Binning LM399s - Page 1
Forum section: electronics / general

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 12:41:58 pm
content: Hello!I made a board to check the performance of some LM399 and I would like to hear some suggestion/critics:That board has 10 LM399H, sharing the same +15V from a bench power supply. The circuit is the simplest from the datasheet, but using the value 6K98 1% for the resistors:Here are some data already collected:The temperature in the graphic is calculated from a thermistor (3000ppm/ºC).The big spikes on the middle coincides with the opening of my lab's window.Here is the standard deviation:Looking at these data, it seems that the position of the references in the board influences the performance.What could be done to improve the performance?Is this setup ok for only to choose the better ones?Should I put the board inside a box, to avoid air flows influence?How many hours is necessary to know the better ones?Regards,Rodrigo.

author: Ian.M
author_rank: Super Contributor
timestamp: August 05, 2020, 12:52:27 pm
content: That's FUBARed. They are so close together that mutual heating is inevitable. Even putting that board in a polystyrene box isn't going to do much to improve the situation.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 01:52:34 pm
content: Are mutal heating that bad for binning LM 399s?That TiN LM399 board use 2 LM399 in contact one with another:https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg703440/#msg703440I could remove 5 LM399 from the board to have it more spaced. Or make another board with the LM399 in various rows.

author: Ian.M
author_rank: Super Contributor
timestamp: August 05, 2020, 02:05:56 pm
content: The ones on the ends are in a significantly different thermal environment than the ones in the middle of the row.I'd suggest spacing them out so you can fit a block of polystyrene foam, with a cut out for the LM399 over each of them leaving a gap between the blocks. Make all the polystyrene blocks as identical as possible. Support the assembled board and blocks in the middle of a much larger polystyrene box so the whole board is equally exposed to the airflow that will result from convection within the outer box. You'll probably also want a couple of temperature sensors, one in the middle and the other right at a corner so you can estimate the temperature differential across the test jig.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 02:28:39 pm
content: Thank you for the sugestions. It will be a nice experiment.Certainly there is a temperature gradient across the board surface, and it will interfere on the references output voltages. But, supposing a constant gradient, the performance should not be affected. If one reference is, for example, 2C above another one, but that difference remais constant, it should not interfere on the comparison results (standard deviaton).

author: dietert1
author_rank: Super Contributor
timestamp: August 05, 2020, 02:59:26 pm
content: I would wrap the whole board into a towel and exclude any records of spikes. You want to run ovenized references in an environment without or with little air movement. For example all our HP 3456A voltmeters have a 3D printed cover over their reference modules, in addition to the cover the LM399 comes with. Then standard deviation on minutes to hours time scales drops to about 0.1 ppm or even less for a good LM399. Of course that isn't something easy to check/prove. I don't know what multiplexer and voltmeter you are using.Regards, Dieter

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 03:35:21 pm
content: I will wrap the board into a towel, thanks for the suggestion.To reach that sigma=0.1ppm I think that is necessary much aging hours. These are brand new, or at least NOS LM399s.The setup I'm using is:Power supply: Agilent E3631AMultiplexer: Keithley 706 Scanner with 7064 Low Voltage Scanner CardVoltmeter: Keithley 2002

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2020, 03:44:18 pm
content: If you don't mind re-doing the Vectorboard, you could re-arrange the sockets into a regular polygon to reduce the inequality in their thermal environments.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 03:53:28 pm
content: Yes, Ian.M suggested something similar too.A circular pattern would be the best. Or put the LM399s forming a very sparse matrix.I did that unique line of LM399s to avoid air wire, as all traces could be done only using solder. And didn't use sockets to avoid contact problems.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 04:08:35 pm
content: Here are the data from a period of about 9 hours, without splikes:The better reference with sigma=0.33ppm.I wrapped the board into a towel, maybe that pattern of decreasing sigma with the position will change. I don't know if it is temperature related.

author: Badwolf
author_rank: Contributor
timestamp: August 05, 2020, 05:02:48 pm
content: HelloUnless you have something that produces a continuous air flow from the LM 399 number 1 to the 10, I don't think the temperature difference between all of them is that important.Moreover we can see a gradient from 1 to 10, so I would think for a power supply / current problem.Can you reverse your power supply and put it at the opposite side?Even if the currents are weak, there can be interactions with this type of card

author: dietert1
author_rank: Super Contributor
timestamp: August 05, 2020, 06:04:07 pm
content: So you are almost there. 0.33 ppm of 6.9 V is only 2.3 uV. So another thing to watch out would be thermal voltages at the scanner input connection and in the link from scanner to voltmeter. To check that i would wire a short that i can measure with the voltmeter through the scanner so see how stable the residual voltage is. Or if you have two spare channels on each end of your "1" .. "10" lineup, you could try and see whether the "1" end is worse.Another consideration would be EMI interference: using shielded cables, adding blocking capacitors on both heater and reference output of each LM399. Andreas wrote up lots of tests, including the LM399:https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684070/#msg2684070Regards, Dieter

author: bsdphk
author_rank: Regular Contributor
timestamp: August 05, 2020, 07:25:15 pm
content: "The ones on the ends are in a significantly different thermal environment than the ones in the middle of the row. "Mount them in a circle ?But still: They shouldn't be in free air, cover them up with something which does not touch and does not leave enough free space above for the thermals to get too strong.

author: Andreas
author_rank: Super Contributor
timestamp: August 05, 2020, 07:48:22 pm
content: Some general rules for LM399 (since your results are still much too "noisy")- use a well stabilized power supply (also for the heater).- best PSRR is given with heater voltages >= 15V- use bypass capacitors 100nF directly at the zener (+ heater) pins- shield pins from air flow (not only on the top but also on the bottom side of the PCB).- do not tilt the LM399 this may give several PPMs deviation. (see pictures here)https://www.eevblog.com/forum/metrology/project-pimp-a-keithley-2000/msg1106829/#msg1106829- for highest stability use LM399s with a zener voltage near 6875 mV @1 mA (gives lowest TC between heater off and on)- keep EMI sources (LED lamps, SMPS, WIFI, Mobiles) away- and of course you should give the LM399s minimum 14 days run in time before doing measurements.with best regardsAndreas

author: maat
author_rank: Regular Contributor
timestamp: August 05, 2020, 08:07:41 pm
content: Typically, the first thing, that you should be looking out for, when binning LM399 is popcorn noise. This is rather a rather frequent problem. For my university project I was binning LM399s and I had to throw our 10 out of 25. (All but 4 came straight from Mouser).Attached I have a log of one the Zeners:To add to the confusion, I tested each diode in an amplifier circuit, which amplified the Zener voltage by (20/17+1) to about 15V. Since my 34470A does not have a 20 V range, I was usinganotherknown good Reference at 15 V (tested against a Fluke 5440B) and measuring the differential voltage. So keep in mind, that the (white) noise is roughly 2.2*sqrt(2) of a single Diode and the popcorn noise of the zener has to be divided by the gain (~2.2). The data was recorded by a 34470A in a stable enviroment (< 0.5 K) using 10 PLC and Auto Zero = On on the 1 V range. The Zeners were burnt in for 100 h prior to testing.So you will looking for popcorn noise on the order of 2 µV for your 6.95 V reference.This is what a good example looked like:Once you have weeded out the worst offenders, burn in the the rest for a few weeks, then start monitoring.Another option is to use a low noise amplifier and have a look at the 1/f noise of each diode.

author: Andreas
author_rank: Super Contributor
timestamp: August 05, 2020, 08:10:31 pm
content: Quote from: rodpp on August 05, 2020, 12:41:58 pmmany: there may be some "popcorn noise" which shows up only from time to timesee comparison of 2 LM399s here (Ch7 has large initial ageing whereas CH6 has up to 2 ppm popcorn noise)https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg796829/#msg796829with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 05, 2020, 08:19:20 pm
content: Quote from: maat on August 05, 2020, 08:07:41 pmshure that your instrument has no popcorn noise?You should have at least 2 instruments to decide wether the pocorn comes from the zener or one of the instrumentsIt took very long time until I recognized that my K2000 has sometimes popcorn noise too.with best regardsAndreas

author: maat
author_rank: Regular Contributor
timestamp: August 05, 2020, 09:16:48 pm
content: Quote from: Andreas on August 05, 2020, 08:19:20 pmPretty sure (Of course I can never be sure). I measured the first sample against the Fluke 5440B for a week, both with a 3458A and the 34470A (and for the fun with a DMM6500 to see if it could do the job) The noise is fairly typical for the LM399. Any dips you see is most likely other people in the lab.To further reduce the impact of the DMM it was running on the 1 V range. So unless the front end amp is popping its corn I doubt that I would even see any popcorn noise at all using this setup.Anyway here is a weeks sample of which I have many more:I also screened the op amps on the test board for popcorn noise and as I couldn't believe the number of diodes I was throwing out, I also measured the 1/f noise of both good and bad diodes. There is a fairly distinct difference. Measuring the 1/f noise actually gives the fastest results (within hours), but makes for less dramatic pictures in a thesis, so I took the time-domain approach.

author: bsdphk
author_rank: Regular Contributor
timestamp: August 05, 2020, 09:36:36 pm
content: "- do not tilt the LM399 this may give several PPMs deviation. (see pictures here)"Isn't that just the plastic-housing touching the can and the leads at different points depending on the direction of the force of gravity ?

author: rodpp
author_rank: Frequent Contributor
timestamp: August 05, 2020, 11:24:46 pm
content: Wow! Thanks for all contribution! I'll address each one.But before that, I would like to show the results after the @dietert1 towel suggestion. It improved the results significantly, probable reaching my setup capabilities.Here are the best one hour of data collection after wrap the board into a towel:The best reference now with sigma=0.06ppm, or 414nV.Note that the temperature is not stabilized yet.The specification of the Keithley 7064 Low-Voltage Scanner Card that I'm using says:"Contact Pontential: < 1uV differential voltage, input to output with copper leads (<200nV typical< 1 minute actuation)."Another observation is that there is no more a pattern where the sigma decreases with the position, the [performance x LM399 position in the board] is more aleatory now.The graphic above is of a very limited data set, only 6 samples. But looking at all data collected, it is very clear the difference after the holy towel:

author: rodpp
author_rank: Frequent Contributor
timestamp: August 06, 2020, 12:20:50 am
content: Quote from: Badwolf on August 05, 2020, 05:02:48 pmYes, it is very easy to put the power supply at the opposite side. It is one more good test to do.Quote from: dietert1 on August 05, 2020, 06:04:07 pmYes, it is possible to check the scanner channels. The board I'm using has 20 channels and I'm using only 11 (10 LM399 and the thermistor). I connected the odd channels to the LM399s and the even channel to ground (except the last one, connected to the thermistor). Another test to include in the to do list...And thanks for the link, I'll read it.Quote from: Andreas on August 05, 2020, 07:48:22 pmThe power supply is direct from the Agilent E3631A to the board green borne. See below the last 24h of the power supply voltage measurement, it's from 14.993-14.998:I'll check the noise with the oscilloscope.I'm using Vcc = 15V.Yes, I'll add the capacitors. One more in the to do list.I'll observe all your considerations to improve this test.Regarding the run time before doing measurements, today is the second day that that references are running. The LM399s I'm using are new ones, never used before. So, it seems that I must wait some days before any conclusion.Quote from: maat on August 05, 2020, 08:07:41 pmI'll pay attention in the noise. I'm using NPLC = 10, Auto zeron ON and filtering 10 samples (sequential, not moving average).Quote from: Andreas on August 05, 2020, 08:10:31 pmInteresting graphic, looking at the graphic scale (total of almost 2 years) the CH6 popcorn noise seems somehow sporadic.

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2020, 08:28:39 am
content: Quote from: rodpp on August 06, 2020, 12:20:50 amPopcorn noise can be sporadic on a scale of hours to days. It took me a month to track it down in a Tektronix made integrated circuit.Since it is related to processing, I am surprised that any modern analog IC would suffer from it.

author: KK6IL
author_rank: Regular Contributor
timestamp: August 06, 2020, 09:11:56 am
content: A few days ago, I set up a crude breadboard to burn in some LM399's, 3 board pulls from China, and 4 NOS parts I got as samples back in the dark ages, National date code 630 (1976?).One of the NOS parts gave erratic readings on a 3456 set for 6 digits, 100 cycles, jumping from 7.10373 to 7.10394 from one reading to the next. Photos of the noise viewed on Tek 7834 with 7A22 plug-in, 100 uV/division, not the 1 mV reported by the readout. Lots of 50 uV short spikes - occasional long duration changes. I wouldn't have been surprised if one of the China parts was bad, but wouldn't have expected it from a factory sample. Anyone ever seen a problem like this? Any possibility it will get better with aging?I moved the part from the far right end of the pegboard to the far right, replacing the wires and 7.5K resistor (15V power supply) - no difference.Yes, a crude setup - way beyond help from a towel. I just wanted to get a few weeks aging on the parts, and see how much the voltage changed.John

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2020, 09:32:30 am
content: Quote from: KK6IL on August 06, 2020, 09:11:56 amIt looks like popcorn noise to me but make sure it is not in the 7A22 or 7834. The usual giveaway is that the amplitude shifts by a common amount, but for variable duration and timing.The photograph below shows the popcorn noise that I tracked down to a channel switch in my Tektronix 2230. It sure looks digital and that is what I initially thought but it is a very analog process. It was absent for days at a time which made tracking it down very difficult.I would not count on it getting better.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2020, 10:19:21 am
content: Quote from: David Hess on August 06, 2020, 08:28:39 amThe popcorn noise is usually attributed to single quantum states (e.g. electron occupation at some defects or impurities) that have a relatively large impact because they sit a some sensitive point. One would not note most of the defects with only a weak effect, but they can still contribute to 1/f noise.While impurities can be reduced, the other prime candidate are defects at the oxide interface, that may not be avoidable. With smaller structures there may be less defects, but more of these can have a large effect.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 06, 2020, 11:33:31 am
content: The pictures anexed using the forum tool was a mess. I uploaded the pictures using Imgur and updated the posts. Sorry for the incovenience.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 06, 2020, 02:04:02 pm
content: Today I received some LM399H, and to my surprise they came without the thermal shield:I already contacted the vendor to return it.Is there any reason to not return it, even if the seller offer a discount?

author: dietert1
author_rank: Super Contributor
timestamp: August 06, 2020, 04:04:19 pm
content: Difficult to answer, since you didn't tell the numbers.If i had one of those i would try and make a thermal insulator with a 3D printer. It would look more like those covers used with LTZ1000 references, something that gets fixed to the board. The original LM399 cover that touches the metal case and is free for minor movements, is a good solution yet one may still improve it. What if the so-called popcorn events were simply movements of that plastic hat?When soldering those references to a large board i would build in some bends into the wires, to take away board bending movements from the reference case by elastic deformation of the wires. I mean something like an s-curve. Bending the wires needs care to avoid stress on the hermetic seals of the metal case.Regards, Dieter

author: rodpp
author_rank: Frequent Contributor
timestamp: August 06, 2020, 04:37:36 pm
content: The price will be around US$2.5.I could buy ones with the thermal shield for around US$6.So the naked ones are less than half the price.Unfortunately I don't have a 3D printer.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 06, 2020, 09:35:46 pm
content: I found a 3D printing service here that is not expensive. I'll keep the "naked" LM399s and order the thermal shield from that 3D print service.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2020, 09:49:24 pm
content: The original caps are a special high temperature plastics. For a printed cap one may consider a slightly different cap, that does not touch the metal can.

author: dkim5nu
author_rank: Newbie
timestamp: August 07, 2020, 12:20:51 am
content: Where can you get these for $6 a piece???

author: wraper
author_rank: Unknown
timestamp: August 07, 2020, 12:27:16 am
content: Quote from: Kleinstein on August 06, 2020, 09:49:24 pmShould get ABS as minimum, PLA most likely will become soft as it has 60oC glass transition temp.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 07, 2020, 01:10:53 am
content: Quote from: Kleinstein on August 06, 2020, 09:49:24 pmYes, or a heat resistant material. I suppose that it supporting around 90ºC is ok.Quote from: dkim5nu on August 07, 2020, 12:20:51 amIn the brazilian eBay, NOS components. In eBay there are some too:https://www.ebay.com/itm/LM399H-PBF-Fixed-Shunt-Voltage-Reference-6-95V-5-4-Pin-TO-46-TU/133128046030?hash=item1eff0cc5ce:g:2sYAAOSw~YldP4QaQuote from: wraper on August 07, 2020, 12:27:16 amThe print house suggested ABS, Petg, or PLA HT (High Temperature).

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2020, 03:03:50 am
content: Quote from: Kleinstein on August 06, 2020, 10:19:21 amMy favored model is that 1/f noise is the accumulation of different levels of popcorn noise and generated by the same mechanisms.

author: 2N3055
author_rank: Super Contributor
timestamp: August 07, 2020, 06:58:17 am
content: Quote from: David Hess on August 07, 2020, 03:03:50 amI also think that is most probable explanation...

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 07, 2020, 07:41:46 am
content: Quote from: rodpp on August 06, 2020, 02:04:02 pmVery strange that the bare can IC is sold, I've never seen that before. The plastic cap is made of Polysulfone, VALOXTMof General Electric, because the chip runs at 90°C, which lets most other plastics get brittle after short time. Therefore, 3D parts from random plastics will not work.My only very old stock part looks like this. I assume from this old logo, that it is also from National semiconductor, bought from a regular distributor in between 1992 - 1995, don't remember any more the exact date.I opened the plastic cap (broke a small edge off), and the metal can is completely blank, no imprint on the top, nothing on the side.Maybe yours are fakes or rejected ones for both reasons, coming with no plastic cap (which is difficult to fake) and with an imprint.FrankPS: The old logo was changed in the era of CEO Gilbert F. Amelio, who took over in May 1991.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 07, 2020, 08:11:31 am
content: It may be a good idea to check a few before ordering more plastic caps, just in case they are fake.Chances are the chips are reclaimed / un-soldered parts. In this process the cover may get damaged / removed. I have seen pictures of old chips without the case, that did work OK.

author: wraper
author_rank: Unknown
timestamp: August 07, 2020, 10:00:51 am
content: Quote from: rodpp on August 06, 2020, 02:04:02 pmWhere did you get them? There is a chance they are salvaged ICs pulled out of their cases, with new legs welded on and marking made right on the metal. And as they stayed in the case previously, they would look new. Not to say you should check them as they may be duds to begin with. I don't see any mention of part number without a heat shield in the datasheet.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 07, 2020, 03:48:20 pm
content: Thanks @Dr. Frank, @Kleinstein and @wraper.After your comments I checked carefully and the chips seems to be salvaged ones.I didn't open the bag, because there was a chance that I could return it.The legs are silver, not gold. It is not very clear, but it seems to be welded.So I assume that these chips are salvages.Even considering that aging in references is desirable, and the price is 1/3 of my other options, I'll return it.

author: guenthert
author_rank: Frequent Contributor
timestamp: August 07, 2020, 04:04:18 pm
content: Quote from: rodpp on August 06, 2020, 02:04:02 pmYou could use them in a temperature controlled enclosure (oven), w/o the internal heater, like a 329. Given that you then can chose a considerably lower temperature (say 45°C, instead of 90°C), long term stability might actually improve. Of course you don't want to pay more than for a 329 then ...I'd buy a few.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 07, 2020, 04:20:38 pm
content: When using multiple units together, it may not be so bad to not have the cover. A common cover for some 2 or 4 units may be OK. The original cover that has contact to the case, but can still move is also not ideal. So an external non touching cover may even have some advantages and may not need to be as temperature stable if there is not too much external isolation.Without the heater it would compare to the LM129, so it still has the hermetic case, which is a big advantage over the plastic of the LM329.New welded legs could be a problem, as there may be additional thermal EMF that may depends on the board temperature.

author: rodpp
author_rank: Frequent Contributor
timestamp: August 10, 2020, 05:47:23 pm
content: Today I returned the LM399 without the thermal shield.Here are the data from the last three days:Next step is to perform the suggested modifications in the above posts.

author: dietert1
author_rank: Super Contributor
timestamp: August 10, 2020, 07:32:12 pm
content: When you start working on your board, please make two separate ground networks, one for the reference parts of all LM399s and a second one for the heaters. This will improve your setup.If the heater current of a LM399 changes by 10 mA for example and your common ground network has 0.1 Ohm from that LM399 to the reference point that you use for voltage measurements, you get a voltage error of 1 uV. Such errors may very well add up to 1 ppm or so. Better you have two separate ground networks. Current in the reference ground network will be small and near constant. The heaters don't care for uV or mV voltage differrences on their ground network.Regards, Dieter

author: Svgeesus
author_rank: Regular Contributor
timestamp: August 13, 2020, 07:32:48 pm
content: Or better yet, if you have a bipolar supply available: run the heater circuit across the positive and negative supplies and keep it away from ground entirely. There is an example in that datasheet.Also amplify the output voltage and use that to generate the zener current in a feedback loop, so that it is as stable as possible (there are plenty of examples of that, too).

author: rodpp
author_rank: Frequent Contributor
timestamp: August 13, 2020, 11:59:54 pm
content: Yes, after reading your post I checked the datasheet and I could use that symmetric source circuit. The power supply I'm using is symmetrical.Regarding the amplifier with feedback to generate the zener current, I'll do that in the future. This experiment is only to select the betters LM399.Early today I modified the grounds of the board as sugested by @dietert1 and added capacitors at the zener outputs. Let's see how it will behave before changing to the symmetrical source, if necessary.