Metrology 101: The Effect of Cables and Shields on Traceability

by Jesse Morse

If a person making a dc or low frequency measurement fails to select the proper cables, or to properly connect the shields the traceability chain for that measurement can be violated. This can happen because the cable-to-connector circuit may introduce serious errors into the signal that arrives at a test tool, or that is supplied by the calibration standard. Under certain conditions, the magnitudes of these errors may even be larger than the desired signal.

Various hazards that cables and connections can present to the traceability chain are described and discussed. A review of a general model of a measurement setup is given along with a discussion regarding the types of cables to consider in specific dc and low frequency measurements. Also presented is suggested shielding and guarding techniques for precision measurements, which can prevent or minimize introduction of errors into a measurement due to common-mode and normal-mode signals.