Fluke 8845A AC current has poor CMRR - Agilent better? - Page 1
Forum section: electronics / testgear

author: pierre
author_rank: Newbie
timestamp: October 20, 2013, 10:09:02 pm
content: My first post from sunny England, please be gentle. I've spent a lot of time developing a mains distribution unit for broadcast/IT installations and bought an 8845A for final R&D testing and calibration. My main use was to be AC current and voltage with other highly accurate measurements as a bonus.The first time I connected the Fluke in series with my current measurements, the 240V/50Hz mains voltage showed 6mA on the 1A/3A/10A ranges with no current flowing. In the first day of use I came to mistrust the Fluke so much that I put it back in its box and carried on with R&D work thinking I must be doing something wrong. I then borrowed a Voltech PM1000 which made much more sense and showed I could reliably calibrate my current transformer measurements from 1mA to 20A and more.Returning later to the Fluke, with a completely different wiring setup, I again found the minimum 6mA displayed on the 1A/3A/10A ranges which share the 10A fuse. Ranges using the 440mA fuse were okay, but the 440mA fuse isn't much use for all but the smallest of mains loads due to inrush blowing the fuse. In any case minimum 1% spec of the 1A range is 10mA which should be accurate to 1.4mA and in my case was displaying 16mA for 10mA.I asked Fluke UK what the issue might be, put the meter away again and waited a couple of months for an answer. I was performing some tests on an audio amplifier, the 8845A came out again. Now instead of 6mA for no current I was getting 140mA. Then I realised I only needed the Fluke common connected for this phantom reading. Looking at the output of the bridged, class D (cheap PA) amp there was 355kHZ @ 1.2Vrms with respect to earth. With a mains iso transformer feeding the Fluke, the 140mA phantom measurement was greatly reduced and it became clear the Fluke has a CMRR problem, not least because 355kHz is well outside its 10kHz bandwidth. Of course at this point I realised the original 6mA displayed on mains also only required one wire connected to the 8845A.Writing to Fluke again with video examples, I explained the 8845A AC current measurement appeared less than useful in the real world. They have now come back to me with a surprising reply, that they can replicate my results (with 8846A) and I ought to simply consider a different manufacturer. Fluke tested Agilent 34401A/34411A and did not see the same issue.The supplier has been very good and agreed to a full refund, I am tempted to buy a Voltech (or whatever Tek are providing post takeover of Voltech) but I like the idea of a general purpose meter that also gives me accurate mains current and voltage measurements. Are the community surprised by the 8845A sensitivity to common mode and/or out-of-band signals? Would the Agilent 34461A be a good idea? Did I read the Agilent resorts to displaying zero current below its minimum spec, something I have seen on a Fluke handheld that should really display ---- rather than 0.00A if it is simply below 20mA.Thanks for your thoughts beyond my rather limited bench meter experience of the 8845A.Pierre

author: mzzj
author_rank: Super Contributor
timestamp: October 21, 2013, 08:29:12 am
content: Quote from: pierre on October 20, 2013, 10:09:02 pmConnecting current terminal of the HP 34401A to live 230vac 50Hz and the meter displays 0,05mA. Having no connections at all shows the same value at the display.

author: lewis
author_rank: Frequent Contributor
timestamp: October 21, 2013, 11:13:21 am
content: That is most unusual, I would not have expected Fluke kit to perform in this way.Quote from: pierre on October 20, 2013, 10:09:02 pmMost isolation transformers pass through the earth connection. Out of curiosity, did you remove the earth connection?Quote from: pierre on October 20, 2013, 10:09:02 pmVery very surprised, it renders the meter almost useless if this is not a unique fault with your unit (and the one Fluke tested!). Agilent 3468, 3457, 34401A, 34461A do not have this issue. I think Robrenz on here is a big fan of the Flukes, would be good to hear what he has to say.It would be interesting to see if this is a LF CMRR issue or out of band noise issue - it's possible you have lots of out of band noise on the mains. If you have a signal generator, try running a 50Hz sine wave into one of the current inputs with sig gen ground connected to mains earth very close to the meter. An PA audio amplifier (not bridged or class D) can provide voltage gain if the sig gen can't tease a result out of the fluke. Try it at 20kHz too.

author: EEVblog
author_rank: Unknown
timestamp: October 21, 2013, 12:42:10 pm
content: Quote from: pierre on October 20, 2013, 10:09:02 pmWowIs there anything at all in the specs somewhere that say what the CMRR is in current mode?

author: pierre
author_rank: Newbie
timestamp: October 21, 2013, 05:33:22 pm
content: Thanks mzzj, I believe the 34461A I am considering has a max 3A input like the 34401A, with an additional 10A input. At least if the 34461A 3A matches your 34401A it partially overcomes the 440mA limitation of the Fluke. The 1A and 3A ranges of the Fluke using the 10A fuse sounds useful where the load is varying from zero to 3A through 10A, also claiming better accuracy at 1A/3A than the 4mA 'due to range' accuracy of the 34461A 10A. In the real world however the Fluke doesn't seem to deliver on those 1A/3A ranges. Based on what you and lewis have said, I will check with Agilent and then likely go that route.lewis, I haven't performed all the tests I might as I'd seen enough to be sure there was an issue. When I emailed and then later sent the videos to Fluke I expected a dialogue rather than "our 8846A does that too [355kHz, no comment on 50Hz], Agilent doesn't, buy somewhere else". I will have to leave further tests to others as the 8845A is packed up to go. I felt the ridiculous effect of the 355kHz might imply out of band on the mains as you suggest but I don't think my mains west of London is special and the Voltech doesn't measure any bizarre harmonics. I have a client east of London who connected his 8845A common only to the mains and measured a similar 6mA on a meter bought a year before mine. My iso transformer doesn't pass earth through, I tried with and without earth connected and can't remember the exact results but what appears in the video (link below) is without earth to the 8845A. I was just concentrating on the fact that the power supply / chassis made a difference to a phantom reading I wasn't expecting to see anyway.Dave, I can't see anything in the extended specification to suggest an issue, no mention of CMRR and what would it be with respect to anyway- earth? Wouldn't I expect common mode to be a difference on the input legs not wrt the unit's chassis or supply? Technically Fluke could say zero current is below 1% minimum so the Fluke can display 6mA, even 600mA if it wanted to. That falls down when you try to measure 1% ie 10mA and get 16mA on the 1A range. Equally they could say the spec to 10kHz means you have to filter above 10kHz yourself but that would be nonsense.Part of my recent email to Fluke UK follows and includes links to videos. My aim is not to bash Fluke although I seemed to do all the work on this and they weren't too interested. More importantly I don't want to make the same mistake again with Agilent. A colleague suggested I buy a precision resistor and use the VAC range but where does that end? It has an ammeter, it should work.Fluke have been very clear in their response which I probably should not quote in full without their permission, noting "similar errors", "no induced error" on the 34411A and 34401A, "customer better off with a different manufacturer's meter"----- excerpt email to Fluke follows --Mains live to common,One leg of amplifier output to common, the 140mA indicated on the 1A range are clearly not flowing,Amplifier again with isolation transformer feeding the 8845A highlighting the poor isolation inside the 8845A,I accept your specification refers to "sinusoidal signals with amplitudes greater than 5 % of range" and that "for inputs from 1 % to 5 % of range, add an additional error of 0.1 % of range". There are few people who would expect for less than 1% that the 8845A should be able to display nonsense. Even if this were true, 1% of the 1A range is 10mA and I measure 16mA for 10mA mains current flowing, limit should be 11.4mA?One year 0.1% reading + 0.04% range at 50Hz on 1A range1% of 1A range is 10mA0.1% reading + 0.14% range0.1% reading + 1.4mAThe amplifier 355kHz should have no effect at all but swamps the front end of the meter. Even if we accept the meter's CMRR is poor, the bandwidth is meant to be 10kHz. This should mean frequencies above 10kHz are filtered by the 8845A, not that my real world signals must be filtered in some way before measurement.AC Filter BandwidthSlow .................................................................... 3 Hz to 10 kHzMedium ............................................................... 20 Hz to 10 kHzFast..................................................................... 200 Hz to 10 kHzIf the susceptibility at 355kHz is linear, only a very small common mode voltage at this frequency would force the 8845A outside specification. On the 10A fused AC current range the meter seems near useless to me. Actually worse than useless, as it has now cost me far more than its purchase price in my chargeable time.

author: robrenz
author_rank: Super Contributor
timestamp: October 21, 2013, 07:54:33 pm
content: FWIW here are all the ranges of my 8846A on ACI 200kHz filter 100NPLCLow terminal connected to hot side of US 110V 60cycle with standard dmm probePicture name is range.

author: lewis
author_rank: Frequent Contributor
timestamp: October 21, 2013, 09:18:18 pm
content: Quote from: pierre on October 21, 2013, 05:33:22 pmYeah, that was a long shot.Quote from: robrenz on October 21, 2013, 07:54:33 pmJesus! So the bottom 3-4 digits on a 6.5 digit multimeter aren't to be trusted? What happens at, say, 200uA load on the 1mA range for example?

author: pierre
author_rank: Newbie
timestamp: October 21, 2013, 10:23:16 pm
content: Quote from: robrenz on October 21, 2013, 07:54:33 pmVery useful robrenz so those of us with 230V nominal (often 240V) suffer a proportionally large error compared with 110V users. Although even your 1A reading looks like you couldn't measure 1% ie 10mA within spec. The daft thing about this is I designed a mains unit which measures one or two inlets and fourteen outlets, all with current transformers and my own RMS code which can measure 1mA, 2mA, 3mA etc up to 80A peak so ~60A at 1.4 crest factor. I wouldn't claim outstanding accuracy over the range but it measures well against the Voltech and my Fluke 117. This 8845A issue, assuming it is an issue and I haven't missed the point, seems like something that should have affected plenty of people in the past. Perhaps it is the way I am adding load from nothing that is unusual hence it hasn't been spotted.The 355kHz wasn't something I was much bothered about, it just added weight to the idea that something was wrong as first experienced with mains current. I am pleased I posted here - for a long time I thought it can't be Fluke, it must be me.Pierre

author: Unknown
author_rank: Unknown
timestamp: October 21, 2013, 10:44:59 pm
content: Quote from: pierre on October 21, 2013, 10:23:16 pmYou can't conclude this from a '0V' measurement. For example many true-RMS meters will have an offset (with shorted inputs) in ACV mode. The RMS of the input signal and this offset is well within spec for any value within the specified range (usually 5%-100% of range). That footnote in the manual that states that the spec is only valid above a certain minimum value is there for a reason.RMS(0.0026A, 0.01A) = 0.01033A, well within the +/-0.421.4 mA allowed by the 24h specs for 10 Hz - 5 kHz.Edit: fixed tolerance

author: pierre
author_rank: Newbie
timestamp: October 21, 2013, 11:07:50 pm
content: Quote from: alm on October 21, 2013, 10:44:59 pmSorry, not sure what you are saying about 5%-100% of range, this isn't a cheap handheld meter. Fluke specifically say what correction to apply 1%-5% of range as above which gives +/-1.4mA at 10mA on 1A range. In my case the standing 6mA made it into my 16mA displayed for 10mA actual although of course I couldn't predict what would happen. I agree I can not conclude how robrenz' meter might react several thousand miles away but I would be surprised if their 2.6mA no current reading fell within 8.6mA to 11.4mA for 10mA flowing. Even if the 8845A does behave on 110V, it might be onerous for folk like me to use a 230/110 stepdown transformer.Agilent 34461A is also specified from 1%.

author: Unknown
author_rank: Unknown
timestamp: October 22, 2013, 12:06:30 am
content: A 16 mA reading for a 10 mA current is clearly out of spec, I was not commenting on that. Robrenz's reading seems fine to me, however, if you take into account what trueRMS (Root Mean Squared) means.I made an error calculating the maximum allowed error in the previous post, it's 0.1% * 10 mA + 0.04% * 1A + 0.1% * 1 A (because where between 1-5% of range). So it's even allowed 1.4 mA error(not shure where the 1.4 mA figure comes from).sqrt( (2.6 mA)^2 + (10 mA)^2 ) = 10.33 mA, well within the 1 mA allowed error. So I don't see why you would suspect any problems. I expect that if you do the same calculations for 5% of range with the tighter specs, it would still come out as in spec. A measurement below 1% of range is meaningless, however, since there are no specs to compare it to. It could be indicating 1 A with shorted inputs and it would still be correct as long as it would somehow be back to 10 mA +/- 1 mA once you increase the current to 10 mA.Edit: fixed tolerance.

author: pierre
author_rank: Newbie
timestamp: October 22, 2013, 01:01:53 am
content: Quote from: alm on October 22, 2013, 12:06:30 amSadly I know all to well what RMS means, I've spent years writing tight, fast code to calculate it and auto-range part way through mains cycles that have already begun measurement. I fell at the last hurdle when I wanted to check the final product with a Fluke meter.I agree it is amazing how large an offset one can have, and how coarse measurements can be, for example to measure a 230V sinusoid to 0.1V accuracy. This is because the larger samples contribute much more to the MS calculation than a small offset. However there is a difference between a non-zero reading for a reason and a fundamental flaw which the 8845A/8846A appear to have. robrenz was kind enough to capture the display with local mains live connected to meter common, this isn't a zero discussion but a discussion about 8845A common mode rejection. Fluke themselves have agreed there is a common mode issue.As explained, my 8845A carried a near 6mA zero current offset through to a 16mA reading for 10mA. On that basis I said it "looks like" robrenz couldn't get on track for a 10mA reading. It might be a London thing, but I felt "looks like" was a vague implication rather than scientific proof. Perhaps one for robrenz to comment on given it is their meter. You are right to echo my comment that technically the 8845A can display what it pleases below 1% but most seasoned professionals would be surprised to own a meter that displayed for example 10mA at zero and 10mA at 10mA.Bit confused again why you don't know where the 1.4mA comes from, having just said the specs are there for a reason. From the 8845A extended specs for 1A range 5%-100%, 10Hz-5kHz, accuracy is 0.3% measurement + 0.04% range. For inputs from 1% to 5% add an additional 0.1% of range, you end up with 0.3% measurement (not a lot at 10mA) + 0.14% of range. Take 0.14, divide it by 100 to move from percent world to real world shifts the decimal point two places to the left so you get 0.0014 of 1A which is 1.4mA.Apologies, I appear to be repeating myself here alm.

author: lewis
author_rank: Frequent Contributor
timestamp: October 22, 2013, 01:15:21 am
content: Quote from: alm on October 22, 2013, 12:06:30 amRobrenz's meter shows 2.6mA on the 1A range. The maximum error I'd expect would be a little over 1mA on this range with an input current below 5% of full range. I can't find anything in the manual about minimum input current, for example Agilent 34461A is specified for inputs above 1% range AND >10uA.

author: Unknown
author_rank: Unknown
timestamp: October 22, 2013, 01:24:51 am
content: pierre:You're right about the 1.4 mA figure, I somehow had an extra mental zero in the of range spec.If the common mode signal is uncorrelated with your input signal, then the two should add up as RMS. Even your 6 mA offset could almost be in spec in that case. If they are correlated they would add up linearly. So depending on whether you're measuring something in phase with the meters mains circuit or not you'd get a different offset. Measuring the current from a function gen/AC calibrator should give the correct value, and that's what they'd do during calibration. Did your measurements involve a signal in phase with the mains net?I'm guessing one reason why it wasn't discovered earlier is that these meters are not typically used to measure current on high-energy circuits, so the only common circuits where you'd have this problem is on the secondary side of a mains transformer in a linear power supply.Quote from: lewis on October 22, 2013, 01:15:21 amI'm pretty sure pierre and I already agree on this, but for your benefit: the 1 mA spec only applies for values above 1% of range, so the offset below that value is pretty much irrelevant. And depending on the nature of the offset it may very well disappear once the current approaches 10 mA. Just compare this to ACV measurements on most true RMS measurements with shorted inputs (surprisingly the 8846A does much better than average in this regard). Keithley even has an FAQ statement that this offset is normal and that you should not use the null function to remove this offset.

author: pierre
author_rank: Newbie
timestamp: October 22, 2013, 01:35:45 am
content: Quote from: lewis on October 22, 2013, 01:15:21 amlewis, Fluke do have themselves covered in the extended specs with >5% then a 1%-5% adjustment, implication anything below 1% it doesn't matter what is displayed. Purely technically. Personally with all those digits I think it should have a go at saying something that is near and this still doesn't forgive the 355kHz problem.I see no other conclusion than the Fluke does not meet its specification, the time I have put into this however does give me some comfort in specifying accuracy for my own product and an appreciation that my competitors' specs are nonsense. Now I just have to convince my customers that my figures are true and honest while others aren't. Oh, and I also need to direct Agilent here and ask them whether their meter will answer our concerns.[edited to remove the word sadly which I used in a previous post, the whole experience has clearly made me quite sad!]

author: lewis
author_rank: Frequent Contributor
timestamp: October 22, 2013, 01:36:17 am
content: Quote from: alm on October 22, 2013, 01:24:51 amAgreed, (it's late): "For inputsfrom 1 %to 5 % of range, add an additional error of 0.1 % of range." Glossed over the 'from 1%' bit mentioned in the Fluke manual and in previous posts.

author: johansen
author_rank: Super Contributor
timestamp: October 22, 2013, 06:57:55 am
content: Quote from: lewis on October 22, 2013, 01:36:17 amwhat people forget is that a digital meter is just a ratio measurement.in the case of an analog meter you are measuring two opposing magnetic fields against a spring. assuming that the permanent magnet and the spring and the resistance of the coil of wire don't change.with digital meters you are measuring the difference in voltage between the unknown and a reference.but you are limited to a set of "steps"below one part in 128 of full scale.. go ahead and drop 8 bits.. they all read zero anyways..the same goes for the top part in 128.thus your unknown is 8 bits less than that which is theoretically possible.most digital meters should be calibrated at mid range for this reason, and the closer you get to mid range the more accurate the absolute value will benon linearity is a given but you can't read beyond the bits that are significant.a 6000 count meter reading to one part in 100 is about all you can expect for example.

author: philpem
author_rank: Frequent Contributor
timestamp: October 23, 2013, 07:48:07 am
content: Quote from: pierre on October 20, 2013, 10:09:02 pmWell, if Fluke are suggesting we buy Agilent's TrueVolt meters, who am I to argue?