Please explain uA (micro) on DMMs to me - Page 1
Forum section: electronics / testgear

author: Timur Born
author_rank: Regular Contributor
timestamp: September 20, 2015, 09:48:25 pm
content: Hello everyone!I don't know the specific inner workings of DMMs, and thus would like to get an explanation of how to properly use and interpret the uA (micro) readings compared to mA. It seems that there are some possible obstacles to better know about, which I assume to be based on how uA measurements are implemented internally?!Please someone explain the following observations to me:Fluke 289 measuring a low level noise signal, fundamental frequency of this specific reading around 130 Hz (I did others in the kHz range), various harmonics present. All measurements taken from the same (!) source.uA AC(5000 uA range)Max 674.9 uAAve 50.1 uAMin 233.3 uAmA AC(50 mA range)Max 34.22 mAAve 2.43 mAMin 12.86 mAuA DC(5000 uA range)Max 676 uAAve 587 uAMin 225 uAmA DC(50 mA range)Max 34.60 mAAve 30.59 mAMin 12.95 mAAs you can see there is ahugediscrepancy between the mA and uA numbers! Shouldn't the uA dial position show OL for max and min? Instead it shows some arbitrarily different numbers?!It seems like the 289's AC measurements cannot deal too well with the noisy signal, as the AC average looks messed up (and frequency readings are either zero or too high). But it can follow the low fundamental frequency changes in DC quite well (even measure the frequency). Still the numbers between the mA and uA dial positions differ to such extremes that I wonder which of these numbers I can trust at all?PS: Some weeks ago I measured about the same source (likely slightly different load) using a Voltcraft VC890 and it measured around 2 mA on the mA dial position and 30 uA on the uA position. So the basic issue was the very same.

author: Matje
author_rank: Regular Contributor
timestamp: September 20, 2015, 10:22:19 pm
content: Quote from: Timur Born on September 20, 2015, 09:48:25 pmYou are right about that.Quote from: Timur Born on September 20, 2015, 09:48:25 pmCurrent measurement works by introducing some resistance into the circuit, then measuring the voltage over this resistance. The actual resistance used will vary by current range, maybe between 0.1 Ohms (amps range) to some 100s of Ohms (microamps range). So by changing the current range you change the resistance, thus the circuit being measured, thus the measurement. Also see "burden voltage".

author: pelule
author_rank: Frequent Contributor
timestamp: September 20, 2015, 10:46:17 pm
content: Matje is right with his explanation, but may have left open questions.In general a DMM uses an ADC (analog to digital converter), which be default is able to measure only voltages, in fact comparing an unknown voltage to a know voltage (the reference) and displaying the ratio. The best accuracy you get as closer the unknown voltage is to the known voltage, let say, reference volatge is 10V. As smaller the unknow voltage is, as more errors you get (noise etc).To measure a current, you need to convert it into a voltage, mostly done by a shunt resistor (the basic law of Ohm: Voltage = Current X Resistance).This voltage is the "burden" Matje mentioned. As smaller the current, as bigger you have to choose the shunt resistor or need a volatge amplifier to get the unknow voltage close to the know voltage (thats, what Dave does with his uCurrent project)Have a look to Daves µCurrent video, this explains it very well and easy to understand.Also you may want to read the µCurrent description on Daves µCurrent product page.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 21, 2015, 05:36:02 am
content: More videos regarding burden voltage.One from Alan.One from Martin.

author: EEVblog
author_rank: Unknown
timestamp: September 21, 2015, 05:45:44 am
content: I published an article on it:http://alternatezone.com/electronics/ucurrent/uCurrentArticle.pdf

author: VK5RC
author_rank: Unknown
timestamp: September 21, 2015, 06:04:20 am
content: Are the AC readings correct? The 'Ave' is less than the minimum value in both sets! My brain hurts!

author: Electro Fan
author_rank: Super Contributor
timestamp: September 21, 2015, 06:57:33 am
content: A Fluke article on Burden Voltage using the 87V for some examples:http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/burdenvoltage

author: hayatepilot
author_rank: Frequent Contributor
timestamp: September 21, 2015, 07:10:34 am
content: I think to measure a "low level noise signal" one would be better off with using a oscilloscope + a uCurrent gold.This way one can see the waveform and make better calculations.Quote from: VK5RC on September 21, 2015, 06:04:20 amThe AC Min values are are probably the actual minimum values, meaning you have a signal going between 233 and 675 uA with a average AC component of 50.1uA .At least that is what i'm reading out of those measurements.@Timur BornWhat is this source you are measuring?Greetings

author: Timur Born
author_rank: Regular Contributor
timestamp: September 21, 2015, 10:50:41 am
content: First of all: thanks to all the replies and even video links.I did read about burden voltage some weeks ago when I started looking for a multimeter that might do what I need. Turns out I even stumbled across the uCurrent pages then. I took note of what it generally means, but didn't fully dig my head into it, because I don't think it affects my usage case.The thing is: I am currently not driving a circuit while I am measuring, I am measuring ground lines of a PC vs. mains earth under different PC load conditions. Even when a circuit is inserted, it is an just amplifier + speaker (+ cables, either including or excluding USB/FW devices in between). So any kind of DMM induced voltage drop doesn't really concern me from the destination circuits' point of view. The amplifier + speaker circuit will always work as intended, just at different volumes (down to its own noise floor).Right now I don't care so much for my measurements, but I am rather trying to bring expectations and understanding of a DMM together. Why do I get something grossly different like 587 uA (average DC) in one dial position (uA), but 30590 uA in another dial position (mA)? Is any of these two results correct? If so, how do I know which one?Or does the uA result just show the least significant numbers of the mA result (30,590 vs. 00,587 seems to imply this)? I would expect the DMM to show "OL" when the current is so much larger than the selected range!?Interestingly Martin's video demonstrates my culprit. At 17:40 he switches the dial position to uA while sending 200 mA (= 200,000 uA) to the Fluke. At 17:52 he lowers the current to 20 mA (= 20,000 uA). The 87V's maximum range is 6000 uA in the uA dial position, so I would expect the screen to go "OL". Instead it's just showing some arbitrary numbers in Martin's video.Which leads me back to my original question: "How does uA measurement on DMMs work?" or maybe rather "What results to expect on screen?"I find this even more problematic with the AC measurements. As you can see the average is considerably below the max and min peaks. Even worse, the average AC mA value is *within* the maximum range of the uA position, while the peaks are outside that range. And without peak measurements I wouldn't have any clue about that. That being said, I trust the peak measurements too much, because of several observations (especially differences between mV and V dial positions even when numbers are *within* range of both).Thanks again to everyone!

author: Timur Born
author_rank: Regular Contributor
timestamp: September 21, 2015, 11:20:00 am
content: Quote from: hayatepilot on September 21, 2015, 07:10:34 amI thought about getting an oscilloscope, but that would be kind of a one trick pony for me, just serving me for this single usage case. I wouldn't use a DMM that much more often, but at least I can think of several other usage cases, including out-of-house ones.And for anything involving "see the waveform" I am likely better off using a (professional grade) audio interface + software. These interfaces sport over 100 dB dynamic range, low THD(+N), high sample-rates and software suited to the audible range that I am interested in.Still a DMM would be generally useful and thus I'm checking how much of use it is for my special usage case on top of that. But apart from the latter I am trying to get a grip of how to interpret different results at different dial positions.QuoteI'd like to know more about this, too. Do AC numbers on a DMM only include those components that cross zero? How is DC offset handled? Should AC+DC be a sum of both components (it is not here, rather the numbers match the DC ones with my usage case)?The 289 at least offers to measure frequency of DC voltage (and even gives me some meaningful numbers), the 177 offers frequency only on its AC position. Unfortunately AC frequency measurements are completely useless for my usage case on the 289, whereas the VC890 did return some meaningful numbers when current was used as basis for frequency measurements.QuoteGround lines of computers under different load vs. mains earth. The ground lines noise increases with certain load conditions, especially when high performance GPUs are heavily loaded. This noise travels over any kind of shielded cable (USB, FW, XLR). Then the noise is turned into audible noise in amplified speaker systems (Yamaha HS80M) that are powered via three-prong power-plugs (aka mains earth connected).But even without any speakers connected you get audible noise in the same frequency range from the power sections of graphic-cards (and mainboards). I don't know (yet) if the latter originates from coils or capacitors.

author: deadlylover
author_rank: Frequent Contributor
timestamp: September 21, 2015, 01:08:13 pm
content: Just to echo the other posters, it is burden voltage that you're dealing with.For the Fluke 289 here are the burden voltages for a few ranges taken from the manual;5000uA range = 102uV/uA (102 ohms)50mA range = 1.8mV/mA (1.8 ohms)5A range = 0.04V/A (0.04 ohms)Always keep in mind that when we measure something with our test equipment, we're changing the circuit being measured. In this case, we're quite literally shoving a resistor in series with the circuit.Ideally, we want our instruments to be as transparent as possible (to borrow lingo from the audio world =P), so it would make sense to use the 5A range all the time because it has the lowest burden voltage. But when measuring low currents we will simply run out of resolution, necessitating a jump to a lower range and the troubles that come with it.Unless you're trying to take accurate current measurements, burden voltage is usually not that big a deal except when dealing with low level voltage sources....like what you're trying to measure.It's late here and I'm half asleep so I hope someone will correct me, but from a quick glance I think the burden voltage of both the 5000uA and 50mA is sucking your voltage source completely dry, as your average measured AC values have the same burden voltage of around 4-5mV. The meters can't read any higher than what they are currently at for both ranges because there's 'nothing left to measure'.....if that makes any sense.Try switching to the 5A range and see how you go.

author: hayatepilot
author_rank: Frequent Contributor
timestamp: September 21, 2015, 02:01:16 pm
content: Quote from: Timur Born on September 21, 2015, 10:50:41 amDo I understand you correctly, that you take your DMM, put it on current and put one lead to the PC chassis (mains earth) and the other lead to the ground line of the PC's power supply?Are you trying to measure the current in ground loops?QuoteI think you are measuring a (very low) voltage source with the current ranges instead of the voltage range:Using ohm's law driving 30.59 mA through 1.8 ohms gives a voltage drop of 55mV.587uA through 102 ohms gives 59.9mV....Those voltages are remarkably similar.One could even calculate the internal resistance of your "voltage source": it is about 0.166ohmsGreetings

author: Timur Born
author_rank: Regular Contributor
timestamp: September 21, 2015, 03:32:25 pm
content: Quote from: deadlylover on September 21, 2015, 01:08:13 pmWhile I understood that part already, I obviously underestimated its impact. In practice the resistor is nothing more than an attenuator for the audio signal, higher resistor = lower audible signal.QuoteAnd this is the part I underestimated. I did the practical test and connected the powered speakers to the PC via shields/grounds only. PC -> USB (Madiface XT) -> XLR pin 1 -> Fluke 289 -> XLR pin 1 -> speakers (HS80M).Using the 10A connector on the Fluke unsurprisingly leads to the highest volume of audible signal, range makes a measurable but no so much audible difference. Using the 400 mA connector leads to an attenuated signal at mA setting. At uA settings the signal has been attenuated below the noise floor of the speakers (or maybe further).QuoteThis one I did not follow. Please explain to the half brain-dead over here.QuoteIt seems that there still is enough left for the meter to measure, there's just very little left, though. To give an example (average, auto-ranged):mA idle = 0.36 mAACmA load = 2.03 mAACuA idle = 13.9 uAACuA load = 47.8 uAACSo there's still juice left, just not much.

author: Timur Born
author_rank: Regular Contributor
timestamp: September 21, 2015, 04:13:52 pm
content: Quote from: hayatepilot on September 21, 2015, 02:01:16 pmFirst of all I should clarify that I am not genuinely interested in the current, but rather voltages and frequencies. The reason why I even started to use the current measurement settings on a DMM is, because on the VC890 the current setting enabled me to measure (high) frequency where the voltage settings failed. On the Fluke 289 the only setting working for frequencies is the mV DC one, and that only works for low frequencies (<400 Hz or so).But once I had a look at the current readings I wondered about the discrepancies and how to properly interpret the results. It's a learning experience and thus a thread about learning how to use DMMs correctly.For my usage case I am not so much using the ground lines of the power supply, but the ground lines connedted to the mainboard (USB, DVI, DP, screws). The results can vary by which ground line you connect to and by which the load state of the system. Usually high performance graphic-cards are the one stand-out part that induce heavy noise on the ground lines, even audible through thin air from their own power section (coils, capacitors). The noise is frequency modulated by load (usually frame-rate = fundamental frequency, i.e. 1800 fps = 1800 Hz).And while the travel-paths and counter-measures are the very same as with ground-loops, this is not a mains hum ground-loop problem. Things get more complicated by the fact that even short differences in various cable runs can lead to lower (cancellations) or higher (summing) noise levels at the speakers end. A display connected (or not) to the same computer that has its three-prong power plug connected to the same or a different power string can make an audible difference.QuoteI think I would have to collect some more numbers to follow that. The latest numbers I posted don't seem to add up to that?! I usually never need this math, so I'm not into it mentally.

author: deadlylover
author_rank: Frequent Contributor
timestamp: September 21, 2015, 09:11:06 pm
content: Quote from: Timur Born on September 21, 2015, 03:32:25 pmYup, let's have a look at the burden voltages from your latest set of measurements;mA idle = 0.36 mAAC (0.648mV= 0.36mA * 1.8 Ohms)mA load = 2.03 mAAC (3.654mV= 2.03mA * 1.8 Ohms)uA idle = 13.9 uAAC (1.418mV= 13.9uA * 102 Ohms)uA load = 47.8 uAAC (4.876mV= 47.8uA * 102 Ohms)We haven't measured the actual voltage level yet, but using the info from your latest experiment on listening for audible changes depending on the current range, if the uA range is taking out the noise then we can just kind of assume it's dominating the voltage source.So now we (kind of) know the voltages to be ~1.42mV idle, and ~4.88mV under load. Notice the burden voltage on the mA range is doesn't quite consume the whole voltage source, and that's most likely why you can still hear some of the attenuated noise on the speakers.As an aside, the burden voltages on the mA range as a percentage of the (assumed) unloaded voltage source are around 45% idle, and 75% load. So if the circuit behaves linearly (a really big if), then the accuracy of the measurements on the mA range will be out by the same order! And that's why burden voltage is such a big deal when trying to make accurate current measurements, it can completely overwhelm the accuracy of the multimeter in some cases.I hope it helps answer your original question on why the uA range doesn't show overload. It reaaally wants to, but it ran out of juice because the current shunt would have to drop more voltage than what exists.

author: Timur Born
author_rank: Regular Contributor
timestamp: September 22, 2015, 08:44:51 am
content: For a fully reproducible measurement I first would have to change connections around my table, disable Windows services and fully control CPU c-states. Without such manners the numbers currently change in the 0.x mA/mV range depending on the current state of everything.That being said, the voltage between the PC's ground and the powered speakers shield connection (XLR pin 1) is around 4.85 - 5.25 mV (average AC), with no connection being made in between the two points (aka DMM in series). So using uA measurement leaves a signal between zilch and around -65.75 dBu. Based on 5.25 mV the mA measurement would leave a signal of -53.7 dBu, so around 12 dB difference between mA and uA settings.The speakers specs list 10 kOhm input impedance, but that doesn't affect pin 1/shield. I measured voltage and resistance of the whole loop with and without the speakers in between (PC -> Fluke 289 Ohm mode -> XLR -> speakers -> power strip -> power strip -> power strip -> PC). Makes little to no difference, so load from the speaker/amplification part seems negligible. Ohms change between around 57.5-60 kOhm on load to about 8-10 kOhm idle, with or without speakers.The changing Ohm readings demonstrate that some effort will be needed to get the readings more stable. The main culprit is using the graphic-card as noise signal generator. Temperature and power limits (TDP) kick in and out, especially the latter. This in turn changes readings. But I am not so much interested in extreme precision (quantitative), but more in generally useful measurements (qualitative). Still the numbers are all too close to the limits of the Fluke 289, so I will give this whole thing another thought. Using a uCurrent gold as voltage amplifier could be a solution, especially since this could give me frequency readings that the Fluke refuses to do atm. Still astounding that the cheapo VC890 (Uni-T based) could do frequencies for currents (it likely was mA range) where the Fluke fails.Anyway, thanks to everyone for the hints and help!

author: retiredcaps
author_rank: Super Contributor
timestamp: September 22, 2015, 07:04:40 pm
content: Quote from: Timur Born on September 21, 2015, 10:50:41 amThe explanation for the above is because the Fluke 87V defaults to AC current when you switch the rotary knob. Martin forgot to hit the yellow button to switch back to DC.The default to AC current is a big annoyance to all users who use the 87V for electronics. The original 87 I and 87 III default to DC current.

author: Timur Born
author_rank: Regular Contributor
timestamp: September 22, 2015, 09:04:55 pm
content: Ah yes, I remember. The good thing about the 289 is that it stays in the last mode used (AC, DC) when you switch between mA and uA. It even keeps the setting over switching off and on again.

author: Timur Born
author_rank: Regular Contributor
timestamp: September 24, 2015, 10:20:01 am
content: Some follow-up: I bought 10 Ohm, 1 Ohm and 0.22 Ohm (smallest available at local Conrad) resistors to measure frequency via mV settings on the Fluke 289. While this works, especially via the 10 Ohm resistor, I found out that I don't need that kind of setup. Turns out that the 289 is *very* susceptible to the polarity of my signals, even when AC voltage measurements are used as basis for frequency. I even managed to have one measurement where the mA DC range worked for frequency measurement.Unfortunately the doesn't seem to be any kind of rule when to use which polarity. One signal may work with the DC component being positive, another will work with the DC component being negative. Sometimes the "wrong" polarity leads to zero frequency results, sometimes it leads to wrong results. In the latter case it's harder to find the correct polarity unless you already know the result (aka source signal). The best clue you can get then is because of values jumping around more, while the "right" polarity makes min/max values stay closer to the average.Generally the Fluke 289 doesn't seem to deal well with noisy - or rather non-sinusoid - signals. Even when the internal 102 Ohm resistor sucks the signal nearly dry in uA settings there still is around 30 uA of signal left, which is still higher than the listed minimum of 25 uA. Still it only shows zero when trying to measure frequency on that. Another indication is that using the low-cut filter on <200 Hz signals does not improve/enable readings, but worsens them.As a result I now ordered an Agilent U1272A to compare against the Fluke 289. It uses a 1 Ohm resistor for its mA range and lists a considerably lower minimum sensitivity level for mA (150 uA). But first and foremost I am hoping that it will generally deal better with not to perfect signals, without shenanigans of having to reverse polarity of the probes and stuff.Based on specs the 289 has some advantages over the U1272A where it comes to pure voltage (and especially DB) measurements. I will compare them and see what stucks. But thanks to your help I can now make more educated decisions.

author: Timur Born
author_rank: Regular Contributor
timestamp: September 25, 2015, 07:03:40 pm
content: Turns out that the shunt resistor of the U1272A in mA range is 2.2 Ohm, not 1 Ohm. So the 1.8 Ohm of the 289 is a bit smaller. Doesn't matter for my measurements, as both cannot produce frequency numbers in that setting. Still the slightly higher resistance leads to slightly lower mA readings, which may be interesting to some. Both DMMs use around 100 Ohm at uA range (measured U1272A: 102 Ohm, 289: 100.5 Ohm).Curiously the extended bandwidth of the U1272A revealed another frequency spike at around 65 kHz, not too bad. Although I still would have preferred if it would display the lower frequency signal that I was producing, since that one had a bit higher level (albeit maybe more noisy).And while I was at it I also compared the LPF with signals below 200 Hz and found that the U1272A is filtering far less aggressive. It is able to display my <200 Hz signals properly, where the 289 does not. On the other hand the U1272A can display over 3 kHz frequencies even with the LPF enabled, so that may be a problem for some people.My take is that I mostly cannot use DMMs for measuring frequencies of my setups and thus need to use an audio interface instead. This will give me far better readings anyway, but also is a bit more of a setup hassle that what I hoped for with quick and dirty tests. Maybe I will order a uCurrent Gold and see what results I can get from that.