Understanding grounding, shielding, and guarding

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.


  1. Tomi Engdahl says:

    How to identify the best power supply for a test application

    Although a review of a power supply’s specifications should always be a part of the selection process, other characteristics should also be taken into consideration. From a user’s perspective, what’s important is understanding a supply’s power envelope to ensure it will be able to deliver power at the voltage and current parameters needed for a specific application.

    For developing, characterizing, and testing circuits that generate or measure low-level signals, selecting the design topology of the power supply and investigating its common-mode current can be essential to ensuring it doesn’t interfere with circuit performance.

    Check Isolation from Earth Ground
    One further indication of the quality of a power supply is the isolation of its output is from the power line. A power supply with high isolation further minimizes noise on the supply’s output. A good level of isolation impedance includes parameters greater than 1GΩ in parallel with less than 1nF and shielded well enough to support less than 5μA of common-mode current.

    Unfortunately, few instruments meet or exceed these guidelines. Low-frequency 60Hz designs may meet the common-mode current specification but fall short of the DC resistance and capacitance figures; switching designs may have low capacitance and higher DC isolation but excessive common-mode current. In some applications, the DC isolation resistance and capacitance are more important than common-mode current.

    One case in which the high impedance is important is when a supply is powering a circuit driven by a linear amplifier. In this situation, the power supply is part of the load on the linear amplifier, and a large power supply capacitance can create stability problems for the amplifier. Alternatively, a supply being used to power a low voltage resistive divider or a very low current measurement circuit may need low common-mode current, regardless of the isolation impedance.

    Generally, the higher the isolation, the lower the noise coupled through the supply from the AC power line. The problem becomes more complex when the application involves other instruments. In this case, insufficient DC isolation in the power supply can provide a conduction path for a high common-mode current from one of the other instruments. For any particular power supply application, it’s crucial to understand the effect of the power supply isolation resistance and capacitance on the DUT, and the path or loop where the primary and secondary common-mode currents flow in order to determine if a noise voltage (common-mode current × impedance) will be developed and whether the noise will be excessive.

    If using a multi-channel power supply, always ensure that the isolation between the power supply channels is greater than the isolation required between the DUT circuits.

  2. Tomi Engdahl says:

    Looking for Common Ground

    The term “ground” should be used more carefully. In our equipment, we distinguish between ground and DC common for good reason. Ground refers to chassis or safety ground to take errant AC voltage to ground instead of through an operator’s body.

    Recently, I discovered that more than one of the major industrial LCD manufactures tie the metal frame (chassis) of the LCD to DC common. When asked about this, they replied, “To reduce EMI.” This may be fine to make the display pass emission standards, but it is just plain wrong from an electronic design point of view. Industrial equipment (the intended market) is often housed in metal enclosures tied to AC ground.

    This one disturbed the nonvolitile Flash memory, causing the CPU configuration data to be corrupted. The operator was rather dismayed to encounter a “keyboard error” message on an embedded system that had no keyboard. The most amusing part of the message was the line that said to “Press F1 to continue.”

    Lesson for the day: Ground and common are not the same, and never the twain shall meet, unless made by monkeys with EMI in their heads.

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  4. Tomi Engdahl says:

    Cable shields

    Of the different choices that are available for grounding a shield braid that encloses a differential pair of signal wires, please consider that the shield braid be grounded only at the signal source, at the input end, and not at the output end.

    As a first thought, and as something that is often advocated, grounding a shield at both ends may result in severe ground loop currents which could adversely impact EMI and isolation properties.

    As a second thought, with the shield grounded only at the output end
    the interground interference signal, Enoise, can induce a differential noise signal between the two outputs E1 and E2 that feed the differential amplifier

    As a third thought, grounding the shield only at the input end averts both the ground loop problem and the time constant mismatch problem.

    Enoise as a common mode signal so that no differential voltage is created between E1 and E2. The A2 differential amplifier is thereby protected from Enoise.

  5. Tomi Engdahl says:

    Grounding for Test and Measurement DevicesGrounding for Test and Measurement Devices

    This KnowledgeBase provides general guidelines for installing National Instruments test and measurement equipment that require a connection to the facility grounding system for the purpose of enhancing electromagnetic compatibility (EMC) performance in accordance with the product documentation.


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