On 11/09/2014 01:50, Mike S wrote:

I'm guessing that typical in this case means the one sigma value so the
three sigma value would be 24ppm. In any case three sigma still only
means 93.32% of parts come within that limit, or 6.7% exceed 24ppm, and
a few could be considerably worse. That compares to a guarantee that all
all parts meet 20ppm. This link:
http://www.gellerlabs.com/LM299AH-20_Case_Study.htm provided in
Andreas's response is very interesting:

"Certified Long Term Drift The National Semiconductor LM199AH-20,
LM299AH-20, and LM399AH-50 are ultra-stable Zener references specially
selected from the production runs of LM199AH, LM299AH, LM399AH and
tested to confirm a long-term stability of 20, 20, or 50 ppm per 1000
hours, respectively..."

So in this case they really do mean a guarantee. And I doubt that
individual testing came cheap. I say 'came' because I wonder if they
still 100% test the 20ppm parts or if they select them using some lower
costs means?

Tony H

Andreas,

Thanks for taking the time to respond. Actually I've seen many of your
postings on eevblog and here - you've clearly done a great deal of work
in this area and would like to thank you for making it available to us all.

On 11/09/2014 06:07, Andreas Jahn wrote:

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.

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.

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!

That's very interesting. I have to agree that the raw data looks
suspect. I wonder what the rejection rate is for this 20ppm selection
and does it mean that non-selected parts have a high probability of
being worse than 20ppm?

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

Am 12.09.2014 um 01:06 schrieb Tony Holt:

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?

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.

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?)

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

On 12/09/2014 06:16, Andreas Jahn wrote:

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.

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.

Yikes! 12ppm for only 30% rh change in a hermetic package?

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'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.

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?

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.

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.

Thanks,
Tony H

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 14/09/2014 18:29, Andreas Jahn wrote:

That's a very interesting find Andreas, thanks. After approx 500 hours,
maximum drift is approx 80ppm, but its impossible to deduce how much of
that is due to long term drift, temperature and humidity changes.

The long term drift specification for the REF5050A tested is 125ppm
(0-1000 hours), 45ppm (1000 to 2000 hours). Those for the automotive
qualified REF5050-Q1 part are 100ppm and 10ppm respectively - way better
after the initial 1k hours.

Regards,

Tony H

On 15/09/2014 17:57, Tony wrote:

One thing that concerns me is that all 5 units seem to drift in sync, so
how much of that is the DVM doing the measuring? or is it just a single
batch with the same date code?

Hi,
I'm late to this thread, but have some experience. Several years ago I did
some research on replacing xenon flashlamps in a high speed fluorescence
instrument application. As the white LED candidates had phosphor coatings I
assumed (don't assume - guess ;-) that they would be too slow. It turned
out that they were significantly faster than the xenon lamps, with sharp
turn-off rather than the tail of the flashlamp.
So don't dismiss white LEDs for modulated applications.

Robert G8RPI.

On 11 September 2014 13:14, Chuck Harris cfharris@erols.com wrote:

Thank you Robert for you experience,

My white LED's did have a persistence to them, so I'm going to change
them to ORG/RED (615nm). I've temporarily using some low output amber
LED's and determined that the caps I changed needed to be changed back
to original values. I also modified the zero control circuit with a 1.25
volt reference centered at zero with two switching diodes to base the
offset at zero for less drift. I,m considering using the Linear chopper
amp Randy's using. But I'm not sure how to incorporate the gain control
at S101B? The over all feedback should be no problem.

Dallas

On 9/15/2014 3:57 PM, George Atkinson wrote:

Hello,

the drift in sync is something that I have with humidity changes and
plastic housings.
And they also have temperature changes of +/-4 deg C in the setup.
But usually the temperature gradient is different with different samples
near room temperature.
The influence may also be due to PCB stress.

with best regards

Andreas

Am 15.09.2014 um 19:35 schrieb M K: