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Measuring Large resistances

Discussion in 'Electronic Design' started by Kreyen, Nov 12, 2009.

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  1. Kreyen

    Kreyen Guest

    I am having problems trying to measure resistance values above
    100MegaOhms. My Laboratory Ohmeter gives unstable values.

    Is ther anyway apart from the usual dc bridges of getting accurate
    resistance measurements.

    Thanks guys.

    Kreyen
     
  2. pimpom

    pimpom Guest

    Are you trying to measure a resistor or something else? What
    range variations are you getting? Did you consider surface
    leakage and other interfering factors? What level of accuracy do
    you need?
     
  3. Kreyen

    Kreyen Guest

    I'm trying to measure 3 resistances to be used for calibration
    purposes.. I'm getting variations of several percent while I'm
    looking into a t least a 0.1 percent accuracy level.

    Kreyen
     
  4. Kreyen

    Kreyen Guest

    Well we got this expensive meter at the Lab at work so I can't see how
    accurate its current abilities are ... as I'm now at home sweet home .
    But I'll check tomorrow and let you know the values. It is a HP and a
    very professional looking device though.

    Kreyen
     
  5. Shaun

    Shaun Guest

    On resistors in those high range, even finger prints on the resistor can
    affect accuracy.
    Solution: Clean them with isopropyl alcohol and let them dry, then try
    measuring them again.

    Some meters have a nanoSiemens range which is 1/Resistance, they usually
    give accurate readings. Other that that a megohmeter that uses high voltage
    to measure resistance is usually used.

    Shaun
     
  6. pimpom

    pimpom Guest

    I was about to suggest something similar to Jan's but decided to
    ask for more details. For that level of accuracy, a bridge would
    be your best bet. With the jury-rigged method, it goes without
    saying that the supply would have to be stable and accurate to
    better than 0.1%, with a similar requirement for the measuring
    instrument.

    Perhaps you could try your lab ohmmeter again, making sure that
    everything is squeaky clean and perfectly dry. In still air.
    Varying thermal gradients might also have affected the readings.
     
  7. 100M isn't that high, well, depending on what kind of accuracy you're
    looking for.

    How much instability are you seeing? How much above 100M?

    LMC6042 has 2fA (typical) input bias current and costs a couple
    dollars one-off.

    A 100M 1% resistor with 50ppm/K tempco runs around five dollars.
    A 1G 1% resistor with similar tempco is maybe double that.

    Most inexpensive DMMs on the lower ranges have very high input
    impedance so the LMC buffer might not be necessary.
     
  8. Fred Bartoli

    Fred Bartoli Guest

    Spehro Pefhany a écrit :
    High value resistors are (depends on the model) somewhat unstable.
    I currently have a batch of Dale 1G/1%.
    They measure fine with 100V bias which is the datasheet measuring
    conditions. When measuring them at low voltage, they're all over the
    place, from +3ish% to +7ish%. Yep, not even grouped...
    On the contrary I've some Caddok rated <0.02ppm/V!

    A friend of mine worked at, IIRC, Vishay/sfernice on that specific high
    value resistors 'feature' and went auditing some of the 'production'
    lines (the quotes are his). He said the voltage dependency was mostly a
    prod issue. High value Rs is a very small niche market and as such,
    production inherited some highest tech tools, like hand lapping and the
    likes. Being labor intensive it was relocated in low labor cost
    countries, with poor buildings (he said, almost backyard :) and less
    than ideal handling cleanliness.

    That could explain a lot some of the strange behaviors...
     
  9. Shaun

    Shaun Guest


    I just measured some Megaohm resistors with my Fluke 867b. accuracy 0.5%

    I measure two - 66 Megaohm resistors at 20% tolerance (not very accurate)

    measured 1 - 65.839 Megaohms
    measured 2 - 69.742 Megaohms

    Also I measured one 200 Megaohm resistor that came out of some medical
    equipment, unknown tolerance

    measured 196.46 Megaohms

    then readings were stable +/- .1 count in nanoSeimens and repeatable.

    You maybe working in an electrically noisy environment, try moving you meter
    to a different location where there is no extra cables around, maybe a noise
    filter would help too.

    Shaun
     
  10. Maybe *your* handhelds.
     
  11. Hmm.. I think you're right on that point. I do have one that does, but
    probably most do not. Benchtop instruments like my Agilent 34401A
    meters do have an "infinite impedance" setting.
    1M? What a POS. What do you expect for dollar store prices. Even the
    Chinese can't make a worthwhile meter for what Harbor Fright wants to
    pay.
    Carbon? AFAIK something like Ruthenium oxide is more common in chip
    resistors. Ohmcraft makes their using a kind of printing method which
    is automated, so it should be consistent. Here is a white paper on
    their process:

    http://www.ohmcraft.com/Datasheets/03_White_papers/WP_HighValuePrecisionChipResistors.pdf
     

  12. Fluke 87-5, for example. DC voltage accuracy is 0.05% + 1 count on the
    6V range.

    But only 1% in the 60nS range (infinity to 16.667M ohm),
    unfortunately. A 100M resistor would have a conductivity of 10.00nS so
    the resolution is pretty much there, but not the accuracy.

    Probably lots of other 4-1/2 digit handheld meters too.
     
  13. Kreyen

    Kreyen Guest

    Hi guys
    Thanks for all the enlightening on the subject.
    The model of the meter I use at the Lab is the Agilent 34401A which
    Spehro incidently mentions has an infinite resistence setting. I
    couldn't find that ... which means I'll have to browse through the
    thick manual. It doesn't seem to deliver more then then a 10V output
    though.
    I suspect it might be measuring the limitations in its accuracy
    readings or the thermal noise in the resistors.

    Kreyen
     
  14. JosephKK

    JosephKK Guest

    Oh drear, it has a 10M ohm input when measuring voltage; measuring
    resistance is entirely different.

    The measurement problems may be exactly that, measurement method. For
    100 Mohm and up i suggest putting the DUT in a closed conductive box
    with holes for the test leads. And use minimum length test leads, not
    touching anything if possible.
     
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