[volt-nuts] LM399 Long term drift specification

Tony Holt vnuts at toneh.demon.co.uk
Thu Sep 11 19:06:56 EDT 2014


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:
> Hello,
> many questions I will keep it short:
> 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.

> 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 

> 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!

> Also its meaningless if you want to have LT or National (TI) parts 
> since LT is the only manufacturer which still produces them.
> With high demands you will also have to sort out the "noisy" references.
> Some "typical" LM399 (all from NS) ageing data can be found on web:
> http://www.gellerlabs.com/LM299AH-20_Case_Study.htm

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?

> http://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg478496/#msg478496 
> With best regards
> Andreas

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:

  – 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 11.09.2014 um 01:00 schrieb Tony:
>> I've just noticed that TI and Linear's specs for 'Long Term 
>> Stability' (typical) are different. TI state 20ppm/1000Hr while 
>> Linear state 8ppm/SQRT(kHr). That's  a big difference - is this 
>> likely to be a real difference or just specmanship?
>> I note that Linear (in Note 4) also state that "Devices with maximum 
>> guaranteed long-term stability of 20ppm/SQRT(kH) are available." 
>> Presumably they would be a special order as there doesn't appear to 
>> be a unique part no. Would they be likely to be much more expensive?
>> Then on page 4 Linear show a graph of long term performance of 44 
>> units (rather cheekily starting the graph at 2 months or approx 1500 
>> hours!). To reproduce something approaching the mean curve using the 
>> formulae (drift ppm/SQRT(kHr)) x SQRT(month * 24 * 30.5/1000), 
>> requires me to use 2.2ppm/SQRT(kHr). That is way less than the 
>> typical 8.5ppm value.
>> To get a curve that resembles the 3-sigma curve requires a value of 
>> 5.7ppm/SQRT(kHr) which is still better than that 8ppm typical figure.
>> I'm not sure how to interpret this; what value would you use if you 
>> were designing a reference that isn't going to be re-calibrated after 
>> the initial calibration and you don't intend to burn in for several 
>> months?
>> Assuming the equipment is expected to have a 15 year life, operating 
>> in a range of 0 to 40C, what maximum total drift would you be 
>> comfortable specifying? I'd prefer it to be less than 100ppm, but 
>> that would require a drift of < 9ppm/SQRT(kHr), but that assumes that 
>> the SQRT(KHr) drift characteristic is valid for periods much longer 
>> than 12 months.
>> Are there any other references, at similiar or lower cost, that could 
>> be reasonably guaranteed to have a total drift of < 100ppm after 
>> 15years?
>> Is it reasonable to assume that there are some types of voltage 
>> reference will always drift, albeit noisily, in one direction 
>> allowing the original calibration to be offset to some extent to 
>> reduce the maximum error over its lifetime?
>> Having looked at several application notes and lots of datasheets, in 
>> those that include graphs of drift over 1kHrs or so of several 
>> 'typical' examples, I have not been able to see any meaningful 
>> correlation between the specified typical 1k drift figures and the 
>> graphs. Eg. in Linear's Design Note 229 (Don't Be Fooled By Voltage 
>> Reference Long-Term Drift and Hysteresis" the graphs of drift for the 
>> LT1461S8 and the LT1790SOT23 show very different drift after 1600 
>> hours - in the range 50 to 130 for the former and approx -5 to +45 
>> for the latter, yet the LT1461 is spec'd at 60ppm/SQRT(1kHr) and the 
>> LT17910 at 50ppm.
>> I realise that I would probably need to contact the manufacturers for 
>> real answers but its been my experience that they aren't often 
>> interested if you're not buying large volumes, and I know that a lot 
>> of people here have a lot of experience in this area.
>> Thanks,
>>   Tony H
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