1V Reference @ 1ppm - Page 1
Forum section: electronics / beginners

author: Rafael
author_rank: Regular Contributor
timestamp: January 26, 2019, 12:22:53 am
content: Hello,Does anyone have any suggestions on how to find / build a 1v @ 1ppm reference?Thanks a lot!

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 12:45:42 am
content: Quote from: Rafael on January 26, 2019, 12:22:53 amI assume you mean: 1ppm/°C. Over what temperature range???

author: Rafael
author_rank: Regular Contributor
timestamp: January 26, 2019, 12:50:25 am
content: Quote from: Wimberleytech on January 26, 2019, 12:45:42 amYou're right, sorry to forget this detail.Range, around 23°C... I can imagine a hard task!Thanks!

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 12:50:56 am
content: Quote from: Rafael on January 26, 2019, 12:22:53 amhttps://www.apexanalog.com/resources/products/vre3050ds.pdf

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 12:53:14 am
content: Quote from: Rafael on January 26, 2019, 12:50:25 amYes, it is challenging. You must curvature compensate a bandgap reference...and pay attention to EVERYTHING.Solution: Buy it. I posted an example product.If you limit the range of temperatures ("around 23°C"), you do not have to do curvature compensation.

author: beanflying
author_rank: Super Contributor
timestamp: January 26, 2019, 01:07:03 am
content: I have used quite a few of the MAX6350 & 6341 (5 & 4.096V 1PPM/C) on different jobs very good initial accuracy and stable over time but can still be tweaked if needed. One was a Mercury Button Cell (1.35V ) replacement for an old HP anchor I own. Buy some good quality Vishay or similar resistors (low temp co) and divide it down.https://www.eevblog.com/forum/testgear/hewlett-packard-740b-dc-standard-digital-voltmeter-(and-740a)/msg1438536/#msg1438536

author: spec
author_rank: Frequent Contributor
timestamp: January 26, 2019, 10:45:27 am
content: Quote from: Wimberleytech on January 26, 2019, 12:50:56 amNice device, but the price: £138UK Ouch

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 01:47:05 pm
content: Quote from: spec on January 26, 2019, 10:45:27 am...you gotta pay for precision! The Maxim parts are much cheaper.

author: mycroft
author_rank: Regular Contributor
timestamp: January 26, 2019, 05:17:40 pm
content: Many, many years ago, i read an article by NBS (now NIST) on using a mercury cell as a voltage transfer standard. The major problem was temperature sensibility. I would like to find this article again. I searched a lot without success. Anyone remembers this article and has a reference?

author: beanflying
author_rank: Super Contributor
timestamp: January 26, 2019, 05:52:49 pm
content: Quote from: mycroft on January 26, 2019, 05:17:40 pmNot the article you wanted but some of us still have some Standard Cells and play with them from time to time.https://www.eevblog.com/forum/metrology/1000-my-(hi)story-of-the-weston-cell-of-the-volt-and-of-being-a-volt-nuts/msg1039421/#msg1039421I had mine out of the boxes a couple of days ago in 35 degrees ambient. It was 0.06mV up on last time I recorded it at about 20C and some of that drift would have been the 34401A. The un-saturated ones had a better tempco than the saturated ones. Generally run in banks and averaged when used as references too.EDIT: NIST is currently offline thanks to the USA shutdown BS but try this search on Google 'nist weston standard cells' will get you some pdf hits when it gets back up.

author: spec
author_rank: Frequent Contributor
timestamp: January 26, 2019, 10:02:56 pm
content: Quote from: Rafael on January 26, 2019, 12:22:53 amHi Rafael,I couldn't find a 1V voltage ref to meet your spec, so the only option is a higher voltage voltage ref, say 1V25, two resistors, and a precision opamp.You could make a miniature oven quite easily and pop the whole circuit in the oven to improve the stability with ambient temperature changes.As you no doubt know, the power supply would need to be pretty fancy, but all within practical bounds.How much current would you want to draw from the 1V reference?By the way, I am no expert on extreme high-spec voltage references, so these are just my thoughts.If you are interested in this approach, just say and we can discuss it some more.

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2019, 10:17:23 pm
content: Quote from: Rafael on January 26, 2019, 12:22:53 amThere is a lot of information in other threads on this forum, in the metrology sub forum.Voltnuttery is expensive, very expensive.But your question is easy to answer - but is it the right question? Hint: state your problem, not your presumed solution. For reasons, seehttps://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2019, 10:19:19 pm
content: Quote from: beanflying on January 26, 2019, 05:52:49 pmI can use my working 1949 Weston standard cells as a thermometer, 40uV/°C

author: beanflying
author_rank: Super Contributor
timestamp: January 26, 2019, 10:26:32 pm
content: Quote from: spec on January 26, 2019, 10:02:56 pmIf it is a 'reference' the answer would be as little current as possible to maintain voltage stability. Op amp buffers can be added but generally they add to errors and tempco problems not help.There is for example a 0.01mV variation on my HP735A Transfer Standard between 10M and the GigOhm impedance settings on my 34401a so if it is being used as a transfer it has to be set to allow for this depending on the intended use. So load matters even if tiny.

author: spec
author_rank: Frequent Contributor
timestamp: January 26, 2019, 10:38:35 pm
content: Quote from: beanflying on January 26, 2019, 10:26:32 pmYes, I do appreciate that that is a very specialist area and that 1ppm is only 1uV in 1V, so what I was going to do was to do the best I can and knock out a circuit and some notes for discussion. To my mind, the opamp is the biggest problem because choppers would be too intrusive.

author: beanflying
author_rank: Super Contributor
timestamp: January 26, 2019, 10:53:33 pm
content: The Maxim Ref I pointed to earlier in the threadhttps://datasheets.maximintegrated.com/en/ds/MAX6325-MAX6350.pdfis about 1ppm/mA typical (seems to be correct from my testing) so a 10k total divider would keep it fairly close to that or better and if you fit up the extra pot shown on the datasheet it can be tweaked up if needed without needing an op amp.

author: spec
author_rank: Frequent Contributor
timestamp: January 26, 2019, 11:14:03 pm
content: Quote from: beanflying on January 26, 2019, 10:53:33 pmI was thinking on similar lines after you said that the loading would be very low. Will have a look at your suggestion.If you had a 1V25 precision Vref (the lowest extreme precision voltage you can get), and connected a precision 250R and 1k resistor across it, as a potential divider, that would produce 1V with a source resistance of 200R (taking the source resistance of the Vref as 0R), which would mean that a 200M load would only result in a delta V of 1uV or 1ppm, all other things being equal that is.

author: spec
author_rank: Frequent Contributor
timestamp: January 26, 2019, 11:42:58 pm
content: Quote from: beanflying on January 26, 2019, 10:53:33 pmOK had a look at this. To have a 10K potential divider to produce 1V from a MAX6325 (2V5), would require 6k north and 4K south. This would give a source resistance of 2k4, which would mean (ignoring the MAX6325 for the moment), that the minimum load for a 1ppm voltage delta would be 2G4.I have also been thinking about the OP's requirement of 1ppm/deg C. But the specification we need is what voltage delta is acceptable over what temperature range and with what delta loading resistance, if there is a delta load- perhaps the load is fixed, which would simplify the design a bit.

author: tggzzz
author_rank: Super Contributor
timestamp: January 27, 2019, 12:01:46 am
content: Quote from: Rafael on January 26, 2019, 12:50:25 amAre you concerned about drift over time? If so, you may need lengthy burn-in times.

author: beanflying
author_rank: Super Contributor
timestamp: January 27, 2019, 01:42:13 am
content: Without wanting to go way down the rabbit hole of voltnuttery a full and complete spec of the OP's requirements would be needed or we are just speculating on the requirements outside the 1V @ 1ppm of ? (voltage, temperature, drift over time etc) and what is the references intended use? So some more info is really needed.Just run over the rough numbers based on the 10K guess I used this morning. -To get the divider of 6 and 4k (assumed dead accurate) you would actually get 0.9996 volts read assuming the nominal 2.5V 10Meg meter so it would need the compensation pot fitting to the circuit to tweak it accordingly if it's purpose is for calibration and if not the input impedance of the driven circuit needs to be taken into account regardless.My couple of tweaked MAX 6350's I built have remained within spec and have been powered on now for over a year so they seem to hold up well after the first week or so of settling. There was some questions raised about the epoxy version available (ceramic is off the market) could be susceptible to moisture over tim but that is well into VoltNuttery @ Andreas level territory (nothing wrong with that at all btw)

author: ArthurDent
author_rank: Super Contributor
timestamp: January 27, 2019, 02:30:58 am
content: I appreciate the irony that someone asking about precision voltage has a tag line of "LTZ1000 for voltages"

author: Rafael
author_rank: Regular Contributor
timestamp: January 27, 2019, 03:17:20 am
content: Quote from: tggzzz on January 26, 2019, 10:17:23 pmThanks a lot for all answers... As a beginner, I need to learn a lot before try some adventures in Metrology section.I have an old russian voltmeterhttps://www.eevblog.com/forum/beginners/31-voltmeter-sch-31-nixie-russian-voltmeter/and I need to calibrate it with 1v reference.@ArthurDent, I really really admire the precision that the component can generate, but for 1V, it gets a little further.edit: Or maybe the answer? Please, be gentle with me...

author: beanflying
author_rank: Super Contributor
timestamp: January 27, 2019, 04:08:55 am
content: Nice NixiesI hadn't seen the thread hiding away in the Beginners section. I have seen similar ones on evilbay from time to time and occasionally tempted.Given the instrument you really need a better 1V source than even 1PPM assuming it's own internal reference is at least that stable. Those cheap Chinese references are reasonable to 3.5 and 4.5 digit meters or as a sanity check on whatever you may build is close to the actual 1V but no more. The initial accuracy of the Maxim references and the easy trimming I would have confidence to better than 4.5 digits against another good meter but not to 6.5 digits.Nothing I have for example is up to 'recalibrating' my Agilent 34401A other than I have a pair of 34970A dataloggers with 6.5 digit multimeter cards fitted that are near the same spec. All of my references held at a static temperature give me a reasonable level of certainty as to how the 34401A is behaving over time and temperature but as it is currently out of Cal it really needs to take a trip to a lab or I need to scrape the $ together for a 3458A and keep that in Cal to keep the rest in lineWhat should be important to you is that YOU are happy you have the required level of precision, accuracy and certainty YOU need unless you go to the cost of getting the meter calibrated at a known Lab. Pick a day or conditions at a known temperature and record it preferably 20-25C. Then raise or lower just the meter if possible and see how the drift is with temperature while the reference hopefully remains at a static temperature. Also watch your connections to the meter you will get several PPM of error with a bad connection. The Russians use an oddball size under 4mmShould you build a reference YES but bear in mind what that reference will give you. The Max will get you about as good as you can get for initial accuracy without breaking the bank. To go to an LM399 or LTZ gives you more ultimate stability but a wider range of initial accuracies so if you don't trust the meter a worse result

author: tggzzz
author_rank: Super Contributor
timestamp: January 27, 2019, 09:02:07 am
content: Quote from: Rafael on January 27, 2019, 03:17:20 amIf you follow the points in that reference, you will find that most people on most forums will be friendly. A small proportion might not, but that's people for you.QuoteYou probably want to calibrate each range separately; a stable known voltage and a 7 decade Kelvin-Varley Divider is a good starting point. You will realise there is no end point in voltnuttery, only an acceptable level of dissatisfactionYou might like to do some initial experiments.For example find a battery which is reasonably stable, keeps its temperature reasonably constant, connect it to your meter, turn on the meter, and record the measured value until it becomes stable. Some meters (particularly old ones) take quite a few hours to stabilise. Keep an eye out for popcorn noise in the meter's reference, and variations with temperature.When at the uV levels, seebeck/thermocouple effects become noticeable.

author: iMo
author_rank: Super Contributor
timestamp: January 27, 2019, 09:47:54 am
content: The answer is simple: there is not such an 1V/1ppm source available in form you would expect (not talking volnuttery capable calibrators). As I can see on the pictures from your link you are already volnut-positive, so you are asking in a wrong section

author: beanflying
author_rank: Super Contributor
timestamp: January 27, 2019, 10:08:16 am
content: As you already have the same reference I jotted down some figures using my matching one this afternoon against my 34401A over a few Temps. Both meter and Reference were in the same environment and you will get an idea that the combined drift is +1ppm/C @ 10V the bulk of this based on other testing will be the Vref as the Agilent is well under 1ppm over this sort of environmental range.Interestingly the buffer used on it is not effected by loading at either 10M or 1G more than 1 least significant digit.Mainly done for me to upgrade the data for it after a years use. These cheapies are far from terrible for3.5 and 4.5 digit meters. You might even do reasonably putting a divider across one of these too just keep the values fairly high to minimize loading but still make your own as well25CNominal 25C 28C 32C old data2.5 2.49905 2.49908 2.49912 2.499055.0 5.00024 5.00027 5.00032 5.000217.5 7.49961 7.49964 7.49968 7.4995510 10.00036 10.00039 10.00045 10.00028

author: beanflying
author_rank: Super Contributor
timestamp: January 27, 2019, 10:10:22 am
content: Quote from: imo on January 27, 2019, 09:47:54 amMore than that without a base standard you have nothing to reference it to so you need to do the best you can and are happy with unlike volt nuttery. You can make all the LTZ references you like and still not know what a volt is.

author: Rafael
author_rank: Regular Contributor
timestamp: January 27, 2019, 01:04:02 pm
content: Accuracy and stability really make me happy. Having an equipment and believing in your measurements fascinates me. This antique equipment, Russian, is in excellent condition (at least cosmetic) and I want to get to a point in having vintage equipment with nixie displays and precision.I've invested a few hundred dollars in parts and calibration devices.But I still do not feel safe to calibrate my other equipment! I have older nixie-based devices clamoring for calibration, such as Meratroniks, Fluke, etc ... They leave any bench happy.

author: beanflying
author_rank: Super Contributor
timestamp: January 27, 2019, 01:25:43 pm
content: Not sure if this is widely known around the world but there is an adage "A man with a watch knows what time it is. A man with two watches is never sure." and that is clearly why a third and a forth and a fifth.. Meter is needed. Welcome to TEA.While I am comfortable with what standards and calibration equipment I have available is more than what I need my journey down the voltnut rabbit hole is heading toward a 3458A and paying keep it in Calibration so I can start with a proper traceable voltage among others. If the budget doesn't allow it this year then my 34401A will get re calibrated by Keysight to get me a little closer to the truth and allow me to go over all of my other gear with good certainty.I have a few Nixie meters in my collection there is just something about the glow

author: spec
author_rank: Frequent Contributor
timestamp: January 27, 2019, 02:59:43 pm
content: Quote from: beanflying on January 27, 2019, 01:42:13 amwhich is -240ppmQuote from: beanflying on January 27, 2019, 01:42:13 amThis seems to defeat the whole purpose of a precision voltage source, or have I missed something?The switch from virtually no load, to a relatively heavy load of 10M has changed the whole aspect of the design.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2019, 05:00:30 pm
content: To calibrate a meter one would likely need both a stable and accurate reference. So just building a reference could result in something like 1 V with < 1 ppm/K TC , but the absolute accuracy would be poor (e.g. like 0.1% or maybe even worse).For a good absolute accuracy chances are a used Hg Cell (e.g. Weston) is probably the best chance if you don't want to pay for a used Fluke 732 or similar. Depending on the meter it may not need exactly 1 V the slightly higher voltage mercury cell may be acceptable too. It would still need a precision buffer

author: beanflying
author_rank: Super Contributor
timestamp: January 28, 2019, 12:56:59 am
content: Quote from: spec on January 27, 2019, 02:59:43 pmIt depends on what you want the divider to show and to what particular DUT. What Raphael needs effectively is a transfer standard using a cost effective method. Not sure of the nixie meters input impedance but as it is a 6.5 digit meter likely to be a lot higher than 10M?My 735A Transfer Standard is sensitive to loading but it has a compensation pot available on the front panel to tweak this nominal 1V if needed. Normal procedure with this device is to Null it against a Weston Standard Cell (known voltage) or similar like I showed previously to set it's internal reference then via the internal divider network set a 1V output. Null meters in common use at the time in theory have an infinite resistance at the Null point so don't load the cell or dividers. Manual is here if anyone is interested and it goes into the theory of operation and calculations for lower impedance meters using this transfer standardhttp://literature.cdn.keysight.com/litweb/pdf/00735-90002.pdf?id=1870825To do this without a known volt source other than the 'claimed' accurate voltages on the 584K and or any other reference he may makes the 'initial accuracy' is the best guide to how close that may be and then test one against the other. So the MAX 6350 will have an initial accuracy of 5V +- 0.5mV in practice it will be better than this spec. Part of the issue is we are trying to make wine from pig swillYou will not get the 'perfect' resistors to divide up the 2.5 or 5V source to 1V so compensation will be needed be that adjusting the input voltage or providing compensation on one or both legs of the divider. The reason I suggested tweaking the Reference is the trimpot (generally terrible tempco) will be fairly unaffected by temperature variation due to it's already narrowed range. The alternate is to do something similar to the 735A circuit and minimize the value of the trimmers in the circuit.If he is to build a new reference I would use the 6350 as a start point as the +-0.5mV initial accuracy is the same for the 2.5V version which would make it twice the error.

author: spec
author_rank: Frequent Contributor
timestamp: January 28, 2019, 03:32:17 am
content: Quote from: beanflying on January 28, 2019, 12:56:59 amThanks for reply(I do appreciate that most approaches would require an initial adjustment to get the absolute voltage spot on.)I see, a transfer standard- that puts a whole new complexion on the design. What I see is an issue though is that relying on the input resistance/characteristics of the meter to be calibrated may wreck even the performance of a transfer standard as the characteristics of the meter input are not known (to the 1ppm level that is). Or are you proposing adjusting the transfer standard every time it is used? Perhaps that is the normal procedure.Interesting stuff about your 735A transfer standard.I wonder if a ref voltage of 1V25 would be acceptable because, if so, an LTC6655 based design could be used and no potential divider would be necessary. Admittedly the LTC6655 is 2ppm but, maybe, keep the LTC6655 at a constant temperature to fix that in terms of the overall performance of the voltage reference unit.On the other hand, if a 1V reference was essential a voltage divider for the LTC6655 could be made to have a lower output resistance and that would reduce the effect of loading delta/unknowns.It is a shame this is not a volume production job, because then we could just phone up the manufacturer, Maxim/LT/AD etc, and get them program a 1V Vref version for us.https://www.analog.com/media/en/technical-documentation/data-sheets/6655ff.pdf

author: beanflying
author_rank: Super Contributor
timestamp: January 28, 2019, 04:04:13 am
content: Just to complete the sort of usage method here is what I did today.Agilent 34401A set to 10Meg Impedance.Adjust the HP 735A Transfer using the meter to 1V (actually ran out of cal range on the front panel so 0.999993)Take that know 'known' (as known as I can make it) voltage and apply it to the 10Meg inputs of the DUT in this case the 121GW.Result the 121GW is horrid and should never be trusted to read anything for reasons at least according to some .....For interest the 735A is well under 1ppm/C (done by longer term logging) and the tests were done at about 28C as per the121GW.The procedure using a homebrew solution would be fairly similar prepare the source as best you can for the test you plan for the DUT in question.

author: beanflying
author_rank: Super Contributor
timestamp: January 28, 2019, 07:04:16 am
content: Quote from: spec on January 28, 2019, 03:32:17 amGenerally for Cal points they are fairly specific. So Apply this then tweak that scenario is normal. So you would need the 1V.Most meters would take the form of 1V, 10V and maybe 100V and 1kV (34401a needs all of these for a full cal) depending on how it is designed to be calibrated just for DCV. The 121GW for example needs 5,50,500 & 600 for the DCV and a couple of others for the mV range for a full Cal of just the DC volt side of the meter. This is why in part you see DC Calibrators to 1000V.The thing to bear in mind is you were to use say the higher accuracy version of the LTC6655 at 1.25V then it's initial accuracy (0.025%) is 1.25 +-0.3mV so divided down the best 'known' accuracy will be higher than the MAX6350 not a deal breaker but if you don't have known volt it is an issue. A little care needs to be taken regards the long term stability figures of both devices and both will tend to settle with use to value X and then need an additional tweak but to what is the questionNow we have scared or driven off everyone to or away from Metrology for ever come on in the Rabbit Hole is large and very very deep

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2019, 07:01:54 pm
content: The Mercury reference cells are just 2% higher than 1 V. So it might still work. With such an old meter this might have well been the reference of choice at that time. Even if one had to divide down to exactly 1 V, it would help starting at just 1.019 V - the divider resistors would be responsible for only 2% and thus something like 12 times less critical than in a 1.25 V to 1 V divider.An accurate 10 V reference and Hamon type divider (accurate 1:10) could be an option too.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 28, 2019, 09:35:44 pm
content: Quote from: Rafael on January 26, 2019, 12:22:53 amIf you are serious there is but one choice: Make a Josephson standard. All other stuff is a crippled compromise. 1ppm is below everything you can buy from renowned companies. And dont forget to join the TEA and the voltnuttery forums.

author: beanflying
author_rank: Super Contributor
timestamp: January 28, 2019, 11:02:49 pm
content: Quote from: Kleinstein on January 28, 2019, 07:01:54 pmCan you still buy Mercury batteries in the EU or only from suspect sources?Building a Hamon Divider for myself too when Mr Pettis catches up a bit

author: ArthurDent
author_rank: Super Contributor
timestamp: January 29, 2019, 03:13:29 pm
content: Here is a photo of the first of my two LTZ1000ACH 10 volt buffered references. It has been powered on and aging for months and is pretty damn stable. When in use it will only be run on battery power to avoid any possible offsets caused by any stray coupling caused by the power lines. Even with all the care I've used to construct this standard it still has to be adjusted and measured and no matter what I do it will fall short of 1 ppm.If you can get 10 volts it is simple to divide it down to 1 volt.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2019, 04:13:26 pm
content: Quote from: beanflying on January 28, 2019, 11:02:49 pmA mercury battery (like those old coin cells) would not help that much, they are more like 1.45 V and relatively high TC. There is essentially no way to get them anymore. AFAIK they are not manufactured anymore and thus would be old and likely bad by now. The point are used mercury reference cells. These are larger and last longer and still sometimes come up on ebay or similar. However they are difficult to transport - not all of them are like transport per parcel and the transport may not be allowed anyway, because of the mercury and acid inside. The cell may still need an extra high impedance buffer and temperature compensation or stabilization.For a Hamon divider one does not need that super stable resistors. The idea usually is to first adjust the resistors chains to be equal and than do the extra connections to go from 3:3 to 1/3:3 resistor ratio. So it needs an extra adjustment step before use. The final accuracy is like the square of the resistor ratio accuracy. So starting with 0.1 % resistors is enough to get to the ppm range. It still needs care to avoid errors from self heating, thermal EMF and leakage.

author: spec
author_rank: Frequent Contributor
timestamp: January 29, 2019, 04:16:19 pm
content: Doesn't a Harmon divider just provide an accurate, and stable potential divider? If that is the case, doesn't that still leave the inaccuracies cause by the 10M Ohm load of the meter under test?Doesn't the same problem of loading apply to all reference cells too? Presumably, they have a significant output resistance (in terms of 1ppm).

author: spec
author_rank: Frequent Contributor
timestamp: January 29, 2019, 04:32:04 pm
content: Can I ask a question of the experts in this field: When you are checking the precision of an instrument, what is the shortest time that the instrument could be connected to the reference source? I have a reason for asking.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2019, 04:37:55 pm
content: A mercury reference cell would need a butter for a 10 M input impedance meter. This usually also applies to a Hamon divider, unless rather low impedance, which would call for self heating problems. However 6 Digit DMMs often have more than 10 M input impedance in the low voltage ranges.

author: tggzzz
author_rank: Super Contributor
timestamp: January 29, 2019, 05:36:00 pm
content: Quote from: spec on January 29, 2019, 04:32:04 pmI would suggest "until the reading is stable and repeatable", where you have to supply the two obvious definitions.Are you thinking of a mechanical multimeter and a Weston standard cell?

author: spec
author_rank: Frequent Contributor
timestamp: January 29, 2019, 07:27:53 pm
content: Quote from: tggzzz on January 29, 2019, 05:36:00 pmMy thought was to use a capacitor to lower the effective output impedance of a precision voltage divider but, in view of your and K's, remarks it is clear that a fair amount of time would be required to do the calibration so that idea goes down the tubes as only a film capacitor could be used. A supercap, which could allow sufficient time, would be too leaky, I think

author: beanflying
author_rank: Super Contributor
timestamp: January 29, 2019, 11:46:19 pm
content: This can just go around in circles until you fall deep into the rabbit hole. When you go beyond 4 1/2 digits of resolution each extra one is 10 times the work to do it right. Until you have a traceable stable known reference to work from you will never truly know what you are reading in particular at 6 digits and greater.So at some point failing that YOU need to create YOUR Volt preferably as close as you can to the 'true' value.For a basic home lab this can be as simple as pick your best meter and what it says rules the others. Worst option so buy/make a Vref.Buy one of the AD584 based references making sure it has the 'known' voltages recorded on it (most seem not to have this currently). Works well offers a range of outputs and is stable against 10M and GOhm meters. Suffers some initial drift from stated figures over time and drifts in the order of 1PPM/C. Should handle a divider to 1V and maintain ok stability and accuracy of its voltage with a small load.Standard Cells of various sorts. SeeRocking Horse PooRequires a GOhm meter or preferably a Null Meter and several cells. Not practical but a stable over time now curio and a sanity check device for me - if you can buy one they are Cool. NIST is back from being shutdownhttps://www.nist.gov/sites/default/files/documents/calibrations/mn84.pdfLM399 and LTX1000 based devices are POINTLESS unless you have a 'known' accurate standard or meter to set them with their initial accuracy varies widely. They will only be as 'accurate' as the device YOU measure them with but generally they are more stable than other References due to Temp but can take weeks to years to settle. They are a can of worms in the Rabbit Hole with you in free fall but always just out of reachRoll your own using the best initial accuracy Reference you can buy sensibly back to the Max series (Best Initial Accuracy) again unless someone else knows of a $10USD one that is better? I have Two in aluminum boxes running from LiPo batteries tweaked to give me an accurate 10V I am happy with for outside use. I do also own a Fluke 515A portable Calibrator which is great to 4.5 digits meters but not so good for 6.5 as it drifts more than the Max based References.Use your best 'known' Reference to 'Null' out a separate supply with more grunt if you need it to avoid loading issues and test it's performance as you load it to see if it remains 'stable' as you load it. Feed that into a Hamon or whatever divider or op amp you like and test away being happy in the knowledge it is YOUR Volt

author: tggzzz
author_rank: Super Contributor
timestamp: January 30, 2019, 01:08:40 am
content: Quote from: spec on January 29, 2019, 07:27:53 pmCapacitors are crap components compared with resistors. Leakage, tempcos, non-linear voltage dependence, dielectric absorbtion, and more.

author: tggzzz
author_rank: Super Contributor
timestamp: January 30, 2019, 01:14:42 am
content: Quote from: beanflying on January 29, 2019, 11:46:19 pmGetting a meter to agree with a voltage source is easy: just turn the knob.Getting one meter to agree with another meter is a little more difficult: open one up and recalibrate it.Getting one of your meters to agree with other people's meters is easy: just empty your wallet.QuoteCells also have another advantage: no popcorn noise. That enables you to infer the noise in a meter's reference.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:04:35 am
content: Quote from: tggzzz on January 30, 2019, 01:08:40 amOh yes I do know that, but there are capacitors and capacitors. Still overall you are obviously right. It was just a thought.UPDATE: just noticed that the LTZ1000 does use a capacitor internally.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:15:33 am
content: Quote from: beanflying on January 29, 2019, 11:46:19 pmAre the electronics in the aluminum boxes in a controlled temperature?

author: beanflying
author_rank: Super Contributor
timestamp: January 30, 2019, 04:32:36 am
content: When not in outside use yes. I have a hacked wine fridge at 25C. and 34970A loggers with DMM cards fitted. Still a WIP and it also has two newish LTZ1000 Refs and the bits for two more coming.If it wasn't out of interest there is nothing I do that requires more than 4.5 digits but having that known 6.5 digit and references gives me very good confidence at that 4.5 has very good confidence. The consideration to jump to 8.5 is a total indulgence and I am still weighing up that or a modern 1/2 rack 7.5 digit option. Either way it would be kept in Current Cal.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:49:19 am
content: CROSSED POSTSQuote from: beanflying on January 29, 2019, 11:46:19 pmIsn't,'They will only be as 'accurate' as the device YOU measure them with'true of any Vref that you make?Both the LTZ1000 and the LM399 are oven controlled. The LTX1000 is top of the chip heap, so why not use it. OK you may not realise it's full potential, but so what. Is it because of the $50 price tag? At a lower spec (0.5ppm) and price ($12), the same goes for the LM399, or is it because the LM399 is a shunt type, which complicates the design, compared to a series type?It would appear that the basic vref of the LM399 cant be that good if it only achieves 0.5ppm/degC performance with oven control. My thinking is that there may be big gains in performance by oven controlling one of the au natural chips we discussed, which as you say, also have an initial absolute accuracy around two orders better than the LM399.https://www.analog.com/media/en/technical-documentation/data-sheets/199399fc.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/LTZ1000.pdf

author: beanflying
author_rank: Super Contributor
timestamp: January 30, 2019, 05:00:02 am
content: In the case of Ref's like the LM and LTZ their initial accuracy varies heaps so without a good meter they are no good for a walkup 'accurate' home built reference. The Max6350 runs rings around them for initial 'accuracy'. So if you have no 'known' source don't build one IMO there is simply nothing to be gained unless you already have a very good calibrated meter which in Raphael's case he gives no indication of.LM399 6.75 - 7.3 V initial 'accuracy'LTZ1000ACH 7.0 - 7.5 V initial 'accuracy'MAX 6350 4.999 - 5.001 V initial 'accuracy' (0.02%)

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 05:16:59 am
content: Quote from: beanflying on January 30, 2019, 05:00:02 amAgree. But at one stage you were suggesting that the Vref be used as as a transfer standard, which implies another precision reference- or have I got that wrong.To put a stake in the ground can I suggest that we consider two approachesTransfer standard (with high output R and adjustment)Stand-alone absolute Vref, with low output R (suitable for a load of 10M or higher) and initial calibration (if required) by resistor AOT.I have an interest in two above.

author: beanflying
author_rank: Super Contributor
timestamp: January 30, 2019, 05:57:16 am
content: With no calibrated gear it becomes a chicken and egg type of thing you still need an 'Accurate' Reference to make that Transfer Standard. So your choices are buy or make the best one you can.Vref best options without going broke.1 - Buy an AD-584 with measurements attached. With no measurements it is no better than your current best meter for Accuracy.2 - Build a MAX6350 based reference and make it YOUR volt. It wil be better than just about any 4.5 digit meter I am aware of for initial accuracy?3 - There is 399 Refs in the for sale section at present for not to many $$ but I am not sure of what there long term aging or stability is as I haven't looked at the project other than a glance in the for sale section? Personally I would stay away unless you already own a meter capable of tracking and looking at its drift (6.5 digits)4 - DMM check or similar other devices are also available online.How you get to the various voltage points to transfer YOUR Volt from there is a topic in itself and really to large for here and outside basic dividers and not Beginner material in particular. Depending on the meter you want to Cal as I put a few posts back you need to hit a huge number of different points even for a couple of meters. This is why I own a couple of Calibrators both AC and DC to 1000V+ but they are not sensible for most apart from their boat anchor dimensions as per the photo below.One other possible option is you go down the path Ian Johnson has with his LM399 based Referencehttp://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-v2-detailusing a good quality DAC/micro on the reference but this is way out of Beginner level and will cost real money.Back shelf is how to go 'part' of the way into the rabbit hole 7.5 digit Advantest on the top shelf, HP735A, Fluke Null meter, Fluke Portable Calibrator, Bottom row HP 740B working but mid renovation 1000V DC Calibrator/Null meter and Valhalla 1200V AC Calibrator. Won't mention my 'slight' time nut addiction either. Plus other assorted 'stuff'. No I don't feel I have a problem because TEA assures me I am 'normal'Yes I am showing off the toys a touch

author: tggzzz
author_rank: Super Contributor
timestamp: January 30, 2019, 08:17:16 am
content: Quote from: spec on January 30, 2019, 04:04:35 amAt voltnut level there are no individual components, only black boxes with lots of hidden and poorly specified internal componentsPeople spend careers trying to minimise and work around that.Silicon capacitors are no different in that respect. BTW, youcanget discrete silicon capacitors in the range 0.3pF to 3.3uF, but watch the availability, MOQ and pricehttps://www.digikey.co.uk/products/en/capacitors/silicon-capacitors/68?k=silicon%20capacitor

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 11:20:51 am
content: Quote from: beanflying on January 30, 2019, 05:57:16 amThank you for such a comprehensive summary.In view of what you and tggzzz say, it might be folly, but I would like to have a go at designing a stand-alone, low-impedance output Vef. So if I get around to posting a schematic, your comments would be appreciated.I have done a couple of outline schematics which, in theory, look promising, but I lack the specialist knowledge and experience that this field requires. My only qualifications are having read the threads on EEV and elsewhere on the net.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 11:25:30 am
content: Quote from: beanflying on January 30, 2019, 05:57:16 amNice

author: tggzzz
author_rank: Super Contributor
timestamp: January 30, 2019, 11:31:40 am
content: Quote from: spec on January 30, 2019, 11:20:51 amFolly? No. You'll have fun and learn things. But personally I wouldn't tweak anything based on its output - first attempts are rarely perfect.You should understand what's behind the specs for professional high quality meters, especially those related to the drift over time.You might like to consider what will/won't happen if you short-circuit your low impedance output.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 11:34:16 am
content: Quote from: beanflying on January 30, 2019, 05:57:16 amYou have mentioned the AD584 a couple of times, but according to the datasheet, it is no big shakes (15ppm) compared to the other chips. So what is the story with the AD584?https://www.analog.com/media/en/technical-documentation/data-sheets/ad584.pdf

author: beanflying
author_rank: Super Contributor
timestamp: January 30, 2019, 11:42:46 am
content: The Acrylic enclosed references uses the AD584KH. It is specced at 15PPM Maximum there is no typical or minimum PPM/C which is a bit strange. Based on what I measured the other warmish day I was getting 1PPM from mine soInterestingly the Initial Accuracy is only Maximum rated tooDo a Search for KKMoon or AD584KH reference you should get plenty of hits here on others that have tested it.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 11:48:12 am
content: Quote from: tggzzz on January 30, 2019, 11:31:40 amYes, well put. I don't actually need to calibrate anything, at the moment. A half-decent voltage reference would just be an interest.Quote from: tggzzz on January 30, 2019, 11:31:40 amOh yes- got that right from the off. I have done quite a bit of precision work, but never designed/built an ultra precision Vref. I spent a year doing the development cal of a precision analog computer at one time, and had access to the highest precision voltage and resistance references, via the company- cost was not an issue.Quote from: tggzzz on January 30, 2019, 11:31:40 amNot much I don't think. But it may introduce a thermal shock to the V Ref Unit, which could take a day or so to recover from.What does concern me is the secondary effects, like galvanic voltage of metals and solder joints- that kind of thing.

author: spec
author_rank: Frequent Contributor
timestamp: January 30, 2019, 12:21:55 pm
content: Perhaps I could outline the V Ref Unit (VRU) design I have been looking at, in the background, recently:Ref Voltage: any from 0V to 10VTarget Absolute Accuracy (max): TBDTarget Tempco (max): 1ppm/degCCalibration: none, or resistor AOT depending on degree of precision required.Material Cost (max): $100USDevelopment Effort (max): Infinite man hoursDimensions (max): Width: 300mm, Depth: 250mm, Height: 150mmWeight (max): 1.5 kgCase: DIY. Material: TBD, but probably a mixture of plastics and aluminum. The case would have at least four thermally isolated compartments. The first compartment would house the electronics that are primarily responsible for voltage precision. The second compartment would house the components that have a secondary effect on voltage precision, the third compartment would house the precision linear power supplies. The third compartment would have an efficient heatsink. The fourth compartment would house the batteries.Power Source: mains and battery. Mains components housed in wall-wart or similar.Power Consumption (max): 100WPower Rails: precision low-noise using specialist linear PSU chipsPrimary Voltage Ref: one of the chips that we have discussedBuffer to Provide 'zero' Ohm Output Z: super precision opamp (I seem to have found one)Thermal Control: Possibly oven controlled primary voltage ref. All relevant circuitry in DIY ovens (simple to do, but the practical side would take some sorting)The VRU would probably require a six-month burn-in period to reach full performance, but would be usable after a couple of days for less precise work.Obviously, the VRU would be powered permanently, either from the mains or battery.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2019, 02:41:06 pm
content: Quote from: spec on January 30, 2019, 12:21:55 pmHave a look at this voltage source, manufactured by a forum member here:http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-v2-detailCheersAlex