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Re: Freaky Amazing DMM?!

Discussion in 'Electronic Equipment' started by Paul, Jan 15, 2009.

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  1. Paul

    Paul Guest

    I've looked at the specs of ~ 30 DMM's today, include a lot of
    fluke's, and never seen anything near 14Gohms impedance. Keithley has
    an electrometer that's probably higher. Most DMM's are around 10Mohms
    (not gigaohms) input impedance. Don't you think 14 gigaohms is a bit

  2. No, it's a good thing, it's like that by design.
    Yes, normal meters have a 10Mohm resistor on the input. One ones with "HI-Z"
    mode remove this resistor and rely just on the input impedance of the FET
    gate and other circuitry which is there. This value varies a *lot* which is
    why they typically don't specify it, they just call it "high impedance"
    mode. E.g. Fluke do not specify the value on their 87 meter, not even a
    minimum (BTW, hold the Hz button when you power-up to get this mode).

    When you need this mode, the input impedance can never be high enough! e.g.
    when measuring very high impedance circuitry (you can buy Gohm range
    resistors for example). Actually, even "normal impedance" stuff causes a
    problem with a 10Mohm input. e.g. you can start seeing errors creep in
    measuring say >10Kohm stuff.

    The cheap Protek 506 & 608 are other meters that have this (not selectable)
    on the mV range. They spec it at simply >1Gohm.

  3. Paul

    Paul Guest

    I'll check out that Fluke. BTW, one thing of significance with my test
    is that the AM-240 was maintaining the charge on the 4.7uF Mylar! IOW,
    if I disconnected the AM-240, then the Mylar slowly discharge at a
    rate equivalent to 5.25Gohms, but when the AM-240 was connected, then
    the 4.7uF Mylar hardly discharge. Actually, it discharged, but at such
    a slow rate, equivalent to 14Gohms.

    So, what shocked me was that AM-240 help the Mylar retain it's charge.
    Polarity didn't matter, which rules out bias current or voltage
    offset. I guess it's a bootstrapping circuit.

  4. The Agilent U1253A also has it. Spec is ">1Gohm".
    There are quite a few meters I've seen over the years that have it too.
    I think I even saw it on one of those $10 disposable meters too.

  5. I doubt it, just a high impedance CMOS input circuit (along with the usual
    protection stuff).

  6. Paul

    Paul Guest

    I think the Agilent U1253A typically lists for $450. That's a bit more
    expensive then the $40 AM240. Do you have model # for the $10 one?

  7. Paul

    Paul Guest

    How could it nearly stop the Mylar from discharging. When the meter is
    disconnected from the Mylar, then nothing is connected to the Mylar,
    and it discharges at a rate equivalent to 5Gohms. So even if you
    connect a DMM that has infinite impedance, it's not going to make the
    Mylar discharge at a slower rate. Somehow the AM240 is *maintaining*
    the Mylars charge. I'm still thinking about this, lol.

  8. Hardly comparing apples and oranges!
    If you are purely after "the cheapest meter that has a high impedance mV
    range" then that's a different ball game.
    No, sorry. These things come and go like the wind.

  9. Paul

    Paul Guest


    That's true, but once you have the kinks worked out it's not too
    difficult to measure sources with megaohm impedance. As you know, you
    have to use low bias current meters or circuits, like an electrometer
    with femto bias current. If say the meter has 10pA, and the DUT
    impedance is 100Mohms, then that's 10pA * 100Mohms = 1mV caused just
    from the meter itself. If the electrometer is 10fA, then it drops to
    1uV. Then you have thermoelectric effects unless you use a balanced
    circuit such as an instrumentation op-amp. After a while of working
    out the kinks, you can get very stable predictable measurements on
    high impedance sources.

    You probably know all of that. It can take some time to work out the
    kinks, but such low signal high impedance sources is usually no big
    deal for a EE who's spent a lot of years working in such a field.

  10. Paul

    Paul Guest

    I have a few $3 DMM's from Harbor freight, but they're 2Mohm impedance
    in the 200mV setting.

  11. Paul

    Paul Guest

    Another test. I just connected the AM240 (while in 400.0mV mode) to my
    Keithley. It measured no bias current. The Keithley resolution is
    10pA. So that's cool. It has 14Gohm impedance, and who knows how much
    bias current, definitely less than 10pA.

  12. James Arthur

    James Arthur Guest

    You're underestimating the insulating power of mylar.

    Try soaking the capacitor good and full, then letting it sit,
    leads in the air, without the meter. Then, after a good
    long time, measure.

    Then you'll see if the meter's been charging the cap or
    not. Probably not.

    James Arthur

  13. A capacitor with flying leads can grab loose ions out of the air, and
    will hold them. It takes a while, but it can register.

    ALL larger sized HV caps usually are shipped, stored, and handled with
    shorted leads.
  14. Whenever I read very small voltages I make sure that my meter leads are

    Twisted pairs on a source and return has amazing noise abatement
  15. Max65

    Max65 Guest

    I know the issue you are talking about. HV capacitors charge
    themselves when their leads are left free, but I'm not sure you can
    state for sure that they grab the "free ions" of the air, since many
    of them are not polarized capacitors, so you should answer this
    question: why +ions should attach to one lead and the -ions to the
    Let me know your point of view about it.
    Have a nice day.
  16. Paul

    Paul Guest

    If the cap is left out in strong electromagnetic fields such as caused
    by local radio stations or wifi etc. then it's possible for the cap to
    slowly charge. These are issues a researcher learns to overcome. I use
    two layers of metal shielding (large, and small) where one is non-
    magnetic (for high frequency shielding) and the other is magnetic (for
    low frequency shielding). Almost all of the time the Mylar cap will
    *not* charge even when left outside by itself, and I live smak dead in
    Los Angeles, California. :) Although it's common to see
    electrolytic charge by themselves when left out. Try placing the
    electrolytic inside a heavy shielded area (faraday cage) and one may
    see a different result. Although it's possible for electrolytics to
    charge even inside a shielded area for obvious reasons.

  17. krw

    krw Guest

  18. GregS

    GregS Guest

    In a radio hobby magazine of years back, there someone gave details how to make
    a gravity detector. More of a gravity detector of other worlds, planets, etc.
    I gave it a try, seems like it was some kind of capacitor integrator
    circuit. So I built it and hooked it up to a stripchart. What
    I got totally surprised me. I got symetrical waveforms of periods
    of minutes or hours. Maybe the damm thing worked.
    The theory of the article suggested when objects lined up, they produced
    varying fields at the detection point. I went no further, but should have measured
    a full day.

  19. GregS

    GregS Guest

    Gee I think I found it.....

  20. They wouldn't "attach", however, if a free ion hits a lead, it will
    become stored on that node's plate. Same is true if a person charged
    with an electrostatic field walks by. Generally both leads will see
    equal pressure, but if one 'sees' the field and the other doesn't,
    electrons can be added to that plate.

    The main cause of capacitor charging up without stimulus is the memory
    effect that the dielectric media has. One can discharge a freshly
    charged cap to 'zero' volts. Check again in a few minutes and it will
    again show some charged voltage.
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