EEVblog #1057 - µCurrent Murphy - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: February 11, 2018, 11:00:04 pm
content: Dave investigates a problem with a recent batch of uCurrents, something that WASN'T supposed to happen!A rare look at a real world combination of design, manufacturing, production testing, and procurement problems.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 11, 2018, 11:52:02 pm
content: With the problematic opamps, what happens if you connect a 100n capacitor from battery negative to output ground? I think the lack of bypass from supply rails to output ground might have been a latent design flaw.

author: IanB
author_rank: Super Contributor
timestamp: February 12, 2018, 12:39:03 am
content: I notice they show out of stock in the shop. Did you withdraw them from sale while you investigate? What will happen to the ones assembled with the bad chip?

author: 2N3055
author_rank: Super Contributor
timestamp: February 12, 2018, 09:00:49 am
content: I had the same problem...I also didn't need high bandwidth, so I added some capacitors.Basically rail splitter was oscillating, but it depended on power source. If battery was new or if I powered it with PSU it worked well..It depended much on internal impedance of the source..

author: EEVblog
author_rank: Unknown
timestamp: February 12, 2018, 09:13:45 am
content: and

author: EEVblog
author_rank: Unknown
timestamp: February 12, 2018, 09:14:08 am
content: Quote from: IanB on February 12, 2018, 12:39:03 amNo, they just happened to be out of stock.

author: TMM
author_rank: Frequent Contributor
timestamp: February 12, 2018, 09:33:16 am
content: Connecting bypass caps to both rails is ill-advised as it creates a PSRR issue. Just one cap to either rail should be fine because you already have another bypass cap between the rails

author: blackdog
author_rank: Frequent Contributor
timestamp: February 12, 2018, 10:30:57 am
content: Hi Dave,Dit you read my remark in the other topic that the feedback capacitor of the last opamp is on the wrong position.It should be direct on the opamp output and not on the output of the device.You will create another pole if you load the output with "to much" capacitens and if the phase margin and/off te power supply rejection of the problem opamp is less,you wil get a nice generatorKind regarts

author: tszaboo
author_rank: Super Contributor
timestamp: February 12, 2018, 10:42:12 am
content: I had a similar issue with simple jelly bean TVS diodes. SMAxxCA. The originaly used, ST part had 1/10th of the leakage current at room temperature, than the replacement part from Taiwan semi. A few hundred board had to be reworked.

author: eV1Te
author_rank: Regular Contributor
timestamp: February 12, 2018, 10:43:42 am
content: Quote from: blackdog on February 12, 2018, 10:30:57 amThis was also my first thought when I saw the schematics, the feedback capacitor for the last OpAmp goes directly to the output where you have an unknown amount of capacitance. This might not be the actual issue in this case but I always try and add a final resistor for some isolation since you never know what it will be connected to.For example, what if the DMM/circuit you are connecting to actually outputs some voltage/noise, will that be coupled back into the feedback circuit?In this case I would make a quick test and add a 1k ohm resistor between the final output of the microcurrent and the DMM you are connecting to and see if that helps. Or move the feedback network to the OpAmps output directly, but this will require soldering.The increased output impedance should not influence frequency response unless the capacitive loading is getting close to 1 nF.

author: MiDi
author_rank: Frequent Contributor
timestamp: February 12, 2018, 11:33:48 am
content: Would suggest it is mainly a CMRR/PSRR problem of the first amplifier stage (second only distributes 1/10).A look into theDatasheet of MAX4239for this turns out that at 1Mhz the CMRR or PSRR are in the order of 0dB rejection (at higher frequencies even worse? Can it go below 0dB rejection?).A CM or PS-ripple voltage at this frequency will be magnified by 110 by the amplifiers, this is more than input signal amplification!1MHz 100µVpp on the supplies or the VGND turns into ~11mVpp at the output.A filter for CM/PS for high frequencies on first amp stage should improve the circuit.A measurement of the CM and PS-ripple on first stage would be interesting.

author: ogden
author_rank: Super Contributor
timestamp: February 12, 2018, 01:55:27 pm
content: Quote from: MiDi on February 12, 2018, 11:33:48 amRight. I vote for thorough stability measurements and analysis of whole circuit. Last stage is especially interesting from stability point of view. Could be so that problem is complex - last stage instability together with inferior CMRR/PSRR of rail splitter, both creating some feedback circuit. "Opamp stability" and compensation could be good topic for video.http://www.ti.com/lit/an/sloa020a/sloa020a.pdf

author: jimmc
author_rank: Frequent Contributor
timestamp: February 12, 2018, 02:24:19 pm
content: I think that the situation is not helped by the placement of the feedback capacitor C4, it would be better to connect C4 to the Op-Amp side of R8.At present R8 and the load capacity introduce additional phase shift in the AC feedback loop via C4 reducing stability margins.http://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.htmlI had a similar problem with a LEM PR200 current probe where moving the feedback capacitors cured the problem.JimApologies to Blackdog, who already mentioned this in post #7.I'm not having a good day.

author: coppice
author_rank: Super Contributor
timestamp: February 12, 2018, 04:38:21 pm
content: I've had bad experiences swapping between makes of the LMV321, LMV322 and LMV324. They vary in a number of ways. Like most op amps described as rail to rail, they don't actually work close to the rails. However, different makes vary considerably in how close to the rails they work. Stability into capacitive loads varies considerably, as Dave has found. They are certainly a go to part for low performance jelly bean op-amps, but you need to try your design with various makes if you intend to swap suppliers during production.

author: analogo
author_rank: Regular Contributor
timestamp: February 12, 2018, 05:48:07 pm
content: Which capacitance decade box is Dave using? I see it is marked with the open source logo, but I could not find it in the usual places.

author: Neilm
author_rank: Super Contributor
timestamp: February 12, 2018, 07:43:02 pm
content: I have had similar problems with certain sensitive circuits, especially with transistors. The assembly notes for the board now read that only a specific manufacture and part can be used. It looked as if some parameters (not on the datasheet) were the cause.

author: Tsippaduida
author_rank: Contributor
timestamp: February 12, 2018, 07:50:47 pm
content: I haven't run into this issue with my uCurrent, but I checked it and it has the "wrong" chip also. My serial is ~428x, don't remember it now. I mailed t he information to Dave in the hopes it will help in narrowing down the affected units.I might order replacement chip when I need to get more parts next time.

author: bitwelder
author_rank: Super Contributor
timestamp: February 12, 2018, 07:56:10 pm
content: Searching on Youtube for uCurrent, I found this old video by electronupdate, where he puts the LM321 die under the microscope... and finds a Fairchild labeled FAN4114A. Interesting.

author: BU508A
author_rank: Super Contributor
timestamp: February 12, 2018, 08:23:54 pm
content: Quote from: analogo on February 12, 2018, 05:48:07 pmI think it is this one:https://www.ebay.com/itm/182642398119

author: ckambiselis
author_rank: Regular Contributor
timestamp: February 12, 2018, 08:29:52 pm
content: Maybe I haven't seen enough chips in my life yet, but every time I see a chip/fet/... with that glossy straight cut edges plastic package it's a fake Chinesium chip I have bought from eBay for some quick testing, otherwise all I get from Farnell, Distrelec, TME etc. have the matte finish rounded corners plastic package.Waiting for some FETs by ON Semi to come from Farnell, lets see what the package looks on them, maybe it's how ON Semi makes the chip packaging.Oscar

author: Dave
author_rank: Super Contributor
timestamp: February 12, 2018, 08:41:17 pm
content: Here, this should fix your design flaws.There are two caps filtering the rail splitter reference, so the voltage jumps directly to the midpoint at startup instead of slowly ramping there.

author: TMM
author_rank: Frequent Contributor
timestamp: February 13, 2018, 12:15:12 am
content: Quote from: coppice on February 12, 2018, 04:38:21 pmI don't think that was the issue here. There was a feedback path via the power supply and the phase shift of everything in that path made it positive feedback. Significant phase shift occurs in an opamp above it's operating bandwidth. Makes sense as the opamps used are ~1MHz GBW and the oscillation was at 2.7MHz. Some variance in the GBW between opamp versions would mean that under certain circumstances some opamps (higher GBW than spec) are stable and others (GBW near spec) oscillate.Quote from: Dave on February 12, 2018, 08:41:17 pmYou don't want to do that because the two caps work as a capacitive divider and any common-mode noise on the rails couples through. Using just one cap gets you an RC filter. There is already a decoupling cap from +rail to -rail so by adding just one cap from VGND/R7 to either rail there will be a high frequency path from the decoupled node to either supply rail. The decoupling caps probably only need to be small to solve the oscillation issue (a few hundred pF will probably do) so there will be no noticeable turn on delay. Even 1nF gives a turn on settling time of a few milliseconds.

author: fusionmkx
author_rank: Newbie
timestamp: February 13, 2018, 02:44:59 am
content: Could you share where to get that the variable capacitor and variable precision resistor devices that you've been using??

author: BU508A
author_rank: Super Contributor
timestamp: February 13, 2018, 06:16:10 am
content: Quote from: fusionmkx on February 13, 2018, 02:44:59 amvariable capacitor decade box: see five postings above. Just scroll up.

author: bitwelder
author_rank: Super Contributor
timestamp: February 13, 2018, 06:19:42 am
content: Quote from: fusionmkx on February 13, 2018, 02:44:59 amSearch for Atlanta Robotics decade capacitance box.http://www.inmojo.com/store/atlanta-robotics/item/decade-capacitance-box/(the page includes the design in Eagle if you want to build it - it's OSHW - and a link to Dave's review a long time ago)

author: fusionmkx
author_rank: Newbie
timestamp: February 13, 2018, 06:51:19 am
content: Fascinating, thanks! This has been a very educational episode indeed.

author: Decoman
author_rank: Regular Contributor
timestamp: February 13, 2018, 03:55:53 pm
content: Not being all that into electronics, my unit #4990 seem to have the one labeled 321T looking closely.

author: donmr
author_rank: Regular Contributor
timestamp: February 13, 2018, 05:51:15 pm
content: I look at this very simply. The op-amp supplying the virtual ground has some limit to it's frequency response. Anything above that limit needs to be shunted through one or more capacitors. You don't need larger caps that this however and I suspect that the 100nF Dave is using may be a bit more than that.

author: amitmey
author_rank: Newbie
timestamp: February 13, 2018, 07:02:50 pm
content: Hello,I found my uCurrent has 321s too and behave the same (3.6mv offset connected to BM235 and 4.7mv when connected to the scope with 5Mhz noise up to 25mv).The offset was cancelled to 0.01mv when 0.1uF ceramic cap was added over R7.Amit

author: Kevin.D
author_rank: Frequent Contributor
timestamp: February 15, 2018, 01:57:36 am
content: Apart from the possible issues created by having Virtual GND which has a source impedance of 270 Ohms :- meaning some fraction (~ 270/Zload) of your output signal appears across R10, your output signal rides on top of this common mode signal who's amplitude increases as Zload decreases and approaches R10, this though isn't the source of the oscillation shown in that video (which at one point looked to be around 2Mhz, well above the lmv321 crossover freq), this though is probably tipping the two unstable Max4239's configurations (u1,u4) into oscillation.Max4239'S are Decompensated op-amps, these are only stable at gains greater than 10. Another way of stating this is that the fraction of Output signal you feed back to the inv input shouldn't be greater than 1/10 .So adding feedback caps (Like c3,c4 here in Ucurrent schematic) which then at some freq (1/2pi C4 Rf) starts to feedback a greater portion of the output signal will cause any NON unity gain stable Op-amp to become unstable.There are a number of techniques available to enable use of NON unity gain stable OP's with capacitive loads . One I won't discuss here is a resistor + cap between the op's inputshttp://www.ti.com/lit/an/snoa486b/snoa486b.pdfHere's one I sometimes use with both compensated and decompensated Op's, being useful when a flat response is desired. First decide what is the max value of Capacitive load that we want the Op-amp to be stable with then Cf can be calculated.I usually make Cf*Rf= Cload*(Rop+Riso). Where Rop is the open loop output impedance of your OP (typically 50-100 but there are exceptions).Then calculate Cs by making Xcf/Xcs = Rf/Rg . Here for these relatively low frequency OP's we can ignore the small input capacitance (few pF) of the Op-amp which adds to Cs .One way to describe whats going on here is that initially the gain is set by feedback resistors Rf and Rg then at some frequency (= 1/2pi Cf Rf) the capacitive divider Xcf/Xcs takes over the feedback roll. Because of wider tolerances in capacitors it's not really possible to match Cf/Cs as accurately as you can Rf/Rg so you won't get a perfectly flat response Over your OP's entire B.W .Addition :- The OP still has to be able to drive the series combo of Cr+Cs : Ct = (Cs*Cf)/(Cs+Cf) and if this get's to large it itself will load down the op-amp, (so then you would try to use lower values by increasing Rf,Rg).Simpler is indirect drive method :- here the capacative load is simply isolated from the Opamp feedback node by a resistor 'Riso'. I generally choose in both methods to make Riso to be = Rop (usually ~ 50 Ohms varying if I want to achieve a specific Zout or working with unusual opamp's) this should be sufficient to stabilize for capacitive loads of practically any size. Making it larger doesn't really increase stability value, it just means higher Zout.The major difference between the first and second method is that in the first method Riso is inside the feedback loop so the op-amp reduces the apparent output impedance to approx (Riso/1-Loop Gain) which is very Low (< 1 milliohm at DC). Then at the comp freq ( 1/2pi Cf Rf) the capacitors take over feedback, above this frequency Riso is no longer inside the feedback loop, thus Zout rises to ~ Riso .With the second method Zout = Riso over the entire B.W , Which doesn't matter here considering the main intended usage (load is usually 10M Zin multi-meter) which results in a tiny measurement error proportional to ~ (Riso/10M).Unlike the final output amp (U4) which sees the load, the first Max4239 (U1) shouldn't need any compensation, just remove C3 to make this stable. ( the few pF input capacitance of U1 creates a pole with R5||Rf but it's well above Fcrossover so no problem).Regards Kevin.DLater Edit :- I want to correct something I posted above, the statement that 'adding single feedback cap's like C4 and C3 always makes Non unity gain stable op's unstable is incorrect. If I had bothered to do the maths before I had posted I would have seen that in Daves Ucurrent the zero created by C3,C4 is at (1/(2 pi 10p 10k) = 1.6 Mhz thats just below the crossover for this op-amp when at a gain of 10. You can of course use single small feedback caps that give phase boost's near the crossover frequency (when loop gain = 1) like Dave did here without being worried what happens at the higher frequencies because loop gain is then well below crossover. So doing this (even in decompensated amps) help's the Op-amp to remain stable with small load cap's or stray cap on the inv input. The problem is when larger values of feedback caps are used to try and compensate for larger values of load capacitance's, it might be ok for fixed load capacitance values in some cases but when the load capacitance is variable then a decompensated OP will become unstable when the load cap is then much smaller or no longer present. This is where the two techniques posted above come in as they can cope with wide variation of load capacitance.

author: marcognon
author_rank: Newbie
timestamp: April 18, 2018, 12:20:30 pm
content: I have huge offset with my uCurrent gold, one of the first batches (bought from the site right after Kickstarter campaign ended). Like 5-10mv even on shorted position, on all three ranges, measured with dmm UT61E, and I tried another two cheap meters.Even I had the correct op amp, I tried replacing the LMV321 with every variant available to me : Texas Instruments, ST Microelectronics, etc.The result stays the same. Output capacitors like in the following video fix didn't helped.Any idea how it could be made to work ?

author: d-digiacomo
author_rank: Newbie
timestamp: June 20, 2018, 07:38:36 am
content: I'd like to purchase a µCurrent GOLD. Could those in stock at present still have the offset issue?

author: marcognon
author_rank: Newbie
timestamp: June 20, 2018, 01:46:57 pm
content: I don't recommend buying it.I tried 2 more op-amps with same result. I think instability is from design, not from out of spec components. Those uCurrent which work are just by luck ... who knows. I purchased mine right after Kickstarter campaign ended so it should have worked because it had a good op-amp.Better design your own and learn something in this process. This is what I'm planning to do.My conclusion is : op-amp design is not so easy as it looks from some enthusiastic videos ...

author: Susceptibility
author_rank: Newbie
timestamp: February 03, 2022, 11:09:15 pm
content: Hi!I know, this is an old topic, but just saw the Murphy video.I am curious about R8 - why is it connected like that? I know that R8 is an output protection resistor, but connected like that, every capacitive load on the measurement terminals effects the feedback circuit. If the R8 is placed after, then the load is seen through the resistor, and may not influence the circuit a way that creates oscillations, even with the "bad chip".

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2022, 10:21:43 am
content: The original way R8 is connected is indeed bad and it is doing more harm than good. The red drawn way is the better solution.AFAIR the problem was more with the virtual ground part and the red modification alone may not solve this, though it can help.