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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