[volt-nuts] LM399 Long term drift specification

[Andreas Jahn]

By the way:

By accident I have found some ageing drift data of REF5050 on Malone's 
homepage:
http://www.voltagestandard.com/Tech_Data.html
For me it looks around 120 ppm drift within the first year.
Unfortunately there is no humidity data with the plot.

With best regards

Andreas

Am 12.09.2014 um 18:46 schrieb Tony:
> On 12/09/2014 06:16, Andreas Jahn wrote:
>>
>> Am 12.09.2014 um 01:06 schrieb Tony Holt:
>>>
>>>> All ageing specs are "typical" if you want to have "guaranteed" 
>>>> values you will have to measure it over a reasonable time. (I 
>>>> recommend min 6 months).
>>>> Every treatment (soldering, mechanical/temperature shock) of a 
>>>> reference may create a new ageing cycle with different slope.
>>>
>>> True. I guess that the new ageing cycle from soldering in an LM399 
>>> is not going to be as bad as that for a surface mount plastic device.
>>>
>> Thats true humidity within plastic housing is changed largely by 
>> soldering. This gives more stress on the die than on hermetically 
>> devices.
>> But why are the instrument manufacturers using sockets for the LM399 
>> (HP34401A) or reference boards for the LTZ1000 for their pre selected 
>> parts?
>
> Well its a lot cheaper to burn-in a device or small reference board 
> than the whole instrument PCB which is why I expect to do the same. 
> The LM399 on the Keithley 2015 isn't socketed though.
>
>>>>
>>>> So 100ppm/15 years outside of "lab conditions" (23 deg , constant 
>>>> humidity) is something that I would not guarantee without 
>>>> re-calibration.
>>>
>>> I had a feeling that would be the answer - though surely humidity 
>>> shouldn't be a factor as these are hermetic parts. The questions 
>>> remains though, what level might you specify - if you were forced to 
>>> come up with a number (ok a guess!) - for non-selected, non-pre-aged 
>>> parts after 15years continuous operation without re-calibration? 
>>> Obviously this is given the context of the presumably limited 
>>> numbers of samples you've tested and I guess you wouldn't have 
>>> bothered to further test early rejects.
>> Sorry this may change from lot to lot. From other (non heated) 
>> references I have very different results. Especially from devices 
>> bought before and after ROHS. And partly even better graded devices 
>> (LT1027BCN) behave worse than standard grade devices (LT1027CCN) in 
>> my temperature range.
>> With humidity and LM399: this should be no issue for the metal can 
>> package of the LM399. Although I have one LM399 (#1) which I 
>> desoldered from a board of unknown age that has a correlation to 
>> humidity (see attachment). Ageing is also in the range of 12-15 
>> ppm/year average with seasonal modulation which follows humidity. But 
>> since this behavior is not typical for my other LM399 references I 
>> would justify this sample as "defect".
>>
>
> Fair enough. I guess only the reference manufacturers are going to 
> have any worthwhile long term drift data on statistically large 
> numbers of devices - apart from customers who aren't going to share 
> the data.
>
>> Humidity: even hermetically parts can suffer from humidity. The epoxy 
>> board swells if exposed to humidity. This creates stress to the leads 
>> of the package and then to the die. I measure around 0.5ppm/% for 
>> plastic parts. In a early publication LT specced around 12ppm change 
>> for a 30% rH change for the LT1236AILS8 hermetically package. This 
>> spec is now removed from newer data sheets. You will only find a hint 
>> to avoid board stress.
>>
> Yikes! 12ppm for only 30% rh change in a hermetic package?
>>>> Although typical drift of pre-aged + selected references will be in 
>>>> the 1-2ppm/year range if properly treated.
>>>
>>> What would you classify as pre-aged? Do they need to be powered up 
>>> or can they be maintained at a suitable temperature? How many 
>>> rejects would you expect to get to get one that achieves 1-2ppm?
>>>
>>> Is it known if the major instrument manufacturers preselect and 
>>> burn-in LM399s themselves for their middle-range instruments? I'm 
>>> pretty sure the top end kit will be all use carefully tested and 
>>> selected parts, but what about a 34401A for example? The basic 
>>> accuracy spec for that is 20ppm for 90 days, 35ppm for 12 months so 
>>> even a 20ppm guaranteed part wouldn't be good enough, especially 
>>> allowing margin for drift in other components. I guess I just 
>>> answered my own question!
>>>
>> From Keithley cal lab you can see that they adjust the instrument 
>> during calibration if they fall outside the 70% window. So I guess 
>> that after 1 year they think that ageing of the components (including 
>> the reference) is below 30% of the 35 ppm spec per year. The part 
>> numbers of the LM399 are not the original ones but a own manufacturer 
>> specific.
>> So they do a pre-ageing. Of course powered up. (perhaps intermittend 
>> like on resistors?)
>
> Interesting. And pre-selected no doubt. I wonder though if heat 
> soaking at say 40C is almost, if not quite identical to powered up 
> pre-ageing? If soldered to a board then its a bit different, as in 
> normal operation the device is going to be warmer than the PCB. For 
> the lower power band-gap type SMT references though I doubt that 
> having it powered up is going to make any difference (assuming there 
> aren't any hot components nearby stressing the PCB). I guess there's 
> plenty of research out there on this subject.
>
>>
>>> I just came across another part which looks very interesting given 
>>> its low cost - the automotive qualified REF5050-Q1. Although its 
>>> only spec'd as 3ppm/C typical, 8ppm/C max, that's using the box 
>>> method over -40 to +125C. The 'typical' chart however, figure 4, 
>>> page 5 shows the gradients to be very flat between 25 and 50. Its 
>>> typical of course, so real parts may be very different aka Vishay 
>>> foil resistors. The 0 to 85C histogram, fig 1 on page 5, do show the 
>>> majority of parts being in the range .75ppm/C to 1.75ppm/C which is 
>>> pretty good, and with luck, in the 25 to 50C range may well be much 
>>> better so a crude heating arrangement may be worthwhile (made easier 
>>> by the 5050's temperature output!)
>>>
>>> I can't reconcile fig 4 with the histograms though; from the chart I 
>>> reckon the 0-85 typical is approx 65ppm/85C = .76ppm/C and for -40 
>>> to 125C is approx 310ppm/165C = 1.88ppm/C. Figs 1 and 2 though show 
>>> modal values of 1.25 and 2.25/2.5ppm/C. Am I doing something wrong 
>>> or are these specs inconsistent?
>>>
>>> Even more surprising is the headline feature on page 1:
>>>
>>> "EXCELLENT LONG-TERM STABILITY:
>>>  – 5 ppm/1000 hr (typ) after 1000 hours"
>>>
>>> Unfortunately that seems to be an error as the 'typical' spec on 
>>> page 4 is:
>>> 90ppm (0-1000 hours)
>>> 10ppm (1000 to 2000 hours).
>>>
>>> The chart (fig 23, page 8) showing 1000 to 2000 hour drift of 96 
>>> parts show the worst case being +25ppm, with the bulk ending approx 
>>> between 0 and 15ppm. I wonder if they carry on improving after 2kHrs?
>>>
>>> That's definitely not the SQRT(1kHr) characteristic and is very 
>>> different from the standard REF5050 which quotes 100ppm (1st 
>>> 100hours), 50ppm (1000 to 2000 hours).
>>>
>>> If you are in a position to pre-age them for 1000 hours that 10ppm 
>>> spec is almost as good as the LM399 and best of all, TI quote a 
>>> price of $1.60 @ 1k parts, compared to $4.65 for LM399s @ 1k from 
>>> Linear. One off prices are rather more at $4.15 from Digikey (part 
>>> no REF5050AQDRQ1) but again is still a lot cheaper than an LM399 at 
>>> $9.92. At $1.60 and .8mA supply current, using 4, 8 or even dozens 
>>> is a realistic proposition to exploit statistical improvements and 
>>> noise reduction.
>>>
>>> Noise is a bit high at 15uVpp. They're also trimmable. Shame there 
>>> isn't an hermetic part though.
>>>
>>> Anybody tried these or spotted the gotchas? Alternatively has anyone 
>>> here evaluated the hermetic LTC6655 for long term drift?
>>>
>>> Tony H
>>>
>> Do you really consider unheated references for a 100 ppm lifetime spec?
>
> I'm not confident that 100ppm is realistic for < $10 after your 
> responses, though I had thought that the LM399 might just achieve it. 
> I didn't say I would use it unheated - if the drift over 0-40C is too 
> great, despite selection, I could locate a heater resistor (or two) 
> next to the reference. The reference has a temperature output and I 
> have an ADC input (and enough power, hopefully not more than a few 
> hundred millwatts - not done any experiments though) available so it 
> should be fairly trivial to keep it within say +/-5C or better. 
> Selecting parts better than 3ppm/C gives an error budget of ±15ppm or 
> less. The real killer is long term drift and this part is the first 
> I've seen spec'd at <= 10ppm (albeit 2nd 1000 hours ageing) apart from 
> expensive metal can parts.
>
> Also at $1.6 I could afford to do some pre-aging and selection which I 
> couldn't with those $5+ devices including the LM399 and hermetic LTC6655.
>
>> And surface mount SO-8 devices which have a factor 3 worse data on 
>> hysteresis (and probably humidity) than the DIP-8 package?
>
> Hysteresis is spec'd at 10ppm first cycle, 5ppm 2nd cycle, so not a 
> big problem. They are typical figures of course so actual budget will 
> need to be somewhat bigger. Since I have a microcontroller which could 
> be measuring it's temperature continuously, there is the possibility 
> to correct some of the hysteresis and even the temperature drift if it 
> can be characterised well enough.
>
> Humidity sensitivity is almost never published, so this is something 
> to think about. Do you have any suggestions as to how much to budget 
> for an SO-8 part?
>
> Did you have a particular reference in mind in a DIP-8 package?
>
>> I do not know where they get their "typical" T.C. curves with flat 
>> area near 25 degrees. Usually they are only measuring 3 points of the 
>> references. (-40, 25, +125 deg C or whatever the temperature range of 
>> the device is).
>
> They may well only measure 3 points of each device or sample batches 
> for QC in production, but the device will almost certainly have been 
> very thoroughly characterised when it was developed, including that 
> double-humped temperature/voltage change curve in the datasheet. How 
> well the bulk production parts still match that requires 
> experimentation but I think its reasonable to believe they will 
> exhibit the same curve shape.
>
>> From theory any bandgap device (REF5050, LTC6655) has to have worse 
>> ageing than a buried zener (LM399, AD586). But also the manufacturer 
>> specific packaging has its influence. The specs in the datasheets get 
>> better the later the datasheets are created for several reasons: The 
>> instruments nowadays are with less tolerances. The silicon for the 
>> dies is more pure than the elder qualified devices. And of course 
>> then marketing demands that every generation has to be better than 
>> the previous one. But this has nothing to do with physics and the 
>> actual behaviour of the devices.
>
> I've seen my fair share of 'creative' or just plain sloppy datasheets, 
> but I'm not sure its fair to say that datasheets are sailing ever 
> closer to the wind. I have seen a number of posts alleging that some 
> recent op-amp datasheets from TI are very poor and a sad reflection of 
> those from the Nat-Semi era.
>
> It does seem that there hasn't been much progress in precision 
> analogue components that push the boundaries, or even negative 
> progress as good components are obsoleted - at least from the mass 
> market manufacturers. I find it hard to believe that there's still 
> nothing to touch the LTZ1000 after all these decades - or at least a 
> part that approaches its performance but is cheaper to buy and use. No 
> doubt due to more and more being done in the digital domain.
>
>> I have my first sample of LTC6655AILS8 measured for T.C. this week. 
>> But for my needs it has a too large hysteresis.
>>
>> With best regards
>>
>> Andreas
>>
> Thanks,
>   Tony H
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