A Low Cost OSHW Voltage Calibration Reference Project

[rhb]

I'd like to suggest people read the introduction to this  paper:

https://statweb.stanford.edu/~donoho/Reports/2004/l1l0EquivCorrected.pdf

I consider it the most important piece of applied mathematics since Norbert Wiener's "Extrapolation, Interpolation and Smoothing of Time series".  There is no need to read the mathematics unless you like such things.  Donoho states the important aspects of the paper in the abstract and introduction in very clear language.

The single pixel camera is a different case. It exploits the mathematics to convert multiple time samples into multiple pixels.  You can find various versions of the paper online.  It appeared in IEEE Spectrum as I recall.  It's really quite cool as they have some pictures.

In the context of a voltage reference one is solving an inverse problem which is an important application of sparse L1 pursuits and what I was doing when I discovered their properties.  However, Foucart and Rauhut only give glancing attention to that.  Candes wrote a paper on Dantzig selector which is essentially the equivalent of what I'm proposing.  One is then using the inverse function as a predictor of future values.

[ArthurDent]

 You can also save a great deal of money on the measurement side of your project. A lot of us can’t afford a bank of HP 3458A meters to fully characterize a $30 hobbyist voltage standard. Other threads and videos have mentioned a feature of the moderately priced HP3457A that can be used effectively. The HP 3457A is sold as a 6.5 digit meter but a lot of people know that the output on the rear panel will give you one more digit (https://www.eevblog.com/forum/testgear/the-mysterious-%277th-digit%27-(hp-3457a-dmm)/ ).

 What I found through lots of experimentation was if you loop this output to the front input of the HP 3457A and run it through the meter one more time you will gain yet another digit. Great way to save money on a high resolution meter and it fits in well with this voltage reference project.

[rhb]

My hope is to design a multirange  calibrator which travels well and can be calibrated inexpensively which is good enough for a 5.5 digit DMM .  The idea being something that can be sold calibrated to a 3458A automatically and which will improve in accuracy over time as additional calibrations are performed.  I have in mind a retail of $100-150.  The DMMCheck Plus I ordered is essentially what I have in mind but more economical to build and maintain as well as better long term accuracy.

Underlying all this is notion of modeling device behavior as a means of improving accuracy using sparse L1 pursuits.  This thread is really about the  engineering tradeoffs for a mid-range calibrator.  While the techniques are the same, a high precision reference requires a more refined device model..  I think it might be useful to discuss applying sparse L1 pursuits as a separate topic in the context of high precision references

[Theboel]

Dear All,
the discussion is really interesting I am not skeptical nor optimistic about price barrier but I think I can learn something if the project can be done in real world.
for me use the best under sun resistor married with the best reference You can buy is easier than made it to be best reference box some one can do.
but may I suggest to use more "acceptable" for some people than plain XYZ resistor what about use PWW from edwin G pettis the cost is not extra ordinary and the performance is not the worst thing we have seen. Yes some people has "bad experience" about edwin resistor but I do not think thats end of the world I am more optimistic if LTZ1000 with Edwin res will more acceptable, "easier" to deal with, more data already floating around even if You need a special format data some one can and will help.
just my 2 cents opinion.
Anton

[eurofox]

I have the intention is to apply a thermal treatment (annealing) to remove the mechanical stress after the machining work, I have a oven and equipment for this as well the knowledge to do it.

With respect to filling with inert gas, it cross my mind but somehow at least in my case I have a buffer to output 10V and need to be tuned in the metal container after aging, this mean there need to be an small hole just above the trimmer and need to be closed hermetical, not easy to do with gaz ....
I will mill under the PCB an area to place 2 packets of silicate gell, they will suck the humidity that could enter te container during the trimming job.

Braze an inner tube or tubeless tire stem to the hole for access to the trimmer.  Then use a metal cap and gasket to seal it after adjustment.  It does require having a tank of dry gas around, but I suspect you've got a bottle of argon or argon/CO2.   Nitrogen is just cheaper.  Assuming everything is dry, the gas just displaces any moist air.  It doesn't need to be under pressure, though it will if the atmospheric pressure drops.  I'm suggesting a tube you can poke into the unit to inject the gas near the bottom.

In fact it is very easy to fill with argon, argon is more than 1.3 times heavy than air this way it make it easy to fill the "container".
I will move the voltage trimmer outside of the container, wires wil pass outside on copper screws molded in epoxy.

I got the PCB in the mail today, got almost al components including resistors from Edwin Pettis, just need to order the LTZ1000.

eurofox

[rhb]

I think keeping the trimmer inside is the way to go.  Topping it up with a shot of argon after adjustment is easy and it avoids noise pickup.  Also the argon will prevent oxidation of the trim pot.  So the temporal stability should be better.

[rhb]

A very interesting design.  It won't change the thermal noise, but it would reduce the current noise and it would result in more uniform temperatures for the resistors and generally smaller temperature gradients across the board.  It would also reduce the shift from turn on to stable operation caused by resistor heating.  Under assumptions of a Gaussian distribution the resistance tolerances are reduced by almost 1/3.

A very useful experiment would be to set up a voltage reference feeding a divider as presented here and a 3 resistor divider with all the resistors thermally coupled using silicone grease in a small thermally insulated enclosure. and collect data along with resistor ensemble temperature for a few days at the maximum sample rate available i.e. PLC = 0.  That would allow comparing a wide range of noise suppression techniques.

[rhb]

The sort of thing that @zhtoor suggested is exactly what I'm looking for from others in this effort.  This is cost -performance engineering.  The sparse L1 pursuits which I hope to employ to substantially improve performance are better treated separately.

EMI suppression suggests that the reference should be completely  isolated from the UI MCU.  That makes the reference a pure analog design project along traditional lines. This makes it very much easier as the important factors are well understood by most people in this group.

Revised outline:

precision reference
buffer amp
biasing  resistors
3-4 bit precision DACs in a Kelvin -Varley configuration so that each range has an approximately uniform set of steps over the full range.
serial to parallel shift register

single input supply line
single voltage output line
single input control line for selecting the output voltage step
single output line for temperature

careful EMI filtering on all enclosure penetrations
4 layer board to reduce noise propagation
smallest possible shielded enclosure filled with thermal grease

I have a rough idea what the issues are in constructing a reference, but I do not know all the details and I  have no knowledge of the various circuit designs which have been used in high end instruments.

So  can we design  to meet the above criteria for a $30 BoM?  That leaves $20 for UI, etc.  Conrad Hoffman's mini-metrology lab is the best analog to what I'd like to do with this project.  It's almost 40 years later which is a generation or two in technology. Can a team of smart people design a better version using more modern devices?  Can we make getting periodic NIST traceable calibration affordable?

Setting up a sparse L1 pursuit to predict and correct errors due to aging, temperature, etc is a major project in itself.  It might not work.  Based on my experience with sparse L1 pursuits that led to my spending 3 years slogging through a lot of math I happen to think it will make an order of magnitude or better improvement.  But it will take months to develop and several years to test.  I'd like to keep that discussion in the "Applications of sparse L1 pursuits"  thread.

Have Fun!
Reg

[thermistor-guy]

hello guys,

how about starting with this lm399 based scheme (initial proposal):-
...
2. the divider resistor r1 is 10x 10k resistors in parallel. https://www.digikey.com/product-detail/en/yageo/MFP-25BRD52-10K/10KADCT-ND/2059114
3. the divider resistor r2 is 10x 100k resistors in parallel. https://www.digikey.com/product-detail/en/yageo/MFP-25BRD52-100K/100KADCT-ND/2059112
4. the zener bootstrap resistor r3 is 5x 100R resistors in series. https://www.digikey.com/product-detail/en/yageo/MFP-25BRD52-100R/100ADCT-ND/2059113
...

On the selection of resistor values: two non-ideal aspects of resistors are leakage current through the body/coating, and contact resistance.

Points 2,3,4 above are low-cost axial metal film resistors. So the contact resistance from the end caps to the metal film will change with age, temperature cycling, etc. I imagine leakage resistance will also change with age, temperature cycling, applied voltage, humidity history etc.

If, for example, the leakage resistance is around 1E10 ohms, and the contact resistance is around 1E-2 ohms, then the geometric mean of 10 kohms minimizes the impact of both factors (1 ppm each).

Contact and leakage resistance may change in ways that are hard to predict. These also may be, themselves, sources of excess noise. Is it worth choosing resistor values to minimize the impact of these two factors?

[rhb]

The datasheet for the suggested resistors shows that they have rather good specs for stability.  The use of multiple resistors in parallel reduces the effect of self heating which is a significant factor in aging behavior.  And the central limit theorem predicts a reduction of  the tolerance from 0.1% to 0.031% and the tempco  from 25 ppm/C to 8 ppm/C.  Not bad for $0.80 + $0.10 of PCB.

The goal here is the best possible voltage calibrator  which can be manufactured in low volumes and sold for around $100.  That requires astute engineering choices. Ultimately, suggested solutions will have to be tested before being adopted.

[cellularmitosis]

When the drift is predicted, will the output be corrected by adjusting the circuitry, or will it print out a correction value which the user applies on paper?

If you are going to adjust the circuit, you could use jfets as variable trimmers.  On my long-term to-do list is a Harmon divider with an “auto cal” button which uses these jfets to balance itself to an exact 10:1 ratio.

[svenskelektronik]

Hi guys.

I've been following this thread and discussing the matter by mail with rhb. As a digital guy I'm enjoying this project as it's brings something new to the table for me. My motivation to participate and follow this project is also due to my interest in OSHW where I want to test some development techniques widely used in FOSS with the hope to make OSHW more widely adopted.

I've made a schematic drawing draft in KiCAD based on the KX LM399 which has been uploded to https://github.com/svenskelektronik/lm399_precision_voltage. Please consider to fork it and submitt your changes by a pull request. The design uses the advatage of parallell resistor banks as suggested by Zia, please feel free to express your opinions on how many you think is sufficient to use. Another thing on the to do list is to determine what resistor to use, MFP Series as suggested by Zia seems like a good candidate. We may start just determine the footprint and then later descide the resistors. Me and rhb suggests the removal of the second LM399 in the design.

[TiN]

And not a single link to a source? That's kinda sad move.

I had some comments on my schematics (and why it's not very useful in perspective of this thread) but I'll hold my thoughts thanks to this. 

[rhb]

I had in mind simply displaying the "correct" value of the output rather than disturbing the operation of the circuit.  To my mind the reference should be  as simple as possible.    I've set a very difficult BoM cost target.  Autocorrection would require a JFET and DAC.  That costs more money and complicates the system error model.

I'm quite impressed by the performance of the basic-lm399 references I was given.  I don't yet know about long term accuracy, but they certainly seem adequate for calibrating a 3478A with a modest amount of signal processing.

The multiple resistor improvement in accuracy and tempco should be 1/sqrt(N) for random variations.  The reduction in current noise for parallel resistors should also follow 1/sqrt(N).  We should calculate the cost to reach a particular level of precision using various parts accounting for PCB space and placement labor.  Reducing the current in each resistor also improves the aging behavior.

I'm unclear what the best approach to implementing the output voltage divider is. I have in mind approximately equal steps across each range from 10 V to 100 mV max.  It would be nice to have a 10 mV fullscale, but I rather fear that would be too noisy to be of value.

If the device were mounted on a dual banana plug in a well shielded enclosure on battery power so that in use it was mounted on the front panel inputs of a DMM would a 10 mV fullscale range be possible with acceptable performance?  Use an external PS for warm up and switch to the battery for the actual calibration.

[TiN]

Usually DMMs don't have 10mV fullscale, that is realm of nanovoltmeters. nV-meters also rarely banana plugs, as these are not optimal for low-level VDC measurements.

[rhb]

I'd like to suggest that the serious thought be given to placing the critical resistors on a daughter board if that is practical.  That way we can experiment with different resistor options, e.g. PWW, etc, without having to rework the board layout and without using excess board space if parallel resistors are not used.  Perhaps use the footprint of the largest precision resistor anyone would want to use as the daughter board footprint and mount the board at 90 degrees to the main board.

[Andreas]

Hello,

the absolute maximum ratings allow only 0.1V in forward direction for the substrate diode.

Note 2: The substrate is electrically connected to the negative terminal of the temperature stabilizer. The voltage that can be applied to either terminal of the reference is 40V more positive or 0.1V more negative than the substrate.

So perhaps another dead LM399.

with best regards

Andreas

[Andreas]

I understand the datasheet in this way:
no pin of the reference is allowed to be more negative than Pin 4 (substrate) by more than 0.1V.

[cellularmitosis]

Very interesting idea zhtoor!  I am curious to see if the 399 survived.

I was looking at the Spreadbury paper again recently and was reminded that these refs don’t seem to drift much while turned off (and I seem to remember reading in another thread that Dobkin said this is also true of the 399).  Perhaps the poor man’s route is to have an additional 399 which is only turned on one day per year?

[Andreas]

Very interesting idea zhtoor!  I am curious to see if the 399 survived.

The question is:
is there enough "before" data (zener voltage, noise, ageing rate ...) to judge any abnormal behaviour.

[cellularmitosis]

Very interesting idea zhtoor!  I am curious to see if the 399 survived.

The question is:
is there enough "before" data (zener voltage, noise, ageing rate ...) to judge any abnormal behaviour.

I would be happy to offer up a few silicon sacrifices to find out if a low-temperature 399 is possible

I just need to find a way to retire early so that I can devote more time to volt nutting 

[rhb]

I'd like to repeat my plea for data in ASCII format, especially data from from start up of a new device.  To date I've received very little data.  I'm going to digitize the plots from the HPJ paper on the 3458A, but that is just because finding the data 30 years later is unlikely.

I feel pretty good about the results I got for the thermal noise spectrum,  They seem reasonably plausible.  So I'm on to the 1/f noise model.  But without documented aging data, I can go no further.

[rhb]

It seems to me that the heater should be attached to an aluminum enclosure say 6 mm thick with 50 mm of insulation.   Placing the heater on the LM399 makes a large thermal gradient.  Self heating makes  gradients unavoidable, but they can be minimized.

I'd been working on this off and on all day.  I'll just include it as is.

Some random comments.

The resistors are as much of a problem as the zener.  The critical parts of the reference should be in an enclosure made from 5-6 mm aluminum.  Two U shaped pieces with external heaters attached to two faces and embedded in insulation should be inexpensive and provide a uniform temperature.  Place some thermistors in suitable spots to check for gradients, at least for the prototype.

One might let the user input the expected maximum ambient temperature and set the chamber temperature based on that.  But that would conflict with fitting an aging curve calculated from the first 500-1000 hours of operation.  Aside from temperature, that seems to me to be the largest error source.  There is the question of whether it is better to burn in at elevated temperature or at operating temperature.  I'm skeptical of having a stable aging curve with high temperature burn in, but it might not matter.

Interestingly, a 40 x 40 mm Peltier device is under $3 each on Amazon for 10.  For a shallow enclosure, two might allow setting the operating temperature independently of the ambient temperature.  It's a big power drain, but it might be tolerable for a lab reference if the operating temperature were average ambient.  Quite a bit more complicated to design.

Such an enclosure is easily made using a 20 ton hydraulic press and a very simple die set.  Anneal  before and after forming and then mill the mating surfaces, drill and tap.  Seal the enclosure with butyl sealant to eliminate humidity effects.

In reading back through the thread I realized I'd never properly answered the question about how you get 6.5 digits from a 5.5 digit meter.   You deliberately add random Gaussian noise which exceeds the resolution of the 5th digit.  You take two long measurements and crosscorrelate them.  The noise does not correlate so the cross correlation only sees the DC value.  It's apparently called "dithering" in metrology.  The limitation is the length of time it takes to make a measurement.  TANSTASFL.

The early part of the aging curve has the most curvature.  It might be that lowering the operating temperature would extend the duration of the large curvature region.

My original premise was that if you built a device of the best, low cost, parts and then measured the behavior during a 500-1000 hour burn in, that you could describe future performance sufficiently well to pick up an order of magnitude improvement.  Such a burn in would include temperature excursions to quantify resistor TC, tolerances and aging. in addition to the reference and buffer amp behavior.

Fluke believes that the observed hysteresis effects are due to stress on the die and have patented temperature annealing the die on cold startup on the 7001.  I'm not convinced that is sufficiently innovative to warrant a patent, but patents are really just a license to sue others.

[cellularmitosis]

i would think to maybe boil a bunch of lm399's or give them some kind of an un-powered cyclic thermal pre-treatment which takes care of
this factor before going in with electrical burn-in / characterisation. (anybody here with die packaging/attach experience? MisterDiodes maybe?)

This approach seems to have worked in 1960 for regular zener diodes.

[montemcguire]

That basic process could be called 'annealing', and as you state, it can be done by thermally cycling the assembled circuitry, powered or unpowered. Aside from stresses inside of the actual semiconductor devices, there are also sources of mechanical stress caused by soldering during assembly. The thermal coefficient of expansion of copper foil is generally different than that of the PCB substrate and any of the devices soldered to the PCB, so if a PCB is being reflow assembled, the solder joints will harden first, and then all of the components will cool with usually different thermal expansion coefficients, introducing mechanical stresses to the assembly. By temperature cycling the assemblies close to the plastic temperature of the PCB substrate, a portion of these stresses can be relieved, probably minimizing long term drift caused by mechanical stress on the components.

Another approach would be to use conducive epoxy to assemble a circuit, so that the problem of the solder solidus temperature being so far above the working temperature is avoided. Epoxy can bond near the thermal set point temperature, so there's no reason why the assembly process needs to 'build in' mechanical stresses to all of the components.