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AADE L/C Meter drift..

Discussion in 'Electronic Design' started by Robert Baer, Jan 28, 2013.

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  1. Robert Baer

    Robert Baer Guest

    Insert battery, decide to test and find this one works up to at least
    2.5uF.
    Next day,

    Power up
    Wait for "ready measure u"
    Select Cx
    Press Zero (reads 0.0pF)
    Let go and it reads -0.06pF (???)
    Let it sit, see it drifts upwards;by day end reads 2.82pF.
    Power down and re-try..now after pressing zero,it typically reads 2.58pF.
    Press Zero (reads 0.0pF), Let go and it reads -0.06pF (???)
    Drifts up as before.

    WTF???
     
  2. Owen Roberts

    Owen Roberts Guest

    Mine drifts a bit more then 3 pf in a hour. Still best bang for the
    buck I have found..
    The repeatability is decent, in the short term, and that is what I
    need.

    Steve
     
  3. If I had sold it to you, I'd apologize profusely, refund your money
    immediately, and send you a prepaid shipping box with which to return
    it. Then I'd sign you up for my competition's mailing list. :-|

    -- john
     
  4. Robert Baer

    Robert Baer Guest

    Usually it takes 2 zeroing attempts to get zero.
    The drift, while apparently seen by others, is irksome and begs for
    explanation as to why it exists.
    To paraphrase the saying, just because 20 million Frenchmen see this
    problem, does not make it right.
    Hell, even the first Fairchild uA709 op amps did not drift like that..
     
  5. Apples and oranges. Do you realize how tiny 0.06pF is?

    It's the capacitance of a fraction of a millimeter of
    twisted pair wire. It's the capacitance of less than
    one millimeter of (narrow!) PCB trace over a ground plane.
    It's the capacitance between the open contacts of a small
    low-capacitance reed relay. It's less than one tenth of
    the gate capacitance of the smallest, lowest capacitance
    discrete switching MOSFET I know of (SD214).

    It's... well, I suppose you get the picture by now.

    Jeroen Belleman
     
  6. Fred Bartoli

    Fred Bartoli Guest

    Jeroen Belleman a écrit :
    Yeah. You don't know enough MOSFETs :)
     
  7. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    very well said.

    there is another prefix for so small values, 0.06 pF is 60 fF,
    Femtofarad. The next one is aF or Attofarad. Both are so small that only
    capacitances within an IC require these units.

    Bye
     
  8. Please, enlighten me!

    Jeroen Belleman
     
  9. Robert Baer

    Robert Baer Guest

    Check, spacing of red/input terminal is extremely close to a switch
    (for starters).
    Concerning range switching, the MCU does that by changing
    frequency..which seems to put a hole in that analysis.
     
  10. Fred Bartoli

    Fred Bartoli Guest

    Jeroen Belleman a écrit :
    Sorry, I was just joking...
     
  11. Robert Baer

    Robert Baer Guest

    Put a scope and Eput meter on it, found it is just an oscillator at
    least similar to -a- schematic; range switching is/in software for display.

    Q: Obvious why frequency changes WRT capacitor, but why does the
    amplitude run from around 500mV pk-pk (if i remember right) at lo
    capacitance to around 8mV pk-pk (if i remember right)?
    "schematic" indicates i was looking at the top of the parallel LC..
     
  12. Robert Baer

    Robert Baer Guest

    Check.
     
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