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What is ground loop and how to prevent?

Discussion in 'Electronic Basics' started by Dummy, Sep 11, 2003.

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  1. Dummy

    Dummy Guest

    What's ground loop actually? Ground current exists? Why does it happen
    and how to prevent?
  2. As you know, there are no perfect conductors in the things we build. If you
    are wiring a circuit, it is sometimes difficult to get all the ground wires tied
    to a single point, and you will occasionally end up with a loop in your ground
    In some applications, this will not present a problem. But in other cases,
    in particular in audio amplifiers or sensitive sensor and instrumentation
    amplifiers, you will have a significant difference from one end of the loop to
    the other.
    To illustrate how this causes problems, take any complex circuit board and
    measure the supply voltage right where the power is connected to the circuit
    board. Then, with the ground lead still at the ground connection, read the
    power supply voltage at the farthest chip on the board. You may see a half volt
    difference or more in some cases.
    Now imagine that you have a standard "star" type ground arrangement and
    decoupling capacitors on each chip- no problem, ground noise will be pretty much
    taken care of. But in a loop you end up with two problems- one is that their
    can be a large difference across the loop which may actually drag ground up a
    quarter to a half volt in parts of the circuit that otherwise would be
    unaffected. The other is that a loop acts as an antenna and can also pick up
    switching transients.
    For many logic applications, the only real problem you may see is either
    "brownout" on some chips or a loss of noise immunity- which can lead to "hair
    trigger" or sporadic operation. Faulty clocking or state changes can happen
    But for audio applications, the ground loop can be death- unexpected tiny
    noises become avalanches of audio garbage, adding pops and clicks, whines, and
    hum to the final output.
    And one final item- in some cases, a ground loop can actual resonate or
    oscillate. You can only imagine what this can do to sensitive components that
    want a clean regulated supply for operation.
    In summary, avoid the ground loop. It can cause vague, hard to track down
    problems in digital systems, and outright terrible performance in audio and
    instrumentation. Stick with "star" or single-point ground, and try really hard
    to isolate digital ground from analog ground when both are present.


    Chip Shults
    My robotics, space and CGI web page -
  3. Andrew Paule

    Andrew Paule Guest

    Even with the topology that you are suggesting, there are still ground
    loops, they cannot be avoided until every ground is in the same point
    spatially There is a limit to transport speed for electrons in Cu
    (about 0.67c - 1ft/ns) and any other known material that we use - no
    system that has been built is without ground loops (the potential
    differences caused by currents flowing in a non-point system), we just
    try to minimize them and their effects. Audio is a good example, but
    things are happening slowly there, try getting ground loops out of a
    chip at 1/4 wave RF frequncies (how long is a quarter wave at 50GHz) -
    0402's are bigger than that and will cause ground loops there.

  4. Marlboro

    Marlboro Guest

    That's what we always facing!!!, everytime we develop a board, (we
    deal with hi-resolution video,) we spend most of the time to fix this
    kind of noise.
    In video it causes terrible humming, windstorm, and sometime the ugly
    sparkles noises. I tell to the PCB guys (well I deal fpga and general
    R&D) always
    keep digital and analog ground separates, and keep in mind that the
    return currents are top killers and must be well isolated from analog

    But you now, some how it still end up noisy, since those PCB guys tend
    to make
    an easy job. He always questions why 2,3, or more power/ground plans,
    why not
    a single ground plans for all :(, while we ussually have 3 big
    sections: input
    analog, digital (DSP/FPGA), and output analog, plus several
    A/D, D/A,... Anyway, a pro PCB designer must be tough/experience/and
    most of all he need to learn!!!
  5. Another thing that causes problems, within analog, is supply currents for
    push-pull output stages. Each supply rail is supplying current primarily on
    only half the cycle, so its current is a very distorted version of the
    signal. If that current is allowed to couple back into the input - for
    instance, if the ground side of a supply filter capacitor shares a PCB trace
    with the ground lead of the input for an inch or two - then there will be
    substantial distortion introduced into the output.
  6. Andrew, you are right. I did not mean to imply that this would be the end
    of the problem, simply that you must be very circumspect about how you lay
    things out.
    In the microwave and higher frequencies, you must use stripline design
    methods to even come close. Every component then has other parasitic functions,
    where resistors become inductors, capacitors become resistors, and inductors
    show capacitance.
    My note was aimed primarily at the "basics" type readers in the groups-
    those who are learning enough to make projects that work, but are plagued with
    odd problems that just will not go away. Now some of them may see an answer, or
    at least a better understanding, of what is going on.


    Chip Shults
    My robotics, space and CGI web page -
  7. JeffM

    JeffM Guest

    ...get all the ground wires tied to a single point...

    I think that the problem in understanding ground loop
    may be that the term "ground" is so poorly understood.
    "Ground" means "the zero-volt reference for everything".
    If zero isn't really zero,
    you can't guarantee what will happen.

    Any time you push current thru a wire,
    there is a voltage drop.
    If you have one "ground" here
    and another "ground" over there,
    there will be a potential difference (voltage).
    Chip's mentions that a star ground
    is the classic way to deal with this.
    I've heard this called a Mecca Ground
    (though a google search doesn't find this to be a common usage).
    Low-impedence ground planes also try to minimize this.

    Andrew points to the propagation delay
    in circuits with fast risetimes.
    This is another reason for reducing distances
    and trying for a clean one-point ground.
  8. Andrew Paule

    Andrew Paule Guest

    Guess you had better either do your own boards, sit with the designer,
    or be firm with written specifications for the board.

    I had not heard "sparkle" for a while - old A/D's used to do this, and I
    saw what you are talking about on the output stage (JFET) of Kodak CCD's
    (still there) - make sure that the reset pulse is doing it's job, and
    maybe CDS the thing. Try isolating your ouputs (cut the trace or pull
    the resistor, hodl some nominal voltage on the A/D and see where it's
    coming from. CCD's have enormous current loads on the transfer gates
    (some of the philips ones require more than 11A for short periods, this
    feeds through to the output stages if there is not enough juice
    available locally).

    First thing I do anymore when doing anything above 30 MHz is to do the
    ground diagrams, max length versus current at nodes etc. Power supplies
    for these things have to be decoupled viciously - figure a good LC to
    avoid feeding it back on both the ground and the high (low) side. I
    found the only way to get beyond 65dB with CCD's was to generate local
    ground and VCC's, using good video op-amps for feedback to the power
    supply for isolation. The star configuration that you are talking about
    (multiple planes) with seperate grounds for analog and digital suggests
    that the digital section is not decoupled enough - I ran ECL (how do you
    say switch current) parts with 150 Ohm termination to ground on the same
    ground as an A/D (ECL at 50-3200MHz, 12 bit A/D (73db) sampling at
    40MHz), no problems as long as the decoupling was done well.

  9. Guest

    Example "earth loop":-

    jack lead earth
    | |
    | |
    |power cord earth |power cord earth
    | |
    | |
    mains plug mains plug
    | |
    mains earth

    60Hz radiation from e.g. a mains transformer can induce "mains hum" in
    the loop.

    Audio signals travelling along the jack plug lead e.g. from a record
    player to an amplifier will now have mains hum added to them(via their
    earth "return" (ironically, the screened jack lead makes it worse*).

    If you have an older "modular" hi-fi system with separate mains cords,
    you will find that only one of them has an earth, the others will only
    have L and N.

    *because the screen *prevents* mains hum on the jack lead's central
    signal wire: this would otherwise *oppose* and tend to *cancel* the
    pickup in the screen.

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