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- เผยแพร่เมื่อ 10 ก.พ. 2018
- 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.
Follow-up fix: • µCurrent Offset Voltag...
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I see the problem in the schematic. The output stage uses a resistor from the output of the op-amp to the output pin. This R, plus a capacitive load adds a high frequency pole which causes excess phase shift and thus oscillation of the output stage. Move that feedback cap to the op-amp output instead of the board output and increase it a bit, I'd guess 100-1000pF. This was a latent design problem. Spice the circuit and you will see the marginal stability of that stage with a C load.
A marginal design is like a blind man walking along the edge of a cliff. If he drifts one way, no problem. If he drifts the other way a tiny bit he falls off. The parameter that is changing slightly is the op-amp phase margin, changing slightly from batch to batch. Fix the design, don't select parts. Hope this helps.
Dave Erickson
This ^
Yeah, late reply but that was my first guess at what was happening just by seeing the schematic and the offset voltage. Strange offset voltage is the classic sign of oscillation!
@Duncan Julio instablaster =)
@Duncan Julio you are welcome :D
My reply is 3 years late but this is definitely true. The phase margin is pretty low for the MAX op amps. Looking at the datasheet, at x100 closed loop gain (40dB), the phase margin (by looking at the gain and phase vs freq) is actually NEGATIVE with just 100pF of load! And this is indeed at the MHz range of freqs, which is what Dave saw. I mean, I don't know why the other op amps fixed this tho... why most of the boards don't suffer from this...
Send the chips to electronupdate for him to decapsulate and compare.
SaNjA2659 yes defo do that
Yes, seconded. Definitely do that.
Another vote for this! (And I bet electronupdate would love to do a comparison between the two different chips, it’s a great puzzle, and would be a perfect fit for his unique skills and methods :-)
dave has a wafer inspection microscope, couldn't he do this himself?
SaNjA2659 i
Murphy should be paying rent with all the time he spends in the lab.
Not that rare! After 35 years of EE design, I can tell you that there are no equivalent parts. Even parts from the same manufacturer can change over time and lead to this type of problem. High volume production has to test for all of the 'corner cases' or you WILL always have gottcha's!
Once its in the customers hands those gottcha's cost big bucks to fix. This is one of the significant problems with 3rd party manufacturing. I've got lot of stories about problems similar, some are significantly more weird than this.
Yeah, this isn't quite the weirdest I've seen either.
R15 is upside down, electrons have fell out
No, he's in Australia. They'd only fall out in the northern hemisphere.
In the West Island they kill themselves instead.
At least they didn't invade the neighboring components and razed them to the ground plane.
Are you going to add a capacitive load to your production test jig?
I think not having a capacitor between V- and GND or V+ and GND is actually a design error, as it puts the complete AC output current through the virtual ground supply opamp.
I'd like to see Dave just add that bypassing cap between there on a unit with the bad LMV321 chip
There’s no controlled impedance between the output of that op-amp and the power lines, and when you connect capacitive loads the impedance goes negative enough. The chopper op-amps are like impedance converters in this circuit. You can make it oscillate in ltspice too. Basically: you were lucky with the original parts. The circuit is under-designed. It’s not unlike designing stuff with discrete transistors and being lucky with beta. Or with differential in-amps and being lucky on linearity (been there, done that, still get a tear in my eye when I think of the brouhaha that resulted from that on mission of mine). I doubt it very much that the chip is fake, but even if it was: it’s certainly good enough to do its job most of the time, so the design is sensitive to whatever is different. And it can be cheaply made insensitive. Thus my assertion of under-design.
This really links in conceptually with your videos on what to charge for products, considering BoM, shipping, admin...and now considering potential product recalls and reworks.
I have the same issue on both of mine, serial numbers in the 4100 range from November 2016!
I bought them with the intention of looking at measurements on a scope but was very disappointed to see 100mV of noise on the signal which made it totally unusable.
Connecting it to a DVM it worked perfectly so I put up with it thinking it was just a poor design and not intended for connecting to a scope.
Now I know different and appreciate you doing a vid on this Dave with a great explanation of the cause. I should have queried it back in 2016!
My favorite eevblog in a while. Love a good mystery!
This is a great video Dave. As an electronics enthusiast, it's oddly discouraging however as I've now had a glimpse at a level of electronics troubleshooting that I'm sure i'll never reach. Really cool to hear you walk through the progression of troubleshooting and techniques to isolate the problem. Thanks again.
This is great! You finally get a repair video that is a bit of a challenge and it's your own product! Kudos! Wonderful video!
Thanks. it was very interesting, couldn't hold back sharing it.
It always makes me laugh out loud seeing your hands move in reflections. I love seeing how passionate you are about electronics.
I think it would be really interesting to see these parts tested in isolation.
Dave I really enjoy your videos. There is a slight amount of ego to your presentation but I rather enjoy it and under stand where it comes from. I was a professional automotive technician for 26 years and under stand the swagger and have come to appreciate quality workmanship and pride of doing it. I have had to switch professions into electronics and have learned more in 1 year from you than years in school. Thanks and keep the videos and rants coming...
First-world lab problem: too many scopes :D
This video is a masterclass example of general troubleshooting processes. It was very interesting, and frankly educational, to watch Dave go through the steps. I'm going to bookmark this one, this is great tutorial material
Thanks
I think that this video was very entertaining and I like it, but it’s not exactly the most straightforward troubleshooting process.There is an inherent trade off between an entertaining and watchable video and an efficient troubleshooting process. Most of my troubleshooting sessions would surely make for the most boring of videos: I measure a few things, ponder for a bit, measure some more, then simulate the problem (usually), fix in simulation, apply the fix, verify model-to-reality match of the fix, and call it a day.
“Mysterious” DC offsets in analog circuits are very often due to oscillations, so it perhaps should be the first thing tried. It’s useful to have a wideband “listener” for this: a 20kHz narrowband sampler - e.g. an old HP or Tek sampler box, with output to a speaker amp. You’ll hear oscillations rather clearly no matter what the frequency is (up into GHz). If you just get the input probe close to the circuit under test. I’m also skeptical of not having local decoupling loop on the output amplifier: one cap output-to-inverting-input, then a resistor from output to the output node, and another feedback from there to the inverting node: sometimes you can get lucky with a tiny chip inductor that would have desired ESR and high inductance to be DC feedback but not AC feedback.
Thanks, Dave. I really enjoy these videos. I'm not a trained EE, just a lowly systems integrator (which makes me little more than a glorified technician.) I hate to say that (at least where I work in the defense sector) this level of debug is starting to become a bit of a lost art. I work with a lot of EEs who I'm not certain have ever built anything outside of MultiSim/OrCAD/whatever. All well and good when everything works as expected, but they tend to flop and twitch when things go awry.
Videos like this, and your test equipment and basic electronics principles videos have really helped me professionally. I appreciate all that you're doing and always look forward to the next one.
tychosis "My simulation says that the circuit works, so it works, I don't care what your bench data says." If I had a nickel for every time I've heard this ..
Yep. If we were selling a simulated product, that'd be great. Unfortunately, we're not there yet.
Brilliant detective-work to track-down this fault. Thankyou for sharing!
This is the reason why big companies in the automotive market like Continental or Bosch require a customer nofication from the supplier whenever something is changing in the device; and I'm not talking about the design or the technology process but even a change in a transistor dimensioning counts. And they usually want to have proof that the parts that they will receive will still be in specification.
As an IC test engineer in the automotive industry this is quite a spill you have Dave :)
You should defenetly call in to Mouser / digikey or even call in On- Semiconductor / fairchild to let them know that this problem exists.
This is an fatal production error that they realy should correct
I wouldn't be so sure that the IC is at fault. In my opinion the circuit design is at fault and has marginal stability. All we are seeing here is a slight variation in opamp behaviour. All of them probably meet spec.
Well that's not quite right. Unless I can demonstrate actual oscillation of the opamp entirely on it's own within the claimed cap load spec, then it's really an application specific problem and not their fault.
I tracked down the designator codes which are on p12 of the latest data sheet. They don't match.
They totally know it already - it's on their Frequency vs C chart.
Interesting video! Looking forward to your tests of the "similar" parts.
Dave's voice hit Top 'C' at least three times during that first segment. You can tell this one was a real doozy to solve!
Wireline
he is trying to test the mic amp for oscillation as well :-)
He's so excited that he can't hide it!
gorillaau and you know, you know you know, I think I like it.
Thank you for absolutely brilliant peace of information.
I used to work for ON Semiconductor. Do yourself a favor, Dave, and stick with the Texas Instruments part.
A really interesting issue. I just checked my own stock, and I have some LMV321M5 (Nat Semi, marked A13 like your TI ones - these were purchased 2010 prior to the acquisition), and some LMV321WG (Diodes Inc, marked BX0jD). I might just swap in the Diodes Inc part on my uCurrent Gold (#00624, with LMV321 marked RC1F) and try this same test with my 121GW.
OK, I did some testing with my uCurrent Gold. First I checked it as-is with my 121GW, and was surprised to see 1.7mV. But I realised the 121GW was still on the AC mV range from an earlier test (FYI it read 1.7mV 35kHz). When switched to DC mV it read less than 10uV, as expected. I then swapped in the Diodes Inc LMV321WG and it read about 3mV, but the PCB was still quite warm from the hot air. Once it had cooled, it again read less than 10uV, and it stayed reasonably stable with additional capacitive load at about 70uV with 1uF. Finally, I pulled out one of my early (non Gold) uCurrents, and it used an LMV321 marked RC1K. It gave a reading of about 20uV with a shorting bar installed. So it appears the Diodes Inc part is a potential alternative if needed. I'm quite glad about that, because the original LMV321 went flying of the desk when I desoldered it :)
Thanks Kean. So that's now 4 out of 5 known good parts.
A capacitor from the negative rail to the +ve input of the voltage divider opamp is normal, taken from one end of the decoupling cap, to avoid unity gain stability problems. So is using wide short tracks for decoupling capacitor connections for the same issue (unity gain stability) 15:56 shows the teeniest of tracks which could have been a lot shorter and wider. Not saying that would cure it, just that it's normal practice to help keep Murphy away!
Had a very similar problem with the MAX4239 oscillating at low / no differential signal in a similar current sense circuit which previously worked fine (albeit single supply rail). After many approaches we ended up changing the opamp. Would be interested to know if replacing the MAX4239s with older also addressed the problem
Good problem and troubleshooting Dave!
In my younger days as a summer intern at an electronics company, we had a mysterious problem in a discrete uvolt
analog to frequency converter that was affected by too much lead in the silkscreen paint that caused leakage paths between components!
It taught me that you have to take everything into consideration and not discard what you might believe could never be the problem or, at least contributing to the problem.
Woah, first I've heard of that one.
Could you share where to get that the variable capacitor and variable precision resistor devices that you've been using?
Learned alot from this video thanks for that break down it was legendary!
This was very interesting. Great video!
*Interesting video with added value!*
Dave, could you explain why your DMM and the Keithley have such a "high" reactive load? Does it have a purpose? A side effect from something desirable?
Dave, have you considered just adding bypass capacitors between your virtual ground & the power pins on the 321? Seems to me that that would stabilise it. (Lowering your voltage divider resistors from 100K might work too, but of course that'd increase power consumption.)
Can you remove the oscillation with decoupling capacitance between the virtual ground and the + & - rail ? maybe pop a couple of 1uF in ?
He could probably do it by creating a new dominant pole (add a large output cap like 10uF or 100uF between the virtual gnd and battery negative).
I agree. I am an analog engineer and never put bypass caps between (+) & (-) supplies without first bypassing each individually to ground.
No u wouldn't've, switching noise is a digital problem.
Very informative even though I have suffered seemingly harmless production substitutions myself. It’s terrible with DRAM and flash where the manufacturers do buyouts and rebadges monthly
Once I was repairing LCD monitor where backlight was not working.
But CCFLs were ok, high voltage PSU also but it's driver was cutting it off after one second, probably detected that CCFLs are worn out a little bit.
At some point I hooked up my multimeter to HV output, and bingo, backlight was just working with my leads attached to the output.
And these leads have some capacitance.
Soldered 1pF 1kV ceramic cap to the output and monitor fixed...
Hi Dave, something I had show up on multiple occasions. Always attributed to stray capacitance that the fab house doesn't gurantee equal amounts of board capacitance in the order of 4pF. I got boards that exhibit 50 to 80 pF that I have to compensate for in design. Need to add as well, chip manufacturer designs components to specs valid in house which are at times harder to mimic in test. The closer the specs to acceptable margins means probably out of spec performance.
So this shows as out of stock on your store page. Is that because the problem you identified is being fixed? Any idea when this will become available?
Interesting vid. Part swapping is convincing evidence of the oscillation correlation but does not identify the underlying oscillation mode. For example, is the LM321 actually oscillating or causing the Maxim amps to oscillate through the Vgnd/load interaction? Your comments suggest that the Vgnd circuit action is trivial. Maybe the DC action is trivial, but the AC action is certainly not. Would be very interesting to learn the underlying oscillation mode. Thx.
I read on analog.com that when using a virtual ground you need to place a capacitor in parallel with the output of the resistor divider to preserve power supply rejection. So maybe whacking a cap across R7 would fix the problem.
Here is no issue with PSRR, it is an issue of missing a low impedance path from supply to load. This could be fixed by placing a shunt cap after the 270ohm resistor.
wrong, see the followup video on eevblog2
Are you going to change the test jig/procedure?
Isn't the avoidance of ringing/oscillation part of op amp 101?
I hope you have fixed this. 3:30 - The protection resistor R8 should be at the output pin, not before the feedback loop, right? Here the output load can feed in unimpeded noise through the C4.
why do you have such small paths? Would it not be sensible to have thicker lines between the components when you have the space on the board?
BTTF reference in your multimeter part numbers? YESSSS.
What is the device, a microammeter to be connected in series?
Is there a microvoltmeter available instead? To trace small currents in circuits, detecting voltage drop in pcb traces .
At the return path of U2: LMV321, pin 4 to pin 3. Would adding a compensation i.e. R&C pole & zero do the job?
What does a capacitive load say to an opamp?
hasta la visat, phase margin
Screwups happen. Digi-Key were kind enough to send me a new batch of ICs at no cost when they screwed up my old order and I got some async binary counters instead of 8-bit transceivers, and with no need to return the incorrect parts, since I was working in small quantity and the shipping was worth more than the chips themselves.
“Rings a bell”. If I recall, one Fluke meter has 4 nF input capacitance on the low ranges. Caused some surprises until I figured it out.
Motorola MJ802 used to be one of our favorite robust transistors and then it suddenly changed to be anything but. After various queries, we found that Motorola had moved the manufacturing to another bigger plant. Bigger wafers and other changes in the process. BAH! We went around and were promised great robustness by a company called Solitron for their version of 2N3771 or something of that number series. We tried and they failed on a short notice. When we pursued that issue, Solitron engineer eventually said that that we must stop using their transistors, because our application with 1 to 20 Hz pulsing caused thermal cycling stresses on the chip to base bond (soft solder connection). Would be fine with DC or over 100 Hz pulsing. Thank you very much for that clarification!
We never found a good replacement for the old MJ802, although MJ15015 survived a little better than any of the other substitutions.
GREAT JOB
On the output op-amp, the capacitor should go from the op-amp's output pin to the inverting input. This provides a negative feedback that is not delayed by capacitive loads.
For best results, one should try to buy from the producer. Also, one should try to select a producer that continuously supports it's ICs. TI has an online store and they do seem to care about quality preservation over shipment.
What changes in circuitry would be needed to compensate for all chip variance? if possible
If you are going to make a video showing the problem by directly adding a capacitive load to the output, can you also test when the TI part starts to oscillate?
Tom I could of swore he did, at around 100uF?
Dave - i recently bought one no: 5692 How do I know if this has the potential issue ? (can you confirm what batch numbers could be effected)
Hi, i like this little capacitance-"box". Thinking about building one or by one if it is not too expensive. Is this type available to buy somwhere?
Decap the chips!
Where can I buy the "µCurrent"? (preferred in Europe)
Time for “Approve Supplier” specification paperwork and new test procedure with capacitive load
Got really interesting here, because it is a real life problem. Sometimes in my job it is just hard to find the time to get in this deep with a problem.
Looking at the schematic, I would be wary of the high impedance present at pin 1 (the non-inverting input) of the LMV321. I would have liked to see the scope examine the 321's output at pin 4 when the MAX4239 opamps were oscillating to see whether the oscillations were present there as well. If not, it's difficult to see how the different 321s could cause the oscillations. If oscillations were present on the 321, then my first instinct would be to decouple pin 1 to each of the supply rails to reduce the ac impedance at that point.
The LMV321M5 is the former National Semiconductor IC while the LMV321IDBVR is the TI one. They may be even still made at different plants because of effects exactly like you found. Imagine TI buying ON Semi and then only making oscillating LMV321 chips at the ON Semi plant, that would piss some engineers right off. So at one point TI may officially declare the LMV321M5 obsolete and introduce the LMV32IDBVR as a successor.
I found some Fairchild data for chip markings and it does not even have the correct format of marking.
www.fairchildsemi.com/Assets/-topmark/dateCode2.ssi
The data sheet on on-semi right now has device markings in it and the format does not match that either. So I have no idea what that chip is but I doubt it is either On-Semi or Fairchild as it should be.
I have always trusted TI, Maxim and Analog Devices over other companies, i feel like they have met or exceeded their specs more often than even LT
Why is there a 100nF capacitor between V+ and V- in two different places in the schematic (C1 and C2)? Is the second one just a duplicate and thus is it redundant? Thank you for clarifying!
Because you always try to decouple the supply as close as possible to each chip, so that the inductance of the pcb tracks won't reduce the effectiveness of the decoupling. C1 will be physically located close to the MAX opamps and C2 close to the LMV321. In digital circuits prior to LSI, it was common to have a 100nF decoupling capacitor every few chips, so you could end up with numerous, apparently redundant, decoupling capacitors on a single board.
Noisy boy...
Dave's been watching AvE, I'm calling it right now.
I have come across opamps that, in a dual rail config, were not happy with a single decoupling cap across the rails, needing one cap per rail. Could it be something similar here?
This is not a usual split rail problem. Decoupling caps on the output of a virtual ground output are caps directly on the output of the opamp, that's BAD, and is the reason why the opamps can oscillate to begin with.
EEVblog I do not understand why it would ossilate when there is 270R on the output anyway. However If a cap was on the v-gnd opamp output (after R10) would the return path not be more direct than capacitance on the uCurrent output terminals skipping R8?
Personally I always find 47R adequate to buffer opamp outputs and keeps nice low output impedance. Telling the master how to suck eggs!
Output resistors are not magic, they just move the pole a bit, not totally eliminate any capacitive load issues.
I think any opamp should be decoupled against the LOAD COMMON terminal, i.e. the virtual ground here, not across the supply pins. I'd suggest trying that and see, if that improves. It should.
As I suggested earlier on Patreon side, please try 100nF or 1uF cap from virtual ground to both +V and -V. I am quite sure that tames the oscillation. It is quite clear that the oscillation is produced by the combination of LMV and MAX opamps, not just from LMV. And if I am right, please remember that I was the first one to notice that!
The reason is that capacitive loading of the uCurrent output creates low impedance current source (in higher frequencies) to virtual ground from +V and -V (which are coupled together by a cap) and this creates high frequency signal to LMV input. Two 1uF caps with 270 resistor creates low pass filter in kHz region which should be enough to tame oscillation. Or even two 100nF caps.
I'd like to state another thing: According to your schematic your feedback path in the OpAmp-amplifier has 9kOhm||10pF resulting in a 1.8MHz cutoff frequency. Notably, your oscillations are in the range of that frequency!In your µCurrent specs a -3dB-bandwidth of 300kHz. So you may additionally think about increasing the capacity from 10pF to 33 or 47pF. I am quite sure that would add additional stability and it may make the batches better comparable: Your 10pF are effectively increased to 12 to 15pF by the pads and line capacities - a quite big range that is significantly reduced by a larger cap.
There is no surprise that Fairchild part oscillates. It has a big hunch on it's Frequency Response vs C chart. From my experience it is a clear sign that it will oscillate on that frequency under some conditions. And TI datasheet has no hunches on it's Gain vs Capacitive Load chart.
Hold on, datasheet doesn't tell you what what the variant of the chip does? How else are you supposed to know? Or is it normal to have fingers crossed getting a reel that it'll work, can't believe that.
20:04 The Mouser datasheet seems to be / link to the Fairchild datasheet of the LMV321. The OnSemi datasheet (from their website) does indeed have the list of markings on page 12.
www.onsemi.com/pub/Collateral/LMV321-D.PDF
silly question, but you do not sit while working at the bench?
I know some are (they are often pilloried relentlessly for their forthright honesty) , but ...
Iimagine if car makers, politicians, PRESStitutes, public servants, fakebook, computer companies, drug companies and medical administrators were *ALL* this honest.
What a wonderful place the world would be!
I am so electronically illiterate but for whatever reason, I watched the whole video... It's like the adult version of Dora the Explorer. Every time you say AHA! I feel like I'm about to be enlightened and still, I feel like im in the dark..
Stuff like this makes me wonder how any consumer electronics ever actually work out in the real world. No way you could possibly test everything, every little crazy combination, etc... wow
Very often "ya takes ya chances".
This op-amp uses Fairchild top marking, according to the binary date code scheme was manufactured in 2006 work-week 16-23.
The confusion is because Fairchild was adquired by ON-Semiconductor, the making scheme between both companies were different and current ON-Semicondutor documentation in the website is a mess.
Hello,
I will give any comments to anyone rarely...but this video just demonstrated the real engineers cabability to give feedback to customer in any situations.. I have noticed that most of the time customers at (carribean area)industrial level are really happy when i tell bad news to them... Some times i have just told that this is fuck up, but i cannot fix it now because it is not my scope and they are still like heaven... This is because lot of service engineers dont have guts to point up other issues they have encounted..anyway customer is pleased that they know issue an can later fix it...
Will you contact Fairchild to see what they say about the part in your circuit? After all, they can't fix it if they don't know about the issue.
Just to add to your serial number database: in Nov 2016 I reported an issue where "Short" reported a -5.5 mV offset (on one of your multimeters!). My serial number is #004118.
With zero response, I "filed" this dud somewhere. I'll find it and e-mail you direct to discuss what to do about it
You should remove battery before using hot air. Otherwise it may pop right into your face :)
yoksel99 Soldering a 2032 lithium battery makes a good Dave go bad.
Whether or not it’s helpful, in all the projects I’ve done, whenever I’ve used any ST or On Semi. parts, they have always been the weak link, especially their op amps. Anyone else ever find these brands to be oddly unreliable?
Which rail forms the ground-plane of the circuit?
I'm a pretty green designer, so there's probably something I'm missing. Can someone explain to me why R8 is inside the feedback loop of U4, rather than being wired outside the loop, the way R10 is with respect to U2? If the purpose of R8 is to provide output isolation for capacitive loads, doesn't it have to be outside the loop?
Like I asked last video, where do I buy one Dave? Please send me a link
do you think that your virtual ground is happy without any capacitors? I would use some... to positive and negative rail, at least 100nF. you have 270 ohms on your virtual ground where U4 feedback can easily disturb U1 negative input. some slow op amps can handle such problems, but others with faster response can begin oscillating, just imagine these RC1F are actually faster op amps with much better specs, which begin oscillating easily?
Wouldn't a series resistor reduce this, if you are experiencing a problem?
can you short R10 when oscilates?
Hmm, so whether or not virtual ground generator is the "culprit", something is clearly able to feed through the capacitive coupling, since the error depends on a capacitive load on the µCurrent outputs. So this could reasonably be narrowed down to whatever is connected to the outputs.
What sticks out to me is the connection of R8, the 270 ohm resistor. Firstly, this resistor makes the output gain 10.27 instead of 10. But that's at the amp output. The 270 ohm resistor forms a voltage divider and the meter output is at the 10:1 tap. The precision loss from a 1% 270 ohm resistor would be below the 0.05% when taking into account the full scale of the resistor network. So far so good. And none of that would create any oscillation issues anyway, just a small measurement error.
The second thing that sticks out in relation to R8 is the connection of C4. The MAX4239 datasheet calls for a capacitor in series with the feedback element. As it stands, the feedback element is R13+R14+R8 whereas the capacitor is only across R13+R14. This could have two effects: 1) Not limiting the bandwidth of the feedback properly. 2) Somehow feeding interference back onto the input with whatever effects that might have. Regardless, the capacitor technically isn't connected as advised in the datasheet.
Things I'd try:
1) Move the C4 connection directly to the output of the MAX4239.
2) Move the R13+R14 feedback connection directly to the output of the MAX4239 as well. R8 would now be in series with the output but no longer part of the feedback loop. This should only be a problem with a very low impedance load which might sag the output appreciably..
3) Come to think to think of it, 2x 100n for the whole circuit seems a bit anemic given the symptoms. Might want to increase that a bit and see if that stabilizes the circuit.
4) Might also want to add decoupling to the virtual ground as well as some people have suggested.
The tolerance of R8 has zero effect on the output precision...
David Jereb Come to think of it, you're absolutely right. Anyway, that was just an aside to explain my thinking from start to finish.
Seems like the battery has too high of an internal resistance!
Tim in Austin Nothing beats adding switching noise on top of existing oscillation! :-)
It is funny how much all of us always come back to the absurdity of the Batteriser...I find myself doing the same in conversation a lot as well.
So the lesson of the day is keep good records.
Why don’t you try connect a capacitor to the input or output of U2?
I solved this mystery 3 years ago. The output stage is oscillating because of the 270 ohm resistor plus a capacitive load causes too much phase shift. It is marginally stable, so some work, some oscillate. See my reply and the design fix below.
Well, it is not the only problem with oscillation, as I discovered and fixed:
www.eevblog.com/forum/testgear/problem-with-ucurrent-gold/
And in my uCurrent is a RC1F opamp.
www.onsemi.com/pub/Collateral/LMV321-D.PDF page 12 RC1F does not correspond to any code on the data sheet. It's the wrong chip.
No, that's the TI part.
If you had actually bothered to check you would see that the On-Semi data sheet does not correspond to either part number on the chips. Stop criticising typing errors and get a life.
I guess that ON datasheet for for their own (Motorola legacy) production before they acquired Fairchild.
And RC1F is TI chip:
www.ti.com/lit/ds/symlink/lmv324.pdf
actually no the datasheet in the vid has Fairchild data with ON logo. 19 mins 41 secs. So nope.
Why not just decouple your voltage divider to stop it feeding back into the virtual ground amp? After all, the tiniest noise on that power rail will get into the input... it's a DC rail generator, so no frequency response is needed. I guess a 10nF would do it.
O'Toole's law says Murphy is an optimist. When dealing with electronics even the best laid plans of mice and men get fouled up by Murphy.
About 15 years ago we had automated controls on our pumps installed. All so that we could operate them from a remote computer. Every so often we had a pump shut down for no known reason. All of our equipment was designed to be stable even in the event of an EMP event including hardened CPUs designed for use in space. And everything was opto isolated so that nothing should get by. Wrong! It took two weeks for it to show up on the chart recorder but finally we caught the little spike that Murphy was throwing at us. It was getting by the batteries, the filter capacitors in the power supplies, the opto isolators, and triggering a fault inside the controller.
I hate SMD :)
Exactly same story here few weeks ago with DAC chip.... Tiny package, only one letter difference, and bang! The original driver FPGA (with no documentation and copy of the firmware) is became a super-fancy, super-hot egg cooker...
Nasty!
Daiyve, your next video will be a detailed account of your losses and possible damages to your very fine reputation. Or, you could just load up the Toyota and head out to PIne Gap, Australia. Lots of crap oscillating out there!