[volt-nuts] Some questions to zeners (1N823-1N829)

[Andreas Jahn]

Hello Warren  and Volt Nuts:

for the first: from the experiences of Warren my project seems to be 
feasible.
Im working now since 2008 on my "project".
Eliminating step by step the drawbacks of stability.
And learning a lot of real and not ideal parts.

I changed from SMD references with large hysteresis and bad stability to
plastic DIP devices being about a factor 3 better than SMD.
Finally finding out that some buried zener devices have relative low 
hysteresis.
But air humidity was a large issue preventing stability below 10ppm over a 
year.
So finally I try to use devices in hermetically tight cases which
eliminates humidity if carefully decoupled from PCB stress.

So what is feasible with selected parts and a 3rd order temperature 
compensation is shown in the attached picture. (ADC13_TC)
A AD586LJ device as reference within ADC#13 is measuring a heated LM399 
reference via a 2:1 capacitive divider.
X-Axis is temperature normalized to 25 degrees celsius -10/+15 degrees 
measured directly at the AD586 reference.
Y-Axis on the left is LM399 measurement value in mV (divided by 2).
The red line is the measurement value of the ADC. The blue line the 
resulting correction with a 3rd order polynominal.
Y-Axis on the right side is together with the green line the resulting 
deviation in uV after calculating out the tempco of the reference.
Deviation is about +/-2 uV referenced to 3.4V which gives below +/-1ppm 
resulting tempco after correction.
Unfortunately the slope of the uncompensated TC is relative large (about 
1.1ppm). Together with my temperature resolution of about
0.1K/step this will give a relative large temperature step noise of up to 
0.6uV/step in the 5V-Range of the ADC.
But anyway ADC #13 was the first ADC which I can use for ageing 
measurements.

After a run in phase of nearly 1 year the ageing of ADC #13 stabilized.
Currently I compare ADC13 nearly every day with 3 heated references (1 LM399 
= LM_2 and 2 LTZ1000A = LTZ_1/2).
The last half year the ageing is about 0.5 to 1.5 ppm for 6 months compared 
to the heated references.
See picture ADC13_longterm:
X-Axis is day
Y-Axis left is drift in ppm with red = LM399#2, green = LTZ1000A #1, blue = 
LTZ1000A #2
Y-axis right is temperature in degree celsius of the temperature sensor near 
ADC13 reference.

By the way: up to now I could not measure any effect which is related to 
thermocouples.
Ok my temperature step noise is still too high. And probably I am using the 
wrong connectors in my tests:
cheap D-Sub connectors where a metal shield is equalizing the temperature of 
2 relative close neighboured contacts.

With best regards

Andreas


----- Original Message ----- 
From: "WarrenS" <warrensjmail-one at yahoo.com>
To: <volt-nuts at febo.com>
Sent: Sunday, January 27, 2013 11:23 PM
Subject: [volt-nuts] Some questions to zeners (1N823-1N829)


> Andreas
>
> 1st my two cents worth concerning comments of others that are being more 
> practical than nutty.
>
> "it is not easy to achieve these specs",
> Too True,
> If you do not already know this, then I'd suggest you back off and do some 
> major readjustment of your requirements.
> Assuming you already know it is not easy and why, then
> IMHO, what you want to make is doable with the right compromises, some of 
> which being
> A little extra power.
> a lot of extra time
> a lot of test selecting
> some extra circuits or S/W
>
> "your expectations are not realistic "
> True,  if you think it can be done from a data sheet and a few of off the 
> shelf parts in a production device.
> Not true, if you want to go totally nuts and have the time to do it.
>
>
> "The thermoelectric effects causing error would swamp the performance"
> Could be true for some. If you do not already know enough to keep these 
> effects well under <<0.1PPM then should find another starter project.
>
>
> "To keep everything below the 1 ppm/deg C range you would have to put the 
> entire circuit in controlled temperature"
> Too true, But for these requirements, fortunately you do not have to keep 
> *Everything* under 1 ppm/deg C
> ALL you have to do is to keep the sum total of Everything under 1ppm.  Big 
> difference!
> And that is very easy to do, by a fact of ten plus if desired.
>
>
> "long term drift and noise will be intrinsic to the devices, and 
> unpredictable except in a statistical sense."
> Half true,
> Using the *right device*, and a lot of time, noise can be low enough to be 
> mostly insignificant
> and long term drift will follow a predicable slope.
> It ain't easy and it's going to take a lot of time. If you think 
> otherwise, you need a new project..
>
>
> "provide near zero tempco at one temperature only"
> Answer too limited to be useful
> zero TCing a circuit, can provide no voltage difference at any two 
> temperatures, (to the limit of the repeatability)
> Depending how flat you need it between these two temperatures, is the only 
> issue.
> Best to plan on having a second order  TC method as well.
> A total max total deviation of 1ppm and a 0.1 PPM /deg is not too hard to 
> get.
>
>
> "One of the best voltage standard Datron 4910AV (4x LTZ1000) have only 1 
> ppm/year drift"
> OK, so that shows there is at least one way to do it, so it is not 
> impossible.
> Now just need to find the best way for you to do it.
>
> "Don't bother with TC zeners"
> Not a bad idea, but Unfortunately Not a lot of other choices considering 
> your requirements.
> The other choice you have (that you should consider if time is a big 
> issue)
> is to get some three terminal Fluke voltage references.
> If necessary by removing them from old test equipment such as Fluke 731
> Those have already been selected and aged, and you'd just then need to 
> work on the long term drift compensation selection method.
>
>
> "There are lots of nice IC references available"
> True, and if you can live with data sheet specs, they are much better 
> choices.
> But hard if not impossible to find anything that will compare to 1/F pop 
> corn noise, and long term stability
>
>
> "I doubt that any TCZ will match an LM399"
> True when considering a wider temperature range, and it sure makes things 
> easier,
> so a good suggestion if TC was your main problem.
> But TC need not be a problem, and most any good zener will outperform most 
> any selected LM399 in low freq noise and stability.
>
>
> "so would have to be ovenized to get *best* performance".
> True of Any circuit when "best" TC is concerned, so not very relevant,
> The question is can it be made good enough without an oven?
> And the answer is defiantly yes with the right tempco circuit.
> And if you want it even better, can make a very low power mini-oven.
>
> Back to your email ******************
>
>> Do you have typical values over a 64-90 ?F range. Will it be above 1ppm/K 
>> or  below?
> I do not have any data over that Temp range in front of me.
> Over My room changes (about 1/2 that temp range), can keep the Total 
> voltage error down to around 1PPM, (not just /deg)
> If it is an important consideration, I may try and see what happens over a 
> wider range.
> For a backup plan, considering having an addition second order TC 
> compensating method such as S/W.
> Overall compensating to 1ppm / C is no problem,
> which is Total change +- 7PPM over a +- 7 deg C range
> Ten times better is possible over narrow temp ranges like that.
>
> It is the hysteresis and stability that is going to be a limiting factor, 
> and last I looked,
> The zeners I tested had no measurable Hysteresis over a much wider temp 
> range than that.
>
>>>From your plot it would be 0.33ppm per 3 degrees in narrow range i.e. 0.1 
>>>ppm per degree.
> Correct, but not a relevant measurement.
> ANY straight line TC as seen there can be zeroed out.
> That was just a pre-test plot to see if the part was low enough noise to 
> do a more accurate Zero TC.
>
>> By the way: is it degrees Fahrenheit or degrees Celsius (= 3 Kelvin)?
> My world is mostly in F, and since I was not even plotting temp at the 
> same time, pretty irrelevant for that data set.
>
>> If I understand you right then you would not use this device because it 
>> does
>> behave other than the others?
> Your understaning is not correct, not even close. My comment applies to a 
> preselection process of 1N82x parts.
> In the case you stated, your preselection process (assuming you have 
> enough to pick from) would be to select all the parts that are less than 
> say 1PPM, with maybe a 20% yield.
>
> If the yield is too low then make a second pass for parts that are say 2 
> +-1 PPM,
> and in that case, then yes the "best" part would not go into that batch, 
> because it will likely need a different compensation than the others.
>
>
>> On the other side it seems to be the device with the largest ageing rate 
>> of  the 5 pieces.
> likely too little data to be important, Just as likely random luck as 
> anything important.
> Until after you do some pre-aging, (whatever that may mean for that part) 
> I would not even bother looking at that data this early on in the 
> selection process.
>
>
>> So I still hope that anyone has experiences with hysteresis of the 
>> zeners.
> Turn on after power down, and hysteresis, repeatable, etc altogether is 
> under 1PPM.
> As part of my pre-test TC procedure, I hit them with cold spray to 0 C and 
> heat them with a heat gun to ~50C, a few times,
> If they are not repeatable they are not used in further testing.
>
> I have a question about two of your requirements
> AJ> tempco below 1ppm/K
> AJ> hysteresis in the 10-40 degree range well below 1ppm
>
> This suggest to me that what you are really planning on making is 
> something with an overall compensted TC that is well below a 1 ppm/K,
> otherwise your hysteresis requirement does not make a lot of sense to me.
> So if what you are really after is more like < 1 PPM total error over time 
> and temp, all this extra trouble now makes a lot more sense.
>
>
> Now the disclaimer, I have no idea if any of the now available 1N825 or 
> better parts work like I've described.
> It is VERY much a manufacture sensitive thing.
> Short term noise being the biggest rejecting thing I've seen in the past.
> 0.5 PPM (3uv steps) are not untypical for some batches.
>
> If someone that has enough of these parts from a single batch and 
> manufacture to make it worth while testing them, and will make them 
> available to others, then I'll test and report the results and compare 
> them to what I have.
>
> There may be another less direct way to get where you want to go (You did 
> not say where that was)
> Divide the task into two or more sections.
> As an example, One being the low power short term stable DVM device that 
> works over a limited temperature.
> There would be no problem getting 0.1 PPM accuracy/repeatable for that 
> part.
> and a second higher power device that is mostly off, and powered up Just 
> long enough to calibrate the low power xfer DVM above.
>
> And if you can then include some sort of cal exchange program for the 
> higher powered but less used device, this would make a very hard project 
> into a relative easy project.
>
>
> ws
>
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