It is recommended that you either use two different modules or separate the high-level signals from the low-level signals with an unused channel connected to ground.

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