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Electronics term : Float ?

  1. Dec 10, 2007 #1
    I am reading the manuals of an electronics measuring instrument, in which the term 'float' is repeated several times. Can anyone explain this term?

    Thanks in advance.
     
  2. jcsd
  3. Dec 10, 2007 #2

    berkeman

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    Floating usually means not connected electrically. So you can have floating inputs to CMOS gates where the pins have not been connected to any other part of the circuit (a bad thing in general), or you can have a circuit whose logic ground is floating with respect to earth ground.
     
  4. Dec 10, 2007 #3

    f95toli

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    If it is a measurement instrument a "floating" terminal is a terminal NOT connected to safety ground (i.e. earth).
    In many instruments (e.g. most oscilloscopes and ALL instruments with BNC connectors) the ground terminals are connected to earth (this is due to safety regulations, there is no real "physical" reason).
     
  5. Dec 10, 2007 #4
    Not necessarily. I can't recall for sure where, but I have seen equipment with BNC connectors that are isolated from conduit ground.
     
  6. Dec 12, 2007 #5
    Generally floating also refers to a signal that is not with reference to the same ground that is creating the signal. To make it clear let me illustrate, in a opamp the output is floating as it is not with reference to the input Vin, but is in reference to the V+ and V- of the supply terminals of the opamp.

    This also prevents loading the source (Vin) but that's another story.
     
  7. Dec 12, 2007 #6

    f95toli

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    Maybe it was a "custom made" instrument? Or, it was not BNC but triax (or twinax, or another "BNC-like" connector)?
    It would is illegal in most (all?) countries to sell a wall-powered instruments with floating BNC connectors. There is no real "electrical" reason for this (except safety), but the rules say that all accessible metallic parts of an instrument (including the enclosure, all external screws etc) must be connected to earth.
    Differential oscilloscopes generally use TWO BNC connectors for each channel.
     
  8. Dec 12, 2007 #7

    berkeman

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    The exception to this rule is if the instrument uses "double insulated" construction. Then the external metal pieces do not need to be connected to Earth ground. For example, when a portable oscilloscope is powered with its wall transformer adapter, the metal shells of the coax connectors for the scope probes are floating with respect to Earth ground. With some care, the "ground" clips of portable scope probes can be connected to either hot or neutral in AC Mains measurements.
     
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