Floating VS grounded

Discussion in 'General Electronics Chat' started by relicmarks, Apr 5, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    I don't understand really about floating voltages VS grounded voltages

    if your using differential signals they use Floating voltages but why ?

    When the differential signals go to the op amp they get converted from floating to ground , because inside the IC chip the differential signals get grounded

    I'm not understand the potential differences when using floating

    Is floating = difference signals DC or AC?

    I seen DC floating to DC ground converters , but why would circuits use DC floating voltages?

    I'm not understanding really what are Floating voltage dividers or floating voltage drops

    How do you measure floating voltage drops ?

    I would think the using DC floating would create alot of noise/buzz/hum is this true?

    How do you know the potential differences when measuring floating voltage drops?

    Is seems that Floating is DC offset with AC signal because your not grounding the DC offset ?

    Is floating always differential signals? if what does floating mean to AC and DC signals?

    Whats the difference between a floating power supply VS a reference power supply? why would i use a floating power supply?
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A single-ended input picks up interference if its input is not shielded and if it is not grounded.
    A differential input has its inputs out-of-phase so they cancel interference, sometimes do not need a shield (telephone wires) and sometimes do not need a ground.
     
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Floating voltage- A voltage that is not connected by any conducting path (including resistors) to ground or the key voltage reference point

    floating means no reference to anything

    A system where the system ground is not actually connected to another circuit or to earth (though there may still be AC coupling) is often referred to as a floating ground
     
  4. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    A single-ended input picks up interference if its input is not shielded and if it is not grounded.
    A differential input has its inputs out-of-phase so they cancel interference, sometimes do not need a shield (telephone wires) and sometimes do not need a ground

    Yes i understand this is the basics

    But what i don't understand is

    How is 12volts AC ground different VS 12volts AC floating?

    Or 12volts DC ground different than VS 12volts DC grounded?
     
  5. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    I'm mostly really confussed about ""single ended FLOATING""?

    not differential floating, i understand that i just don't get how does single ended floating works and how the voltage drops or voltage potential works with single ended floating, when the signal ended is NOT grounded or reference its floating
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Ground is a reference selected because it is literally global, the earth outside is selected as a reference point for all voltages. Part of the reason is the earth is basically an infinite electron sink, you can dump excess or pull more electrons as needed. I refer to it in my article in work ESD control in the All About Circuits ESD eBook Article. With this reference, plus and minus power supplies are possible, without it they have to be referenced to some other common point, if one exists at all.

    Ground is a all encompassing reference, used in house wiring for safety, ESD control, and electronics as a common reference. An AC signal that is grounded, as in the 12VAC you mention, now has a hot and a neutral lead, The neutral lead no longer has AC as you think of it, and the hot lead is the powered wire. Without the ground it is a differential signal, 180 degrees out of phase.
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    It's really very simple.

    Differential input means that there are two inputs you could connect to and if you did the output would be the difference between them. you can connect any signal, battery, bias voltage to either or both.

    Floating means that if you did connect a bias battery, say -09 volts, to one input - it would be 9 volts DC below the other input. You could extend this and bias the other input to say, +100 volts, there would then be 109 volts dc between the inputs.

    Single-ended means that one of the two connectors of the input is connected to a ground which you cannot alter. The other is still floating and can be altered as noted above.

    Of course the voltages you connect must be within the working range of the device!!


    An earth or ground is a fixed potential, which you cannot alter.

    See this thread for a full explanation.

    http://forum.allaboutcircuits.com/showthread.php?t=8325&highlight=earthing
     
  8. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    So single ended can not be floating? or can single ended be floating or have floating voltages?

    Isn't this a comparator:
    "Floating means that if you did connect a bias battery, say -09 volts, to one input - it would be 9 volts DC below the other input. You could extend this and bias the other input to say, +100 volts, there would then be 109 volts dc between the inputs"

    Can you guys give me more examples of floating voltages or differences of voltages between 2 inputs or nodes?

    I need more examples on how floating is used?

    Whats the difference between floating VS virtual ground?
     
  9. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    My multi-meter or oscilloscope can not measure floating voltages or virtual ground, how can i measure these then?


    Floating voltages means a voltage differences between the 2 nodes?

    Virtual ground should measure Zero volts

    Where are virtual grounds mostly located or at ? any examples of virtual grounds please?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    They either already has differential inputs (DVM), or has the option for differential inputs (as in the case of the Oscope).

    The presumtion of ground just simplifies the schematic, as only one wire is important, though I have seen a lot of malfuntions from ground was assumed but had broken down (as in an open wire) somewhere.

    A virtual ground is any point that looks and acts like ground, but isn't. An op amp circuit, for example, has a dandy example of virtual ground.

    [​IMG]

    Point A looks exactly like Point B, which makes it a virtual ground, and is referred to as such in text books. In every measurable way it is ground, but there is no direct connection.
     
  11. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    differential inputs (DVM), or has the option for differential inputs (as in the case of the Oscope

    How do you set up your DVM for differential inputs? ( with NO ground) it has to be floating or to measure virtual ground

    How do you set up your oscillscope of differential inputs ( with NO ground) it has to be floating or to measure virtual ground


    When measuring virtual ground with my DVM i put my RED probe on the virtual ground NODE and the Black probe on the circuit ground ? thats going to make my DVM have a ground connection which is not going to measure the virtual ground right because know i have a path to ground

    Same thing with my oscilloscope , internally inside the oscilloscope has a path to ground, so you can't measure floating voltages or virtual grounds
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    A differential input is any input that doesn't have a ground, it is that simple. This describes a DVM nicely.

    Most Oscopes assume a ground, which is not differential, but with dual input Oscopes you can use both inputs in the X1-X2 modes, which becomes a differential input.
     
  13. Xray

    Well-Known Member

    Nov 21, 2004
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    One of the best examples of a floating voltage that I am aware of is the low AC voltage that is used to power the filament of an X-ray tube. In most older Dental X-ray tube heads, the filament of the X-ray tube is powered by approximately 4 Volts AC, and the filament electrically sits at 35 KV DC above ground! (See attached schematic diagram). The combination of the two 35 KV windings of the high voltage transformer provides 70KV across the tube from anode to cathode. So, you can imagine how difficult it is to measure 4 volts AC when it is riding 35 KV above ground!

    And here is a simple way of thinking about differential inputs to an op-amp. There are two input wires, call them PLUS and MINUS. The op-amp only amplifies a signal that enters both input wires with a different polarity or phase between the two wires (hense the name "differential"). If you were to tie both the PLUS and MINUS input wires together and applied a "common mode" signal (common mode means relative to Ground), then the op-amp inputs would be receiving the same amplitude, polarity, and phase voltage, and would ignore the signal (because of its common mode rejection) and there would be no output. Therefore, if some interfering signal, such as AC hum from the power lines, were to enter both the PLUS and MINUS inputs with equal amplitude, polarity, and phasing, the op-amp would ignore it.

    Yes, the differential signal can be considered as a "floating" voltage because it is not referenced to Ground. And yes, a floating voltage can be AC or DC.

    I hope this helps, and doesn't add to the confusion. ;)
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    a typical battery powered DVM is "floating" in the way that you define what is ground by connecting the Common lead to something.
    If the DVM or oscilloscope was mains powered, it is possible it will not be floating, because the Common lead can be connected to mains neutral (through resistor).

    The ground is the point which you define as ground (not earth, thats always Earth). Your mobile phone has floating ground, because it is not connected to anything. If you try to measure some voltage in it, you have to connect the black wire to some point, which becomes your point of reference.
     
  15. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    how do you know if the reference is zero volts tho, and not like 5 or 10volts reference?

    yes, a floating voltage can be AC or DC.

    Can you give me examples of Floating AC and Floating DC?

    Is Floating DC a reference DC voltage above or below ground?

    It seems that a Floating DC is DC OFFSET to me since its above or below ground

    Biasing power amp tubes , the bias voltage is at negative -48 volts so this must be some type of floating voltage , because the AC signal input to the tube has DC offset so the AC signal is referenced to a -48 DC volts not ground so it floating right?

    I seen tube preamp stages and transistor stages where the emitter or cathode bias voltage is above ground is this a floating voltage since the AC input is not referenced to ground since the biasing resistors are changing the baising voltages above or below ground?
     
  16. Xray

    Well-Known Member

    Nov 21, 2004
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    If you think of the term "Floating" as a voltage level that is not measured in reference to Ground, then that should be all you need to know. Imagine the signal or voltage "levitating" at some level above ground, such as the main battery in an airplane. You can not measure the battery voltage by attaching one of the meter leads to the earth and the other lead to the battery at ten thousand feet! You have to take your meter onto the airplane and measure the battery voltage while at ten thousand feet, using the airplane ground system as a reference.
     
  17. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    "Floating" as a voltage level that is not measured in reference to Ground

    So i put my RED probe at VCC power node and the Black probe the the floating node?


    Comparators have a VCC at 12volts, and the (-) inverting input will be above or below ground at 2 volts DC, so the AC or DC signal input going to (+) non-inverted input is at zero DC volts

    To measure the floating voltage, i put my RED probe on VCC 12 volts and the black probe on the inverting input at 2 volts DC , the difference or floating voltage should be 10 volts right?

    If i have a voltage divided of two resistors , the input to the voltage divided is 10volts and the middle of the voltage divider tap is at 5 volts referenced to ground

    If i put my RED probe on the input of the voltage divider and across the R1 then its NOT referenced to ground, I will get a floating voltage because its RED probe is at 10volts and the Black probe is Referenced at 5 volts giving me a difference floating voltage of 5 volts across R1

    Can you guys give any more examples measuring Floating voltages?
     
  18. Xray

    Well-Known Member

    Nov 21, 2004
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    Why do you need more examples? What is it about FLOATING that you still can not grasp?
     
  19. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    any AC signals that is RIDING ontop of a DC offset voltage is "Floating" then right?

    If your measure from the DC offset voltage to the AC peak voltage this is a Floating voltage because its a "difference" between 6volts AC peak to a 2 volt DC offset voltage which is a difference of 4 volts
     
  20. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    What is it about FLOATING that you still can not grasp?

    I don't understand When or where or why would a circuit or designer would want to use Floating or a difference of voltages

    Like a Center Tap transformer , where the TAP is not at ground but is Above or below ground

    example : Floating Center Tap transformer

    Center tap is at 5 volts ( floating)

    Top terminal side of the transformer is at 8 volts
    Bottom terminal side of the transformer is at 2 volts

    8 volts minus 5 volts = 3 volts of a difference
    2 volts minus 5 volts= -3 volts of a difference
     
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