[volt-nuts] LM399 Long term drift specification

[Andreas Jahn]

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?
>>
>> 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".

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.

>> 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?)

> 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?
And surface mount SO-8 devices which have a factor 3 worse data on 
hysteresis (and probably humidity) than the 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).
 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 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



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