FLUKE 117 change its input impedance!!!! - Page 1
Forum section: electronics / testgear

author: 001
author_rank: Super Contributor
timestamp: November 22, 2018, 03:14:35 pm
content: HiI experimented with my Fluke 117and input impedance = 10MOhm is for 60V range onlyIt is 9.9Mohm at 600V and 600mV ranges and 11.05Mohm at 6V range

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: November 22, 2018, 03:51:03 pm
content: Where in the manual does it specify the input impedance, what does it say? How are you measuring it?And why do you care?

author: Monkeh
author_rank: Super Contributor
timestamp: November 22, 2018, 04:01:08 pm
content: It is quite normal for handheld multimeters like this to change input impedance with range.

author: 001
author_rank: Super Contributor
timestamp: November 22, 2018, 04:26:23 pm
content: Quote from: In Vacuo Veritas on November 22, 2018, 03:51:03 pmI simply plug it to ohmmeterQuote from: In Vacuo Veritas on November 22, 2018, 03:51:03 pmBecouse it fail measurments at tube grid in the autorange mode

author: Monkeh
author_rank: Super Contributor
timestamp: November 22, 2018, 04:33:55 pm
content: So don't autorange, then.

author: Shock
author_rank: Super Contributor
timestamp: November 22, 2018, 06:14:34 pm
content: My Fluke 87V and 117 show similar measurements to yours when they measure each other. So what you see is probably normal (or at least common to others) unless there have been revision changes. What I gather from what David Hess said (second link) it could be a combination of the two meters involved which show it as 11M, VK2TIL confirmed similar behavior (third link), so take that into consideration as well. As suggested by KC4UMO (third link) you can use the voltage and resistor method for discovering a meters input impedance in auto range, various ranges and at different voltages if you wanted.https://www.eevblog.com/forum/metrology/input-impedance-in-fluke-autoranging-meters/https://www.eevblog.com/forum/testgear/multimeter-input-impedance-riddle/https://forums.qrz.com/index.php?threads/input-impedance-of-fluke-115.133278/https://www.electronicspoint.com/threads/what-is-digital-multimeter-impedance.29116/This link was kind of sad readinghttp://brianhoskins.uk/agilent-u1253b-input-impedance-problem/Would be nice to have something definitive on it.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 22, 2018, 10:52:13 pm
content: 001, I can also confirm that several meters have varying input impedance within +/-10%, although nothing as serious as the article linked by @Shock.Quote from: Shock on November 22, 2018, 06:14:34 pmThanks for the link; this, added to the battery and OLED issues that other users reported here makes me believe the U1253 was not the best design.I checked the manuals for my U1273A and U1282A and they don't have the remark. I will test them later today - fingers crossed they do not present such idiosyncrasy.

author: HKJ
author_rank: Super Contributor
timestamp: November 23, 2018, 09:30:39 am
content: Many meters use a input voltage selector like this:Position 1 is for mV and will have high input impedance. Meters with separate mV selection on the range switch may switch another 10Mohm resistor across the input in this range, especially in mVAC

author: 001
author_rank: Super Contributor
timestamp: November 23, 2018, 03:21:10 pm
content: Quote from: HKJ on November 23, 2018, 09:30:39 amWhy? Different impedance = different readings isnt it?

author: Fungus
author_rank: Super Contributor
timestamp: November 23, 2018, 03:44:34 pm
content: Quote from: 001 on November 23, 2018, 03:21:10 pmThat's what calibration is for.

author: HKJ
author_rank: Super Contributor
timestamp: November 23, 2018, 03:55:24 pm
content: Quote from: 001 on November 23, 2018, 03:21:10 pmAs long as the source is fairly low impedance (That is below 100kOhm), the difference due to impedance is minor.This type of input selector works with an electronic switch and the resistors can be reused for the resistance ranges.

author: bd139
author_rank: Super Contributor
timestamp: November 23, 2018, 04:05:12 pm
content: This is actually a pretty big irritation of mine. I occasionally use DMM internal impedance as part of the divider on something else so for example HV and RF probes and the like. This buggers the whole thing up completely.It got bad enough I actually designed and built my own shonky analogue meter with constant 10M input impedance! (CA3140 front end with divider chain up front driving 100uA meter as a current source.)

author: Fungus
author_rank: Super Contributor
timestamp: November 23, 2018, 05:26:57 pm
content: Quote from: bd139 on November 23, 2018, 04:05:12 pmIf you get a proper "EE" multimeter it will have a very high impedance voltagemode somewhere to avoid this (usually GigaOhms). You can then pit it in parallel with your voltage divider.

author: HKJ
author_rank: Super Contributor
timestamp: November 23, 2018, 05:30:54 pm
content: Quote from: Fungus on November 23, 2018, 05:26:57 pmAs I (more or less) wrote above, it is common in mVDC range, also in cheap meters.If you want it up to around 10V you need a bench meter of the better ones.

author: Fungus
author_rank: Super Contributor
timestamp: November 23, 2018, 05:35:43 pm
content: Quote from: HKJ on November 23, 2018, 05:30:54 pmIf you're building the voltage divider yourself you can choose any voltage range you want.(...and if you're not? Get some resistors and hack the divider!)

author: rsjsouza
author_rank: Super Contributor
timestamp: November 23, 2018, 05:48:57 pm
content: It is certainly something tied to this particular model, as my other Keysights are what I would expect.

author: bd139
author_rank: Super Contributor
timestamp: November 23, 2018, 06:06:55 pm
content: Quote from: Fungus on November 23, 2018, 05:26:57 pmThey usually have some interesting limits there. For example 34401A clamps at +/-17V with Gohm impedance switched in.This is fine if your probe assumes say 2V FSD over 10M however.

author: alm
author_rank: Super Contributor
timestamp: November 23, 2018, 07:17:58 pm
content: Yes, when relying on a constant input impedance it is good to set the meter to manual ranging. Both in the case of meters that switch from > 10 GOhm to 10 MOhm and meters that switch between 10 MOhm and 11 MOhm. Occasionally, the change in reading caused by the change in impedance breaks auto-ranging: the meter will range up, then the decrease in input impedance decreases the measured voltage, so it will range down again, etc.I ran into this issue recently when trying to use the 10 MOhm impedance as a current shunt for nA currents. Had to force the Fluke handheld to mV range to get a 10 MOhm input impedance. Funny thing: The Fluke 189 has both a 5 V and a 5000 mV range, which you'd think is only a display difference. But in the 5 V range, it has a resistance of 11 MOhm, while in the 5000 mV range, it has an input resistance of 10 MOhm (in parallel with what looks like a cap that has to charge).

author: 001
author_rank: Super Contributor
timestamp: November 24, 2018, 06:50:57 am
content: Quote from: alm on November 23, 2018, 07:17:58 pmlooks not good

author: alm
author_rank: Super Contributor
timestamp: November 24, 2018, 10:05:29 am
content: Eh, this is the first time I noticed in years of using this meter. Unless you are measuring values from a high-impedance source (a current source and a high-voltage probe with a 999 MOhm resistor in series are both examples of that), you're not going to notice the difference between 10 MOhm and 11 MOhm. You're going to be hard-pressed to see the difference between an input impedance that is 10,000 times higher 11,000 higher than the source impedance.

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2018, 04:23:52 pm
content: Quote from: Monkeh on November 22, 2018, 04:33:55 pmThat will not always work. On some autoranging meters, manual ranging only affects the display and the meter still changes ranges on the attenuator as needed no matter what the display indicates.Quote from: Fungus on November 23, 2018, 03:44:34 pmCalibration assumes zero source resistance. Without knowing the input resistance in different ranges, there is no way to calibrate out the error produced when the input resistance of the meter changes and the source resistance is not zero. The best you can hope for is that the source resistance is low enough to make the additional error insignificant.Quote from: alm on November 24, 2018, 10:05:29 amFor safety, high voltage probes include a shunt resistance which helps by lowering the output resistance however even with that, Fluke has a warning in their high voltage probe manuals about the error contributed by autoranging meters which have variable input resistance. It is too bad Fluke does not show the same diligence in their multimeter documentation because it is a real problem. A couple years ago I wasted hours tracking this problem down; I finally had to use one of my bench meters to get consistent and accurate results.Quote from: bd139 on November 23, 2018, 04:05:12 pmThe thing which really pisses me off is that it is not consistently documented. At least Fluke should have the courtesy to include the input resistance versus range in the specifications but that would just make their products look bad which they are and this only makes them worse. They include a warning in the documentation for their high voltage probes, at least so far, but no specific values for their multimeters other than "about 10 megohms" or "> 10 megohms" or whatever marketing term is favored now.QuoteManual ranging meters are usually of the other design where the input divider has a constant input resistance. Some autoranging meters like from Amprobe and Uni-Trend are specified to have a constant 10 megohm input resistance but do they really? Uni-Trend says all of their meters do (1) so I suspect they are lying but Amprobe actually says some of their autoranging meters do and some do not which is promising. Apparently all of B&K's autoranging meters have variable input resistance and all of their manual meters are 10 megohms.(1) At least all of the ones I checked. I limited myself to multimeters which use the superior 3/4" banana jack spacing so that standard dual banana jacks can be used. Multimeters which align all four jacks in a row make baby Jesus cry.