EEVblog #1057 - µCurrent Murphy

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.

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[ericksonengineering]

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

[SaNjA]

Send the chips to electronupdate for him to decapsulate and compare.

[mcconkeyb]

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.

[frazer]

R15 is upside down, electrons have fell out

[elektro-peter]

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.

[oswaldjh]

Murphy should be paying rent with all the time he spends in the lab.

[absurdengineering]

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.

[simontrimble]

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!

[andrewchen]

Are you going to add a capacitive load to your production test jig?

[tychosis]

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.

[cgibbard]

I think it would be really interesting to see these parts tested in isolation.

[DJ-Manuel]

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

[DrRChandra]

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.

[fohdeesha]

My favorite eevblog in a while. Love a good mystery!

[irgski]

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.

[codens]

First-world lab problem: too many scopes :D

[KeanM]

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.

[Mirandorl]

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!

[NetworkXIII]

I used to work for ON Semiconductor. Do yourself a favor, Dave, and stick with the Texas Instruments part.

[Catoblepa]

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 :)