Inadequate shielding and bad grounding are often blamed when measurements are inaccurate, especially in high-impedance applications. Understanding grounding, shielding, and guarding in high-impedance applications article tells tells that in fact, shielding and grounding problems are frequently responsible for measurement errors, but many test system developers aren’t quite sure why. Many measurement errors can be traced back to currents from external fields that have become coupled into the measurement test leads.
Understanding grounding, shielding, and guarding in high-impedance applications article explores how ground loops and poor or non-existent electrostatic shielding can cause error or noise currents to flow in measurement leads or the device under test (DUT), as well as techniques for identifying these error currents and preventing them from undermining measurement integrity:
Most measurement errors can be traced to currents coupled into the DUT or into the measurement leads from external electrostatic (high-impedance) fields. Adding an electrostatic shield properly grounded to the instrument can totally eliminate these noise sources.
Differences in the safety ground caused by safety-ground currents generated from line-operated equipment can also cause measurement errors if the current is allowed to flow through the measurement leads. Common-mode current from the test system’s instruments contribute to these errors.
If the instrument common is connected to safety ground with a relatively large resistor, the RF energy will not enter the instrument, and voltages due to EMI rectification can be minimized.
Another worth to read document on this topic is A basic understanding of grounding and ground loops. It tells that in the world of data acquisition if there is one thing which causes more anguish that anything else, it is grounding! The A basic understanding of grounding and ground loops article attempts to provide a “simple” (if ground loops could ever be called simple) explanation of this phenomenon.