Measuring Large resistances
in [Design]
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From: Robert Baer on 13 Nov 2009 11:10 Spehro Pefhany wrote: > On Thu, 12 Nov 2009 20:26:57 -0800, Robert Baer > <robertbaer(a)localnet.com> wrote: > >> Kreyen wrote: >>> On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" <pimpom(a)invalid.com> >>> wrote: >>> >>>> Kreyen wrote: >>>>> 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. >>>>> >>>> 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? >>>> >>> 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 >> *That* is a color of a different horse..the DVM used would have to >> have that accuracy, leaves out handhelds! > > Maybe *your* handhelds. > OK, i will bite; what brand(s) and model number(s)?
From: Spehro Pefhany on 13 Nov 2009 11:18 On Thu, 12 Nov 2009 23:01:30 -0600, Jim Yanik <jyanik(a)abuse.gov> wrote: >Fred Bartoli <" "> wrote in news:4afc7e1b$0$21967$426a74cc(a)news.free.fr: > >> Spehro Pefhany a �crit : > >>> Most inexpensive DMMs on the lower ranges have very high input >>> impedance so the LMC buffer might not be necessary. > >Nonsense;they have the same impedance on their lowest range as the >others,unless they disconnect the input divider.They don't. 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. >(I did that on a TEK DM501 to make cal measurements for a 577/177 curve >tracer.) > >also,I checked a Harbor Freight DMM and it was only 1 Megohm input R. >(and the first one was way out of calibration,read a 1.5v alkaline cell at >1.9v) 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. >> 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... >> >> > >seems like they could just use carbon film deposition on glass substrates >and laser-trim them,all machine handled. 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
From: Spehro Pefhany on 13 Nov 2009 11:27 On Fri, 13 Nov 2009 08:10:46 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: >Spehro Pefhany wrote: >> On Thu, 12 Nov 2009 20:26:57 -0800, Robert Baer >> <robertbaer(a)localnet.com> wrote: >> >>> Kreyen wrote: >>>> On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" <pimpom(a)invalid.com> >>>> wrote: >>>> >>>>> Kreyen wrote: >>>>>> 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. >>>>>> >>>>> 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? >>>>> >>>> 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 >>> *That* is a color of a different horse..the DVM used would have to >>> have that accuracy, leaves out handhelds! >> >> Maybe *your* handhelds. >> > OK, i will bite; what brand(s) and model number(s)? 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.
From: Kreyen on 13 Nov 2009 14:40 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
From: Robert Baer on 13 Nov 2009 21:37
Spehro Pefhany wrote: > On Fri, 13 Nov 2009 08:10:46 -0800, Robert Baer > <robertbaer(a)localnet.com> wrote: > >> Spehro Pefhany wrote: >>> On Thu, 12 Nov 2009 20:26:57 -0800, Robert Baer >>> <robertbaer(a)localnet.com> wrote: >>> >>>> Kreyen wrote: >>>>> On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" <pimpom(a)invalid.com> >>>>> wrote: >>>>> >>>>>> Kreyen wrote: >>>>>>> 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. >>>>>>> >>>>>> 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? >>>>>> >>>>> 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 >>>> *That* is a color of a different horse..the DVM used would have to >>>> have that accuracy, leaves out handhelds! >>> Maybe *your* handhelds. >>> >> OK, i will bite; what brand(s) and model number(s)? > > > 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. > > Thanks. |