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My mother today emailed me that she considers the LM399 thermography movie by Andreas and branadic "art"
Why art? -
FYI there are LM399 (NS) on the bay at $4.60. I bought some a few months ago and they where OK, even if I have not done any deeeeep measurements on them. Seller Polida2008. I have no affiliation whatsoever.
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Galaxyrise posted a link to AN-161 (National Semiconductor / TI), which I found very interesting and didn't know before. We had a discussion about lead length in this thread. Here is what the AppNote says:
"...Thermocouple effects can also use errors. The kovar leads from the LM199 package from a thermocouple with copper printed circuit board traces. Since the package of the 199 is heated, there is a heat flow along the leads of the LM199 package. If the leads terminate into unequal sizes of copper on the p.c. board greater heat will be absorbed by the larger copper trace and a temperature difference will develop. A temperature difference of 1°C between the two leads of the reference will generate about 30 ?A. Therefore, the copper traces to the zener should be equal in size. This will generally keep the errors due to thermocouple effects under about 15 ?V.
The LM199 should be mounted flush on the p.c. board with a minimum of space between the thermal shield and the boards. This minimizes air flow across the kovar leads on the board surface, which also can cause thermocouple voltages. Air currents across the leads usually appear as ultra-low frequency noise of about 10 ?V to 20 ?V amplitude..."
This confirms what I already stated here:
http://dg3hda.primeintrag.org/doku.php?id=lm399_thermographie
Keep lead lengths short. -
What if you did the opposite, keeping the leads at the longest, but using a styrofoam sheet(a thick one, going the whole length) as a spacer.
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FYI there are LM399 (NS) on the bay at $4.60. I bought some a few months ago and they where OK, even if I have not done any deeeeep measurements on them. Seller Polida2008. I have no affiliation whatsoever.
Also bought 4 LM399 from that sellers, still in the sealed bag he put them in, I need to make some small pcb's for them
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A temperature difference of 1°C between the two leads of the reference will generate about 30 ?A.
Keep lead lengths short.
Ok, I have finally evaluated the measurements = .csv tables of Branadic from the LM399 thermograpic measurement.
Long leads without slot: around 5 degres stray within pad + 1,7 degrees from pad to pad
Long leads with slots: around 5-6 degres stray within pad + 2,3 degrees from pad to pad
Short leads without slot: around 9-11 degres stray within pad + 3,1 degrees from pad to pad
short leads with slots: around 12-15 degres stray within pad + 6,6 degrees from pad to pad
So from the measurement the conclusion is that with long leads the stray (=thermoelectric voltage) within pad is lower than with short legs. And the thermoelectric voltage between different pads is lower without slot.
If there should be mechanical issues the slots should be at least around 15 mm from the reference to give the pads the possibility to equalize the temperature.
On the thermography with short leads I have seen up to 5 deg C more difference within pad and 1,4 deg C more difference between the soldering pads
So what is better: having up to 5 K * 40uV/K = 200uV voltage difference within the soldering joint (or 52uV between 2 pads) or have longer leads which can be thermally isolated by other means? So I´m still not shure wether long (good thermally isolated) legs or short legs are really better.
For me only one thing is clear: thermal isolation of the legs and solder joints as good as can.
And no slots (thermal mass on the PCB) to equalize the temperature of the solder joints.
With best regards
Andreas
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Sadly, I blew up my best LM399. (I'm still not quite sure why that experiment destroyed things, either.)
Of the references that survived that experiment, I took 5 with fairly similar voltages and wired them in parallel, averaging their voltages with 5ppm/C 1k resistors. To supply the reference current, I used a LT1236 and some 5ppm/C 3k resistors. I think all the support circuitry drifts are attenuated by at least 1000:1, but I used 5ppm stuff anyway
Inspired by Mickel T's super-insulated reference and the Bob Dobkin comment to keep the heater current down, I also put them in parallel thermally as shown in the attached pictures. (I've replaced that proto board with a proper PCB, but didn't take another picture before taping it all back together.) With a 30V heater supply, the combination of 5 LM399 draw about 15.5mA heater current at around 15C ambient; 3.1mA per LM399, which is a decent reduction in heater current, I think.
I monitored the setup overnight along with ambient temperature changes to get a rough feel for the relationship between heater current and ambient temperature: about 150uA per C. I then put the foam cube inside a cooler and monitored the results. At its peak, the heater current was 20.25mA and the voltage shift looked to be around -26uV; a fine result, I think, and suggests I'm getting better than the nominal 0.5ppm/C tempco.
I want to do the 7V to 10V conversion with LTC1043s, but that involves much faster rise times than I've designed for yet.
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Hello,
interesting results.
Which output voltage do the references have?
(I have always to think of the "sweet spot voltage" between unheated and heated device.)
It is not unusual that the LM399 near room temperature
is much better than over the whole temperature range.
My LM399#2 (National Semiconductor with 6860 mV output)
has around 9uV shift over 10 deg C to 45 deg C.
A part of it (the "hysteresis") are obviously thermocouple induced voltage shifts.
To your sudden die of the reference:
It is winter. Perhaps you have zapped it by ESD.
This is one reason why I place a 100nF capacitor (SMD) directly on the
pads over the zener output and over the heater input.
Besides ESD it also rejects some mains line noise for the case
that I use a linear power supply instead of batteries.
With best regards
Andreas
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FYI there are LM399 (NS) on the bay at $4.60. I bought some a few months ago and they where OK, even if I have not done any deeeeep measurements on them. Seller Polida2008.
I was worried because those were not available a week or two back but you were right about him relisting after the chinese holiday. Since these were available for slightly less than the cost of my meter (self imposed budget for that project) I ordered one. Two friends also got in to take advantage of his combined shipping so cost was even less. Your lead was greatly appreciated. Everybody else wanted $10-$15. Some even charged 2x that or even more than the cost of an LTZ1000. Typical Ebay tricks. -
To your sudden die of the reference:
For the charts I produced awhile back in this thread, I had the 10 LM399 in series driven by a 1mA current source, so they'd all have the same current during measurements. I had my DMM switching between them via switching card. I ran like this for days.
It is winter. Perhaps you have zapped it by ESD.
I tried to improve the resolution of this measurement by using another 7V reference on a different supply. So the switching card was now moving the ground point of that extra reference and the DMM was measuring the difference between each LM399 and the 7V reference. After some minutes of this, the first LM399 in the chain would fail short. By the time I caught it, three were dead.
There was definitely LM399-killing potential in the circuit, but I'm pretty fuzzy on how it was getting carried over from the last to the first LM399. I wonder if the capacitors you suggest would have prevented the damage.(I have always to think of the "sweet spot voltage" between unheated and heated device.)
The resulting average is 7.0689. None of the units I got were under 7V. I didn't record normal operating voltage on them individually, but I believe they ranged from 7.05V to 7.08V. -
I tried to improve the resolution of this measurement by using another 7V reference on a different supply.
What kind of supply?
- Battery?
- linear supply
- switch mode supply
Especially the switch mode supplies partly have rather large capacities (around several nF) between mains line and output. These could have forward biased the internal diode (do not forward bias above 0.1V) within the LM399.
And also linear supplies may have some 10 to 100 pF (parasytic) capacities.
10 Zeners in series (70V) would also violate the max 40V spec of the internal diode if the heater is supplied from a common source.
With best regards
Andreas
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Quote
10 Zeners in series (70V) would also violate the max 40V spec of the internal diode if the heater is supplied from a common source.
They weren't. I did try to take that 40V spec into account, at least, and the failures were closely correlated to the DMM scanning and not the static setup (which had been running for days before I did the scan.)What kind of supply?
Both the LM399s and the 7V were on linear supplies.QuoteAnd also linear supplies may have some 10 to 100 pF (parasytic) capacities.
Some capacitance-related answer certainly seems likely. Parasitic to earth ground, you mean? Hmm... Perhaps I'll try to recreate it with resistors instead of LM399s and see if I can track it down. Seems likely to be educational at the least! I don't want to clutter up this thread with it, though; I'll put something in Beginners if I do play with it some more. Thanks for your input!
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Time to play with LM399 (actually LM199 from one of 2001 boards).
To do: ratio units for conversion 7V -> 10V, 7V -> 20V.
Meanwhile during debugging: some values pretty far from 10.00000VDC
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I Just wanted to share a photo of the LM399 (die marked 199A) die that I found on the web. I couldn't see it posted in the thread anywhere.
The die looks about as asymmetrical as it's possible to get with the heater down the right hand edge. Not sure but I assume the buried zener is the circular deep well in the center??? There's another one bottom right in the heater section (the chip schematic shows two in the heater).
Anyway, hopefully of interest....
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Gents, and experts on LM399 ...
I have a couple of these desoldered from decommisioned equipment .
Unfortunately I only had eyes on the OP27, OPA627s, MAX420s etc on the boards, not the LM399s and therefore didnt focus on these little fellows.
A couple of them however fell of the boards easily when I used my hot air gun (1500 W) to remove the opamps. These later chips have all been tested satisfactory with regards to operation but I have no idea about actual temperature exposure but that they came of the board very easily (thin covar leads are bad thermal conductors compared to an 8 pin DIP leads) indicates comparatevly low thermal stress.
I would estimate that the equipment in place would have been in operation for several years, all chips are datecoded early-mid 90ies.
In your opionion, if these LM399 are in working order at power applied, do you think they are "finished ageing to reach maturity" or did I start something bad by desoldering? Any opionion or guestimate is welcome, if not for learning something new...
And yes, I have already slapped myself...
regards,
Rickard -
Hello,
my experience with references:
every stress to the package (including soldering) may (= most probably, or sometimes may not) start a new ageing cycle.
The chip and the mounting plate of the package (and the epoxy die attach) have different thermal expansion.
With best regards
Andreas
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Last week I finished one of TiN LM399 10V ref using Vishay UXB0204 thin film resistors and LM399 (non A) reference.
Current output voltage is slightly off, I'm waiting for Vishay 1280G pots.
I surprised how stable it is during burnin period (<1ppm). Zener current is set to 4.5 mA ( in datasheet it is set about 1mA )
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Wow, cool. You did more progress on this module, that I had
Would be happy to see it in good use with some measurements. -
Zener current is set to 4.5 mA ( in datasheet it is set about 1mA )
There is no advantage in setting the zener current above 1 mA.
(see statement of Ken).
Maximum stability of LM399 will be at 1 mA.
With best regards
Andreas
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Wow, cool. You did more progress on this module, that I had
Thanks, I do not think so, I also needs to spend more time on LTZ1000. For measurement I needs to finish the climate chamber and some MUX. I would like to assemble additional three PCB with LM399AHs.All with Vishay UXB resistors and same enclosure.
Would be happy to see it in good use with some measurements.Zener current is set to 4.5 mA ( in datasheet it is set about 1mA )
There is no advantage in setting the zener current above 1 mA.
(see statement of Ken).
Maximum stability of LM399 will be at 1 mA.
With best regards
Andreas
It was my dilema. I read the response from Bod Dobkin about 1mA Zener current provided by Ken, but based on the noise comparison in LTZ1000 datasheet I decided to increase it at least for first try. It can be interesting to make noise measurement dependency on zener current like it is in LTZ1000 datasheet. So compare something like 10uA-> 20mA can confirm this. I know that I find reason to buy decent decade box sooner or later and this is the case ;-) -
@Plesa #334,
You should be using either Bourns 3250 or 3290 wire wound pots (these are military qualified) if you want long term stability, film type pots are not nearly as stable over the long term (2-3 years compared to decades) and also require a minimum amount of current to flow through the wiper to keep it in 'good shape'. The WW pots do not really have to have any current flowing in the wiper but 1uA-3uA is suggested. The Vishay 1280G pots may have a bit better resolution but they are not nearly as stable long term and also cost a lot more than the WW pots.
The main drawback of the LM399/A references is noise compared to the LTZ references, about 4-5 times higher but at the same time they can be just as stable over time as the LTZs. If your application does not need the lower noise level of the LTZ then the LM399/A is certainly the lower cost way to go. As Bob Dobkin said, the higher current level does reduce noise but increases drift, there is no way around that tradeoff, if a little higher drift is okay then you are set with the higher current but noise is a very important spec and sets the minimum accuracy floor of the reference, you cannot filter the noise out from a zener so if you need low noise you must start with a low noise source. -
@Plesa #334,
You should be using either Bourns 3250 or 3290 wire wound pots (these are military qualified) if you want long term stability, film type pots are not nearly as stable over the long term (2-3 years compared to decades) and also require a minimum amount of current to flow through the wiper to keep it in 'good shape'. The WW pots do not really have to have any current flowing in the wiper but 1uA-3uA is suggested. The Vishay 1280G pots may have a bit better resolution but they are not nearly as stable long term and also cost a lot more than the WW pots.
The main drawback of the LM399/A references is noise compared to the LTZ references, about 4-5 times higher but at the same time they can be just as stable over time as the LTZs. If your application does not need the lower noise level of the LTZ then the LM399/A is certainly the lower cost way to go. As Bob Dobkin said, the higher current level does reduce noise but increases drift, there is no way around that tradeoff, if a little higher drift is okay then you are set with the higher current but noise is a very important spec and sets the minimum accuracy floor of the reference, you cannot filter the noise out from a zener so if you need low noise you must start with a low noise source.
Hi Edwin,
thanks!! I just ordered tube of 10k 3250 and we will see how they will work;-)
I'm going to build also the LTZ1000 reference to compare them.
So I will go down with Zener current to 1mA.
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I surprised how stable it is during burnin period (<1ppm).
Hello,
I am not surprised. At least after some run in time the LM399 is very stable.
See the LM399 CH6 + CH7 of branadics thermal test.
Picture below is now after 1.5 years.
Blue curve is my LTZ1000A#2 shown as comparison.
see also old recordings:
https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg580443/#msg580443
with best regards
Andreas
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Thanks Andreas.
I put reference metal enclosure into two polystyrene insulation box and I will continue in measurement.
If someone will be interested in the atmospheric pressure dependency, it can be also added later on.
Hermetic chamber is almost ready.
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Hello everybody,
what do you think about this offer:
http://www.ebay.de/itm/LM199AH-883-Manu-NS-ST-Package-CAN4-Voltage-Reference-/191046638531 ?
Nice price, but when i look at the bottom and see those small rings of plastic at the outlets of the pins, it looks like a counterfeit, because i can't find similar looking items when searching the whole internet.
Your opinions?
So it may be better to buy this, slightly more expensive but pre-aged ones:
http://www.ebay.com/itm/1x-LM199AH-883-Precision-Reference-LM199-/111015694020 ?
Best regards
Frank
