Author Topic: A Low Cost OSHW Voltage Calibration Reference Project  (Read 8950 times)

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Offline rhb

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A Low Cost OSHW Voltage Calibration Reference Project
« on: March 31, 2018, 09:58:01 am »
@svenskelectronik and I have been discussing design  and production of an LM399 based voltage calibrator derived in part from TiN's KX design. He owns a PCBA facility, so he is in a good position to manufacture and market an OSHW product without depending upon it for his livelihood.

Initial outline is as follows:

provide 5-10 steps per decade over 3 decades  from 10 V max to 10 mV min

use low cost 1% metal film resistors for the voltage dividers and op amp feedback resistors

provide on board voltage regulation and filtering

use an MCU and an LCD display to show the selected output voltage

cycle through the outputs via a pushbutton

display the  output voltage on the LCD

package in a shielded box with temperature management and measurement

store calibration constants in the MCU flash or EEPROM

burnin units for 500-1000 hours before shipment on a calibration system to get aging coefficients

temperature cycle the units during burnin to get temperature coefficients

fit burnin data via a sparse L1 pursuit to determine calibration constants

at each calibration,  recalculate calibration coefficients for best fit to all calibrations

make the calibration process completely automated other than physical connection to calibrator

The basic premise is that one can build a device whose behavior can be accurately predicted  at lower cost than making a device which is physically as accurate.  For a voltage calibrator there is no reason to prefer a 1.000000 V output to a 0.997869 V output if one knows the latter as accurately as one knows the former. The MCU and LCD allow displaying a variable  output voltage adjusted for age and temperature. If humidity and barometric pressure prove significant, one can add sensors for those.  A key goal is to reduce the cost of calibration to very little more than shipping cost by automating the procedure.

Barring  device damage, I think such a design could achieve the rather ambitious goals I mentioned earlier of 5-10 ppm after 5 years.  Doing this is a bit beyond what one person can do.  I know the math for the calibration process.  But there are many other important details about which I am ignorant.  I think that most things of significance that can be done by one person have been done.  What remains are things that require the efforts of several people.  Hence the importance of open source efforts.

This should probably move to the Projects forum, but I thought that it should be discussed here first.  I'd like to hit a BoM of under $50.  That's pretty much entirely dependent on being able to calculate the resistances as a function of the relevant factors as well as op amp gains, offsets, etc along with the aging and temperature dependence of the LM399.

Yes, I am acutely aware that this is *very* ambitious.  But if it were easy what would be the point?  You can easily buy a better reference, but it's not cheap.

Have Fun!
Reg
 
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Offline MisterDiodes

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #1 on: March 31, 2018, 01:47:26 pm »
Your plan sort of falls apart at:

"use low cost 1% metal film resistors for the voltage dividers and op amp feedback resistors".

Therein lies the problem - those will never come close to being stable enough over time, temperature and humidity.  That's why we use PWW and Bulk Metal Foil resistors that are tens or some hundreds of times more stable.

You might want to do more research - there are already other versions of the cal box you're describing, here on the forum and on the 'Net.  Right down the LCD display, pushbuttons, voltage output etc.  Look around.

 
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Offline TiN

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #2 on: March 31, 2018, 02:25:02 pm »
Sorry, there is a core conflict between the items

Quote
5-10 ppm after 5 years.  Doing this is a bit beyond what one person can do.
That's 1-2ppm/annual, which require careful selection of state of the art parts (LTZ) and permanent fixed deployment.
It's nothing to do one or ten person involvement.

and

Quote
the LM399
You will need to buy and sort thru lot of LM399's to even approach 2ppm/annual. That task alone will take you few years. ;)

Quote
use low cost 1% metal film resistors for the voltage dividers and op amp feedback resistors
That is the main catch. LTZ references are expensive NOT because of 50$ LTZ chip, but because of resistors. And output stage 7V-10V is WAY MORE difficult and demanding than LTZ/LM399 source itself.

Quote
I'd like to hit a BoM of under $50.
Okay, just stable and good performance package in a shielded box with temperature management and measurement will cost more than $50.

You see others expensive and often overspecified discussed designs here on forums not because we want to spend lot of money, but because there are no much other ways.
Design is as stable as weakest component in the circuit.
 :-//
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Offline cellularmitosis

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #3 on: March 31, 2018, 03:40:05 pm »
Good feedback from very experienced members in here, but let's play with the idea just a bit further.

If you were doing down the road of "yes my components will drift, but if the drift is predictable and I can model it, I can extrapolate where my next calibration point will be, to X accuracy", what you'd want is a board where each of the resistors have four test-point pins exposed, so that you periodically power down the board and perform 4-wire measurements on all of the resistors.
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Offline TiN

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #4 on: March 31, 2018, 03:49:36 pm »
Quote
drift is predictable and I can model it


Quote
, I can extrapolate where my next calibration point will be, to X accuracy
How to prove (every time) reality matching the expectations though? .

So what you saying is : instead of forking 50$ for resistor and forgetting about it, I'll spend XX hours every week/month/year to test unstable resistor and wiggle the calibration math to get the numbers to coverge. Sounds like somebody have too much free time  .
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Offline zhtoor

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #5 on: March 31, 2018, 04:37:00 pm »
hello reg,

thanks for stimulating our thoughts with an interesting idea.

now what i understand from your description is:-

1. you model the reference as a "black-box"
2. this black-box is isolated enough to be called a "black-box"
3. this bb produces a voltage output, which is to be modeled against time (drift), and environmental factors (temp, humidity etc.)
4. you come up with an equation like: Vout = f(time, temp, humidity, pressure) whereas time is accumulated in the black-box (infinite battery?)

how about thinking of a common carbon-zinc or an alkaline cell as such a system (with appropriate sensors etc.)?

regards.

-zia
« Last Edit: March 31, 2018, 04:40:00 pm by zhtoor »
 

Offline TiN

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #6 on: March 31, 2018, 05:21:05 pm »
It would be more like Vout = f(time, temp, humidity, pressure, output loading, EMI/RFI condition, mechanical stress, orientation, airflow, setup thermal constant). We talking 1-2 ppm/year, as outline by first post.  :popcorn:
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Online hwj-d

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #7 on: March 31, 2018, 07:07:03 pm »
Yeah,

play arround with your thoughts, discuss it here, build some refs, and then, after a couple of time for further maturation, repeat your project yourself again.

Take it easy  :-+
« Last Edit: March 31, 2018, 07:13:32 pm by hwj-d »
 
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Offline Kalvin

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #8 on: March 31, 2018, 10:51:09 pm »
Here is a recent, practical oriented article published in EDN "Factors affecting reference long-term drift performance":

https://www.edn.com/design/analog/4460470/Factors-affecting-reference-long-term-drift-performance
 
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Offline rhb

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #9 on: April 01, 2018, 01:34:02 am »
Zia is the only person who understood the concept.

I have 30 years of very varied experience with multichannel data analysis and processing.  I was the person people went to when they had unusual problems.  See "Random Data" by Bendat and Piersol for the traditional treatment based on Norbert Wiener's work in the late '30's and early '40's.  I also  recently spent 3-4  years learning about sparse L1 pursuits.  See "A Mathematical Introduction to Compressive Sensing" by Foucart and Rauhut for a treatment of the work of Donoho and Candes who did the primary work on sparse L1 pursuits.  For a less intense summary of sparse L1 pursuits see "A Wavelet Tour of Signal Processing" by Mallat, 3rd ed.

I bought Foucart and Rauhut  in 2013 when I realized I was solving problems I'd been taught in grad school at Austin could not be solved.  That was quite exciting, so I wanted to know how and why this was possible.  It took 3 years and 3000+ pages of text, but I now understand it reasonably well.

A simple example.  A voltage divider composed of two stable metal film resistors.  By stable, I mean that  age dependent effects are negligible.  If those resistors are placed in  an enclosure filled with thermal grease  and a thermistor and a precise voltage fed to the pair. by measuring the drops across the resistors as a function of temperature using a heating chamber I can calculate the voltage drops at any temperature over a wide range at any time in the future.  Moreover, I can do this to the same accuracy or better than a pair of expensive low tempco precision resistors. I am in no way constrained by nonlinear behavior and  can readily accommodate thermocouple effects, thermistor errors etc.

If that is not possible, then using the most expensive precision resistors made is futile also.  Air currents, EMI etc all have to be controlled in the usual fashion.  4 wire resistance measurements are not needed.  Measuring the node voltages will suffice.

The accuracy of estimates of the aging of the LM399, or any other reference, are dependent upon how much history is available.  To make use of that one must recalculate the aging function *every* time the device is calibrated.  I see no evidence that is being done today by anyone.  My assertion is that with 5 years of performance data the error will drop to 2-3 ppm or less.  The initial error will be an order of magnitude larger. 

I spent several days modeling predictions of exponential curves.  What I found was that accurate prediction (i.e. 1% prediction error) was possible out to a time twice the length of the prior history.  I've not yet done this for  a logarithmic function, but I shall. As the exponential and the logarithm are closely  related, I'd expect similar behavior from the logarithm.  A device which is changing by 20 ppm per year which has been properly characterized can be modeled to 0.2 ppm if the prediction error is 1%.

I've read many statements now about the "random" variation of a set of references.  But when I look at the data it looks highly correlated to me.  There is a significant random noise component, but the dominant features are not at all random.

The mathematical side of this requires a lot of data and probably a month or two to analyze.  I've gotten data from TiN but I need more.  I especially need data from Andreas and others with similar data in CSV format.  Most of the work is sorting out how to handle the data and developing filters to throw out bad data. Bad data can only be identified by having lots of data and rejecting things which are 3 standard deviations and more out.

For a super major I extracted from the company database 135 GB of geophysical measurements made in well bores in the GoM in over 19,000 wells.  None of the data had any usable pedigree.  To use it I had to examine how each sample related to nearby samples both in the same and in other wells and then develop criteria for rejecting invalid values.  For the wireline data I had to get time on a seismic processing cluster for  overnight runs.  Voltage references are a cakewalk by comparison.
 

Online chickenHeadKnob

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #10 on: April 01, 2018, 02:46:14 am »
There is a forum member- IanJ (Ian Johnston) who makes and sells a reference which is remarkably similar to the one you are proposing. see forum thread:
https://www.eevblog.com/forum/metrology/ian-johnston-pdvs2/msg1196493/#msg1196493

or Ian's web store:
http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-v2-detail

Of course it doesn't meet your price target and it deviates in other ways, but it can give you an idea of some of the trade offs he made. He has some background on the design and its iterations on his web site.
 

Offline TiN

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #11 on: April 01, 2018, 02:25:58 am »
rhb
Did you see this paper. datasheet and this?

In the end - if you would set target to 4-8 ppm/year, I would be first person to tell you - go all in, and would donate few references for the project.
1-2 ppm/year? Maybe not. Either way 50$ BOM not going to happen :). Theory and math correction are great tools, but they should be in connection with practicality and real-world scenario, which is not as simple as tempco/humidity and pressure correction. I would be happy to see this project move from forum discussion to some hardware implementations, and would support just to get nice box with internal oven, control and monitoring functionality.

In applications like low power reference, precision resistors are used due to single reason - better materials that provide long term stability. To be completely correct, there is NO need for precision resistors in DC reference, what is required - long-term stability (both temporal and environmental). Like you already know, it does not usually matter if it's 7.041843V output or 7.000000V output. If there is 5% stable resistor, it would work just as good. It's just from resistor manufacturer business perspective no sense to make 5% ultra-stable resistor with better materials, than 0.1% ultra-stable resistor. You can ask any resistor manufacturer, and if they kind enough then you'd see price between 1% high-end resistor and 0.005% high-end resistor is actually same in manufacturing, it's just extra testing cost to provide you 0.005% tolerance guardband.  ;) Paying extra for 0.005% resistors in LTZ reference circuit it's just waste of money for nothing.
« Last Edit: April 01, 2018, 02:39:36 am by TiN »
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Offline zhtoor

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #12 on: April 01, 2018, 02:26:44 am »
hello,

valhalla 2720GS is probably the only reference unit which does remotely like suggested here,
by taking out mal-performing references out of an averaging circuit.

regards.

-zia
 

Offline Magnificent Bastard

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #13 on: April 01, 2018, 03:16:40 am »
Rather than design and build a calibrator that has limited functionality (e.g. low DC voltages only), why not change the specifications a bit, and design one that handles 0Vdc to 1000Vdc, 0Vac to 1000Vac, 0Adc to 30Adc, 0Aac to 30Aac, and ohms from 1-ohm all the way to 1G-ohm... ??

The resistors can be directly connected to the output for those meters that supply higher voltage, and also run through a resistance synthesis circuit to produce fractional values in between.

The DCV, ACV, DCI, ACI functions do not need to have zillion-digit adjustability-- only on tenths.  In other words, 1.0V to 9.9V (etc.)  These could be generated by a PWM DAC (with high stability, but low cost and it does not need a zillion bits).

Then, because we are limiting the settings to a small set, each voltage on every setting can be calibrated by a freshly calibrated 3458A (automated of course), and this calibrator would then be good enough to calibrate and/or adjust a 6.5-digit meter.  This way, we don't need to worry about linearity (etc.)-- only stability and repeatability.

Some meters have a capacitance function, and so it might be a good idea to add that as well (and same thing-- some synthesized values could be made out of a few capacitors and some additional circuitry).  The capacitance function of even high-end DMMs is really for indication only, and so these capacitors do not need to be calibrated to very low uncertainty-- a calibrated GenRad LCR meter would be just fine (0.02%).

I have no idea if inductance should also be added.  Maybe someone can find justification for that.

Of course, this is not all going to happen for US$50-- it will be much more (realistically), but I bet it can be done for lest than US$500 with the right choices of off-the-shelf parts.
 

Offline MisterDiodes

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #14 on: April 01, 2018, 03:18:57 am »
RHB:

You started out with:

"...But there are many other important details about which I am ignorant."

But then in a later post you seem to have it all figured out.  I'm sorry, and I say this will all due respect - It might be time to put the books down and start learning.

Or as the sign over my office says:  "Where theory ends and reality begins"

Here's an easy conceptual test you can try right now.  Take a couple of your "low cost" resistors you think will be stable enough and put them under load for 1000 hrs.  Maybe measure TC and humidity effects.  And while you're at it - since you want a variable voltage output look at Power Coefficient change (PC) and Voltage Coefficient change (VC) as well - something most people here overlook because they're too enamored with TCR.  For instance, realize the power in any resistor at 10V bias is 100 times that of the same resistor at 1V bias, and remember that your drift rate even on the best resistors is highly affected by some ppm by EVERY mW power dissipation over time.  A lot of that is dictated by the thermal coupling between the resistor and its environment.

So do you "burn in" at full power condition or low power condition?  You could argue either way is valid.  What if someone runs their device all the time at max output voltage?  Compared to someone that tosses the device in a drawer for months at a time.  How do those devices compare to each other after a year, and how do you predict the end usage condition?  How do you predict usage history??  Or maybe one device lives in a warm humid place, and another device lives in a dry, cold climate.  While they operate the same way in a year or two?

And what makes you think that "burning in" a cheap resistor will get it to stabilize?  Have you ever tried doing this and tried to come up with a prediction model?  You might want to investigate that part very carefully before proceeding.

So - I'm suggesting as your first step of discovery is take some cheap resistors, and measure their ratios on an 8.5 digit DMM (backed up by a stable Vref and Resistance standard).  Since you're going for low ppm you'll need a capable measuring setup.  Measure changes over time, temperature, humidity, power and bias points for starters.  Also make sure your test resistors are mounted on a realistic PCB and realistic enclosure for realistic thermal coupling, otherwise all you're doing is measuring the "dangling in free air" condition of the resistors...which is not really the end use condition.

One thing you'll come across - you get that pesky power-dependent drift due to bias/power point, and doggone it now you realize you've got hysteresis too: You go to high power for a while, and then when you come back to lower power...Shoot...The resistance value never came back to the same place from where it started.  So you go back to higher power...and now it's different again.  And so on.

Again:  At some point you realize why we have stable PWW and foil resistors.   

As we tell our clients that want a precision circuit designed:  You can have it "Fast", "Cheap" or "Good".  In the Metrology world, you pick ONE.  Generally if you want any performance at all at ppm, you pick "Good".  "Cheap" is almost always a complete waste of time. "Fast" is usually never as important as "Good".
« Last Edit: April 01, 2018, 03:38:04 am by MisterDiodes »
 
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Offline zhtoor

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #15 on: April 01, 2018, 04:13:08 am »
Zia is the only person who understood the concept.

thats probably because i am a total newbie to the subject.

Quote
The mathematical side of this requires a lot of data and probably a month or two to analyze.  I've gotten data from TiN but I need more.  I especially need data from Andreas and others with similar data in CSV format.  Most of the work is sorting out how to handle the data and developing filters to throw out bad data. Bad data can only be identified by having lots of data and rejecting things which are 3 standard deviations and more out.

andreas has huge amount of experience with LM399, TiN has *huge* amount of experience in LTZ1000, both of them probably can setup an environmentally perturbed
references (both LM399/LTZ1000) in controlled chambers and stream data to rhb for analysis and building a predictive model based on this streamed data. perturbations to
the environmental chambers can be co-ordinated by rhb/andreas/TiN and then the model / results published here.

maybe alex nikitin can pitch in with a JVR and somebody else does an LT1021BCN8-10 on a standardized eevblog vref streaming server (TiN?)  ;)

best regards.

-zia
« Last Edit: April 01, 2018, 04:21:30 am by zhtoor »
 

Offline rhb

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #16 on: April 01, 2018, 04:31:56 am »
Arguments that "it won't work" are pointless.  The only way to find out is to try it. 

They told the Wright brother's "it won't work".  Rather than trust other people's data they built a wind tunnel and collected their own.  We all know the outcome.

I feel quite confident that no one is applying the mathematical techniques I described to the implementation of a reference.  What I'm suggesting is a non-trivial application of state of the art mathematical techniques.  Will it make a difference?  No one knows.  Or at least if they do, they have not published.  Ten years ago I should have said that this is not possible.  Now I know it might be.  So it's time for an experiment.  This is the experiment.

If there are aging data for cheap metal film resistors please direct me to it.  I shall certainly require it. Should someone wish to collect some I should be most grateful.  I've got a lot of work to do just setting up the infrastructure to make the measurements.  And collecting 1000 hrs of data will take 6 weeks.

Hysteresis would require logging the relevant variable.  I presume that all behavior can be described by the device physics. Furthermore, I presume that the device physics can be described mathematically.  If the rules change by divine intervention, then obviously such an approach will not work.  But there are a few hundred years of evidence that God does not intervene in day to day physics.

It really doesn't matter what the resulting accuracy is.  Whatever the accuracy is, it *will* be better for the price than the result if one ignores the math.

I do need all the data I can get.  I can deal with any limitations of a dataset if I have enough data.  I should be able to show quite a bit with existing data.
 

Offline cellularmitosis

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #17 on: April 01, 2018, 04:50:44 am »
So it's time for an experiment.

 :-+ 

Quote from: rhb link=topic=tte107088.msg1467078#msg1467078 date=1522521116
If there are aging data for cheap metal film resistors please direct me to it.  I shall certainly require it. Should someone wish to collect some I should be most grateful.

Not related to this project, but I've been wanting ageing data on cheap metal films for a while now as well.  My HP 3478A is just collecting dust, and 5.5-digits will probably be enough to get useful data on cheap metal films, so I'll commit a scanner card to getting this sort of data for you (and for everyone).

Looks like these things are just controlled via a 30-pin card-edge connector, probably 5V TTL.  Should be easy to just rig up an Arduino to control a single card in isolation.

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Offline TiN

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #18 on: April 01, 2018, 04:51:33 am »
Arguments that "it won't work" are pointless.  The only way to find out is to try it. 
Yup, as MB explained nicely, time to set keyboards aside and start soldering ;). Sorry If my posts got you wrong thinking, I'm not saying "it won't work", I'm saying it would be very expensive (not in parts cost), long and painful learning process (which may or may not lead onto outcome you desire).

Quote
They told the Wright brother's "it won't work".  Rather than trust other people's data they built a wind tunnel and collected their own.  We all know the outcome.
Perfect example, where outcome (flying planes) came from getting hands dirty and doing stuff. No math in the world would help if we just talk about it.

Quote
I feel quite confident that no one is applying the mathematical techniques I described to the implementation of a reference.
That is what whole multi-M$ metrology industry is doing for decades. Predictions and math theory goes hand to hand for improving uncertainties of everyday measurements. If you look into primary standards theory and experiments, you will find lot of amazing papers and projects around those. But all those cost bit more than 50$ to achieve even 5ppm/year :).

Quote
And collecting 1000 hrs of data will take 6 weeks.
That's just a warm up time ;). Amount of data to collect to correlate for stability, is the reason why stuff in ppm world usually take months and years.

Quote
Whatever the accuracy is, it *will* be better for the price than the result if one ignores the math.
I don't know, but time to prove math works also have the price.

cellularmitosis
Good idea. I was meaning to post my K7168 code (I use RPi to control my card).
« Last Edit: April 01, 2018, 04:53:13 am by TiN »
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Offline cellularmitosis

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #19 on: April 01, 2018, 05:02:52 am »
cellularmitosis
Good idea. I was meaning to post my K7168 code (I use RPi to control my card).

Aha!  Perfect!  That would be great  :-+
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Offline MisterDiodes

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #20 on: April 01, 2018, 05:53:41 am »
I love the references to Wright Brothers.  That also reminds me of Hitchhiker's Guide by Adams: There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss. Pick a nice day, [The Hitchhiker's Guide to the Galaxy] suggests, and try it.


So go try it!

The difference here is that there is people have been trying to achieve any sort of stability with cheap resistors for decades (trying to ignore basic Chaos Theory).  The exact end result: That's why we have PWW and Foil resistors, which are already good solutions to the "flying" problem.  It's not like we haven't taken flight before in the realm of resistor stability.  The underlying reasons of how & why cheap resistors are built on a grand scale is also the reason they are more unpredictable,  chaotic and not well suited for critical circuits requiring good stability.  You get what you pay for.  Always.

RHB,  you are correct here:  It IS futile to expect any device or circuit to remain "in calibration" forever.  That will never happen on ANY device, no matter what components are used.  That's why all precision instruments must have periodic calibration performed.

So when you design a precision device with some target "drift rate" that also must include the target calibration interval, and you also include device uncertainty calculations over say 15 minutes, an hour, a day, 30 days, 1 year, etc.  This is the metrology section, so every measure and spec includes some non-zero level of uncertainty.

The main problem you'll have to overcome:  Even if you had a model to predict drift of some crappy resistor, it would be impossible to predict the future use case of A LOT of variables that affect ALL resistors - Temperature, power, bias point, humidity, stress, time, duty cycle, etc. etc. etc..  Good resistors eliminate or restrict those variables down to a -much- more manageable scale.

So go start testing cheap resistors!  Let us know how the prediction model works out for you - and don't forget to test repeatability after the power level changes!
 

Offline MisterDiodes

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #21 on: April 01, 2018, 06:38:02 am »
One last thought for RHB - nowhere in your dream $50 spec sheet do I see a target spec for noise (maybe I missed it?) - and that's dependent to a certain extent by the resistor technology used.  Even though the '399 isn't exactly the quietest Vref on it's own, you always have to look at the contribution of resistor noise to the system.  That total apparent noise will always limit the usable uncertainty of the device - especially at lower output levels.

Once you learn why PWW and BMF resistors are (sometimes much) quieter, then you begin to understand the unavoidable low-level chaos and hysteresis effects inherent to other resistor technologies.

Have fun testing!

 

Offline rhb

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #22 on: April 01, 2018, 07:14:41 am »
Perhaps someone would be good enough to point to literature references on the various effects on resistors and their magnitude.  They are after all, sufficiently well known that I had presumed the need to model them even if I did not state such.  Age and temperature are first order effects.  After all, if soldering a resistor will change the value, it's rather obvious that significant changes in current will also.

I'm sorry that I can't supply a complete and finished specification of every last detail at the start of an experimental project.  In my experience, doing that is a waste of time. You estimate the first order issues and then go where the data takes you.

The rock properties project for the super major I mentioned was said to be impossible by a bunch of the research staff at the company lab in California.  I did  it and it worked very well. So well that they had me do it a second time.  The first version was only 9,000 wells predominately on the shelf for the shelf business unit.  Deepwater was so impressed they wanted it done for them.
 

Online hwj-d

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #23 on: April 01, 2018, 07:33:26 am »
Quote
I'm sorry that I can't supply a complete and finished specification of every last detail at the start of an experimental project.  In my experience, doing that is a waste of time.

Sorry, I see some different things that represent "a waste of time". 
 

Offline BNElecEng

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Re: A Low Cost OSHW Voltage Calibration Reference Project
« Reply #24 on: April 01, 2018, 07:44:14 am »
rhb, do you have access to IEEExplore? I ask because not everyone does. I did a quick search and there look to be some good papers there I could send you a link to.

In addition to the usual sources, take a look at Vishay Z-Foil datasheets as well as the papers they have published. They have a reputation as one of the leaders in the field of precision resistors. Here is an example:  https://www.vishay.com/docs/28873/predictable-components.pdf

FWIW, I don't think anyone here is questioning your credentials. You asked the question and received the considered answers of multiple forum members.

 


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