Trivial DMM question...

Discussion in 'General Electronics Chat' started by EEMajor, Oct 19, 2006.

  1. EEMajor

    Thread Starter Well-Known Member

    Aug 9, 2006
    67
    4
    Hey everyone,
    I have a trivial, "doesn't really matter" question about DMM's. I have always had some really cheap DMM's, the most expensive one I owned was about $40.00

    I just purchased a new Fluke DMM, model 189. It has a much higher precision then any other I have owned, and I have noticed that it will have readings that "flutter" in the microvolt range when no voltage source is hooked to the leads.

    What causes this?? :confused:
    Just curious.

    Thanks!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    The apparently random readings that occur with the leads open result from small charges induced on the input from stray electric fields. They are "visible" to the meter due to its high input resistance, which allows the tiny currents in the leads to develop a significant voltage across the input resistance.

    That is a bit of a problem with an input resistance of on the order of 10 X 10^6. Imagine the fun when working with input resistances in the range 10^9 to 10^15.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    As beenthere says it's very sensitive. About the difference between a Ford with no power steering and an Audi with power steering. :p
     
  4. EEMajor

    Thread Starter Well-Known Member

    Aug 9, 2006
    67
    4
    Alright, thanks for the info guys! I think I'm hooked on Fluke, good stuff.
     
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