LM399 reference questions - Page 1
Forum section: electronics / projects

author: LordXaos
author_rank: Contributor
timestamp: June 01, 2021, 07:50:19 am
content: Hi!I have designed and built a somewhat unusual LM399 reference, at least as far as I can tell. It does usually work quite well already, voltage staying within a few µV for hours on end, but there are a few issues I couldn't get to the bottom of, despite a lot of consideration and Internet/forum search.One thing is that I quite often found mention of the LM399 initial aging phase, which apparently experienced engineers can easily identify, but the clues ended around there. It would appear there is some more popcorn noise than later on, and the little I could gather is that this would be some shifting up and down of the reference voltage - but not too clear how much for how long, and if this changes once the reference ages.Another issue plagueing me is that while the reference stays pretty much spot on for hours, every now and then it experiences a brief spike, where voltage drops somewhat for a few samples (sampling occurs at about 3Hz with the DMM, so on the order of 1s maybe), but then recovers back.While I do believe this is due to some RF (?) injection into the sense connection I failed to recreate this situation in simulation: Whenever I tried to inject some signal into the sense connection it led to symmetric excursions and not just dropping. On the other hand the dips got lesser in scale the better I shielded the connection, so this might just be me not having a good idea how to draw that injection in SPICE.Another explanation might be an instability in the circuit, which I indeed could get in simulation, although this always led to a relatively regular oscillation and a clearly detectable pattern. Maybe it is also some mixture of both, or someone is fooling around with an EMP gun in the neighborhood...There are some further minor questions, but I think these two are the ones that confuse me the most. So if any of you has some idea or pointers to relevant information I would very much appreciate it!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2021, 09:33:09 am
content: The initial aging is mainly in the first 100 hours of operation or so. There is no clear end, just getting slower and slower over time. In the first few hours the voltage can go up and down and change direction. Later it would be mainly one direction and not much back. The datasheet shown 20 ppm typical drift for the first 1000 hours. I don't think there is much change in the noise. Some units may improve, some may get worse, but likely no general trend.From the 3 LM399 I have measured so far the popcorn noise of the LM399 shows as steps of some 5 µV or so, having either the upper or lower of 2 levels. Some samples may show more states, but so far I had only 2 levels (or 3 when for the ratio of two references). How often the jumps happen can be different. I have 1 that jumps about once a minute and a 2nd that jumps about once an hour.Keep in mind that the measurement direktly with a DMM is the combination of the DMMs reference and the DUT. So it may be 2 x LM399 combined. This also applies to the drift, though the DMM may be better aged already.Some EMI effect is possible, but the circuit is not expecially sensitive.

author: LordXaos
author_rank: Contributor
timestamp: June 01, 2021, 10:38:06 am
content: Thanks a lot, that makes the initial aging and popcorn noise a lot clearer!To somewhat compensate for the reference drift of the DMM I do occasional measurements with a second, different one, so at least chances should be low that they drift in unison.The sporadic drops at least partly seem to be related to my mobile phone (not a good idea to have it near the measurement, of course), I mostly fail to understand why the voltage only goes down. Might well be there is no real problem, but as long as I don't understand...I will try building a second prototype to see if behavior is similar.

author: Andreas
author_rank: Super Contributor
timestamp: June 01, 2021, 09:39:32 pm
content: Quote from: LordXaos on June 01, 2021, 07:50:19 amAll is possible. Some LM399 age relatively fast. Others need 2-5 kHrs to stabilize.2 examples of fressh LM399 references against a LTZ1000 here:https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg796829/#msg796829There are several mechanisms: Mainly relaxation of the epoxy glue between package and crystal. And crystal ageing effects.Popcorn noise is mainly created by impurities of the silicon crystal structure.So some devices have it others not. I also have read that popcorn noise within zeners correlates with ageing.One device may also have more frequent noise depending on operating conditions (temperature, current ...).I usually sort out devices with large popcorn noise (> 1 ppm).So it is a good idea to build 3-4 devices and compare them.What do you mean with "sense connection".RF is a problem on any device if you are diving in uV resolution due to rectification effects on the ESD-protection diodes.Here a example how to improve EMI-behaviour of a LM399 with different measures:https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684100/#msg2684100with best regardsAndreas

author: LordXaos
author_rank: Contributor
timestamp: June 04, 2021, 07:19:08 pm
content: Thanks also for the reference, this did give me a couple of ideas where to improve. I have seen some serious improvement already on the way, as the design started out on breadboard with obviously a lot of susceptibility to RF interference, and this got a lot better once I moved to a PCB, and again with a metal enclosure and shielded wiring. I do admit that my filtering caps are ... let's call it emperically chosen... and not really up to the task - I did solve quite some issues by rubbertaping a 150nF WIMA MKP10 to the DMM side of things, to catch some of the noise.But anyways, the rectification struck a bell and stuff made sense all of a sudden, it does explain the noise issues I have, so it looks as if - apart from thermal noise and whatever, this is at a reasonably low level - the major problems arise from the mobile phone or some neighbor WiFi interfering; the second prototype interestingly seems more immune to that.The "sense connection" is where I connect the reference to the DMM, there is no buffering on there, so any interference couples into the reference - which I did somewhat intentionally, as the reference will likely be used within some other device later on, where RF will be shielded by some other means anyways, so as long as I can attribute any large scale errors to RF interference I am goodOverall the goal was staying cheap, mostly, getting most of the long term stability of an LM399 for as little (financial) effort as possible. I do have a couple of contenders to compare to, like a MAX6350, which is less noisy, but the datasheet doesn't give useful long term stability figures (unless my reading skills are impaired), so I would likely combine something like the MAX6350 (or an LT1027 or whatever) which is really low noise with a more stable long term reference, against which it is compared every now and then. Might also be a futile attempt, but it does at least sound somewhat useful.I do agree that RF interference is a nuisance at µV levels, and I can live with that as long as I know it truly is RF interference, and not something else - if there is some inherent effect in the circuit itself which I missed, that would of course change the picture. This is why I want to rule out such stuff or at least be able to safely attribute to some known source.Anyways, thanks a lot the links and details have been quite helpful,Best regards,Bernhard

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2021, 06:48:14 pm
content: Remember that rectification from RF interference may be present in the output circuits rather than the input circuits, which has a special significance with test leads between the multimeter and test point. There may be a place for coaxial shielded test leads here. In the past I have gotten some improvement with wideband chokes at both ends of the test leads.

author: jmelson
author_rank: Super Contributor
timestamp: June 06, 2021, 07:10:55 pm
content: I used to use LM399s a lot. Now, I use LM4140's, and give them about 2.5 - 4.0 V supply. Above 4 V they get noisy.This is the quietest reference I've found. We had a device that had no noise immunity, the output amplitude shifted with supply voltage. I ran tests with a Keithley 7-digit voltmeter to confirm the supply changes perfectly correlated with the output changes. We use the LM4140 and the AD706 op-amp to provide clean power to this circuit, and it works great.Jon

author: LordXaos
author_rank: Contributor
timestamp: June 22, 2021, 06:22:16 am
content: A lot of further investigation seems to confirm that the dips were from RF interference, probably mostly from mobile phones as there weren't any when nobody was home, so that mystery is solvedI also now use shielded cables which lessen the problem a lot, and put the references inside a shielded box as well, which also helps to isolate them from gusts of air.Overall I have built 5 with a success rate of 60% nowguess the constant fear of a two year old sneaking up from behind and trying to touch the hot iron didn't exactly improve my soldering skills, but at least those 3 have survived. Two of them work pretty great and stay within a few µV for several hours, the third one is the oddball and oscillates between two voltages around 15µV apart, with a slight preference for the higher of these. I believe the LM399 is to blame, as I cannot find a correlated signal elsewhere in the circuit.The voltage really seems to jump between those two levels, which themselves are pretty stable... What probably amounts to wishful thinking has led me to believe that with further aging the preference for the upper voltage grew, but I might just make this one a gift for some friend who hasn't more than 4 digits of DMM available...The other two I apparently pretty much fried the same way, both while trying to fix some solder joints, apparently creating a short somewhere and thereby frying the LM399 next time I tried it... it will take some abuse briefly, but doesn't like it if I stare at the current draw in disbelief instead of turning off the power supply... Well, a lesson learnedThanks for all hints and tips,Bernhard

author: LordXaos
author_rank: Contributor
timestamp: August 16, 2021, 06:58:20 pm
content: Quote from: jmelson on June 06, 2021, 07:10:55 pmThese do look interesting from the datasheet. I did some experiments with a few other references also, and I also remember the MAX6350 as relatively low noise and quite stable, though I want to repeat the experiments under more controlled circumstances.Quote from: David Hess on June 06, 2021, 06:48:14 pmI now got two of the references properly mounted in an aluminium box, and the cable to the DMM shielded and connected to the box, which eliminated most of the nasty RF interference, keeping mobile phones away also helped. I now do get somewhat consistent measurements. For the better one of the two the Keithley 7510 shows a standard deviation of below 1µV over about 150 minutes (after warmup), which is far better than I expected.I tried measuring temperature coefficient but the result is largely meaningless I suppose - when I wrapped the enclosure in some cooling gel pack that has been in the freezer before I could get the voltage to change slightly, but not much more than it tends to drift around anyways, plus I don't have any control about the exact temperature this way anyways.The second working reference is slightly worse (like twice the standard deviation) and still exhibits occasional popcorn noise events (at least I believe them to be that), but still way better than I expected it to turn out.I do plan to build a few more (with an improved PCB layout) in the future to see if this was just sheer luck or is reproducible.