HP 34970A manual Voltage Measured = ‾‾‾‾‾‾‾‾‾‾‾‾√ V 2 + Noise

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Chapter 8 Tutorial

Measurement Fundamentals

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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 VY 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

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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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