Grounding and EMI

One of the most common direct coupled noise sources is when the ground which is being used for reference or return is not referenced to earth as expected. This is especially prevalent in sensitive high-gain circuits.

Grounding and Shielding Existing Equipment – How to effectively minimize EMI issues when best practices are not available is a good paper mentioned by NASA Tech Briefs and Automation Weekly.

It tells that undesired signals couple into signals via four coupling factors: magnetic (inductive) coupling, capacitive (high speed voltage changes (dV/dt)) coupling, direct coupling, and radiative (Rf) coupling. Numerous books, articles and white papers are written all the time about this (many books on Electromagnetic compatibility cover those topics well). The material you will find include information on the coupling mechanisms and the sound mathematics of measuring inductance, capacitance, mutual inductance, resistance, and field intensity.

Most of the time the shield can be viewed as a band aid on a wound. Although necessary, one should not ignore the bleeding below. It is important to understand what is causing the noise and whether it can be resolved.

In summary, for protection against low-frequency (<1 MHz), electric-field interference, grounding the shield at one end is acceptable. For high-frequency interference (>1 MHz), the preferred method is grounding the shield at both ends, using 360° circumferential bonds between the shield and the connector, and maintaining metal-to-metal continuity between the connectors and the enclosure. Also safety considerations may require that the remote end of the shield also be grounded.

However in practice, there is often a caveat involved with directly grounding the shield at both ends: it creates a low frequency ground loop. In some cases (usually with balanced signals and differential receivers) the receiving end can be grounded with a low inductance ceramic capacitor (0.01 µF to 0.1 µF), still providing high frequency grounding. When you need to ground a shielded twisted pair cable from one end only, the ideal situation is to ground the shield at the driving end and allow the shield to float at the differential receiver.

If the receiver is a single-ended type, so there is no choice but to ground the coaxial cable shield at both ends.

For video applications where grounding at both ends leads to problems there are few tricks worth to try: Using a humbugging transformer on one end of the cable can reduce the problems considerably while still having both cable ends grounded. A normal single-ended video input can be made to work as differential input when you add video isolator to it. For single-ended audio applications audio isolation transformers are worth to try.

12 Comments

  1. Tomi Engdahl says:

    Return path discontinuities and EMI: Understand the relationship
    Minoru Ishikawa -June 11, 2015
    http://www.edn.com/design/pc-board/4439672/Return-path-discontinuities-and-EMI–Understand-the-relationship?_mc=NL_EDN_EDT_EDN_today_20151221&cid=NL_EDN_EDT_EDN_today_20151221&elq=f16edb6937274a16b479901f7f319cdb&elqCampaignId=26236&elqaid=29978&elqat=1&elqTrackId=287db830eedd4d7d8fab8b58a7ee48fb

    It’s conventional wisdom that a solid, continuous return path provides a better result in electromagnetic compatibility (EMC). This article discusses the relationship between return path discontinuities and EMC.

    A quality signal channel has a nice, uniform trace and a continuous return path from driver to receiver. Disruption to the return path introduces noise, and is typically caused by:

    Changing the reference plane(s) along the signal path
    Discontinuities within the reference plane

    There are two modes of high-frequency current flow

    Normal mode: This is the simpler mode. Current goes along a closed circuit loop, so the total current along the loop becomes zero. The loop is small/narrow enough, so the radiation from the incident current is canceled by the return current.

    Common mode: Noise power goes through both of the traces and, lacking an appropriate, closely spaced plane, something like the enclosure can become the return path. The noise induced by the currents on the signal traces is not canceled by a nearby return current, so strong radiation could occur. This physically larger circuit can act as antenna, so it may cause EMI as well as an EMS (electromagnetic suseptability) issue. The common-mode noise source could be the reference plane discontinuity mentioned in Normal mode.

    Reply
  2. Tomi Engdahl says:

    Well-grounded serial networking
    http://www.controleng.com/single-article/well-grounded-serial-networking/b9b9df50970922c88808d28913c99268.html

    Without proper grounding in a RS-422/485 installation, the user becomes vulnerable to common mode voltage transients that can compromise accurate data transfer and sensor measurements and damage equipment.

    The terms “2-wire” and “4-wire” are often used when discussing RS-422/485 installations. These terms can be misleading because they suggest that there is no need for an additional ground wire. That isn’t the case. Without proper grounding, the user becomes vulnerable to common mode voltage transients that can compromise accurate data transfer and sensor measurements and damage equipment.

    RS-422/485 systems can sometimes communicate successfully without the signal ground. This can happen when the nodes are located in very close proximity and the local ground is at the same potential, as in a controlled lab environment. This method is not recommended because if any significant distance separates nodes—and there is no signal ground—lightning strikes and other electrical noise can cause the common mode voltage to rise to levels that can prohibit communications and result in serious damage.

    Connecting signal grounds on both ends while keeping them separated from the earth ground is not sufficient to prevent issues in long RS-422/485 runs, even if you have a good physical earth connection and external surge protection.

    Reply
  3. Tomi Engdahl says:

    Cable shields
    http://www.edn.com/electronics-blogs/living-analog/4442166/Cable-shields?_mc=NL_EDN_EDT_EDN_analog_20160609&cid=NL_EDN_EDT_EDN_analog_20160609&elqTrackId=a7a526f434524aaebcb0179099f4f145&elq=67fb6fd9fb344b108c38beed0e9d229a&elqaid=32603&elqat=1&elqCampaignId=28476

    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.

    Reply
  4. Tomi Engdahl says:

    HDMI sparking – earth loop issue?
    https://www.electriciansforums.co.uk/threads/hdmi-sparking-earth-loop-issue.18396/

    A bad cable installation destroyed my $2,000 TV and maybe almost killed me
    I never knew it could go this wrong
    https://www.theverge.com/2016/5/18/11705038/cablevision-coax-destroyed-tv-nearly-killed-me

    Reply
  5. Tomi Engdahl says:

    Isolation—An Integral Component in Robotics Motion Control
    https://www.electronicdesign.com/industrial-automation/isolation-integral-component-robotics-motion-control?NL=ED-003&Issue=ED-003_20190128_ED-003_659&sfvc4enews=42&cl=article_1_b&utm_rid=CPG05000002750211&utm_campaign=22965&utm_medium=email&elq2=ef6184faacda44fabd51db8ec1b2278c

    Sponsored by Digi-Key and Analog Devices: To prevent the propagation of dc and unwanted ac currents between input and output, while passing the desired signal, isolation is needed. Here are some ways to ease its integration into motor-control design.

    The transition to Industry 4.0 has accelerated the adoption of robots, cobots (cooperative robots that interact with humans in a common workspace), and other advanced machines on the factory floor to generate higher levels of productivity. In addition, new energy standards are demanding ever-increasing levels of power efficiency.

    Together, these new requirements drive new design challenges in several areas, including networked communications, distributed sensing, and precision motion control.

    Reply
  6. Tomi Engdahl says:

    Video Isolator Circuit Diagram
    https://www.eeweb.com/extreme-circuits/video-isolator-circuit-diagram

    The ground loop problem can be overcome by galvanically isolating the video connections, for example at the aerial inputs of the surround-sound receiver and the TV.

    1
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    Posted Tuesday, April 23, 2013

    These days many more audio-visual devices in the home are connected together. This is especially the case with the TV, which may be connected to a DVD player, a hard disk recorder, a surround-sound receiver and often a PC as well. This often creates a problem when earth loops are created in the shielding of the video cables, which may cause hum and other interference. The surround-sound receiver contains a tuner that takes its signal from a central aerial distribution system.

    The TV is also connected to this and it’s highly likely that the PC has a TV-card, which again is connected to the same system. On top of this, there are many analogue connections between these devices, such as audio cables. The usual result of this is that there will be a hum in the audio installation, but in some cases you may also see interference on the TV screen.
    The ground loop problem can be overcome by galvanically isolating the video connections, for example at the aerial inputs of the surround-sound receiver and the TV.
    ADVERTISING

    Special adaptors or filters are sold for this purpose, known as video ground loop isolators. Good news: such a filter can also be easily made at home by yourself. There are two ways in which you can create galvanic isolation in a TV cable. The first is to use an isolating transformer with two separate windings. The other is to use two coupling capacitors in series with the cable. The latter method is easily the simplest to implement and generally works well enough in practice. The simplest way to produce such a ‘filter’ is as an in-line adapter, so you can just plug it onto either end of a TV aerial cable.

    The only requirements are a male and female coax plug and two capacitors. The latter have to be suitable for high-frequency applications, such as ceramic or MKT types. It is furthermore advisable to choose types rated for high voltages (400 V), since the voltages across these capacitors can be higher than you might expect (A PC that isn’t connected to the mains Earth can have a voltage as high as 115 V (but at a very low, safe current), caused by the filter capacitors in its power supply.

    These capacitors don’t need to be high value ones, since they only have to pass through frequencies above about 50 MHz. Values of 1 nF or 2.2 nF are therefore sufficient. To make the isolator you should connect one capacitor between the two earth connections of the coax plugs and the other between the two signal connections. The mechanical construction has to be sturdy enough such that the connections to the capacitors won’t break whenever the inline adapter is removed forcibly.

    Reply
  7. Tomi Engdahl says:

    Isolation between antenna network and TV/radio equipment
    https://www.epanorama.net/circuits/antenna_isolator.html

    Reply
  8. Tomi Engdahl says:

    DIY Ground Loop Isolator
    https://www.freemansgarage.com/index.php/2014/08/24/diy-ground-loop-isolator/

    Audio-Optical Isolation Amp
    https://audioxpress.com/article/Audio-Optical-Isolation-Amp

    This circuit enables the full galvanic isolation of sound frequency signals with optical transmission in excellent quality, eliminating interference.

    When you connect audio components, interference may occur from various power and signal grounds.
    For safety reasons, screenings are interconnected through the protection ground and the protection wire. The circuit that is formed this way opens a free path to the noise and hum from the power supply network and to the interference that originates from computers and other high-frequency generating equipment.
    By eliminating protective grounds, you could reduce the problem, but this is not allowed, because all power equipment must have a protection ground. Disabling the already existing protection ground can be hazardous.

    Reply
  9. Tomi Engdahl says:

    For DC blocking in audio applications, would you choose an electrolytic or ceramic capacitor? How do you choose the value?
    https://www.quora.com/For-DC-blocking-in-audio-applications-would-you-choose-an-electrolytic-or-ceramic-capacitor-How-do-you-choose-the-value

    Reply

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