general purpose "precision" voltage reference. suggestions ? - Page 1
Forum section: electronics / projects

author: rob77
author_rank: Super Contributor
timestamp: November 08, 2014, 08:53:32 pm
content: Hi Guys,i'm going to build myself a voltage reference - to check my cheap-ass meters against it and calibrate my power supplies (once in a while). i'm thinking about something much much better than a LM336 based reference, but on the other hand something cheap enough. cheap enough means that anything in the same ballpark as the LTZ1000 is a big NO-GOi'm thinking about a reference based on LT1021CCN8-10 (i like THT parts) it's 10V, 0.05%, 5ppm/C , 6uV noise - the specs are good enough for a hobbyist and the price is acceptable for that accuracy - it's approx 6Eur in single piece. and the circuit is very very simple, the reference is able to sink/source 10mA - so no output buffer needed.i was also considering a LM399 which is a overkill but the price is still acceptable (approx 12Eur for the lm399 alone), but then i rejected the idea because of the more complex circuit (precision resistors & precision opamp needed). it would be at least an order of magnitude better than the above.. but i simply don't need such an accuracywhat are you using guys as a "general purpose" voltage reference ? any suggestions ? should i just go for the LT1021 ? or is there anything better i should consider ?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 08, 2014, 09:00:43 pm
content: http://www.ebay.com/itm/AD584LH-4-Channel-2-5v-7-5v-5v-10v-High-Precision-Voltage-Reference-Module-/121200404799?pt=LH_DefaultDomain_0&hash=item1c381b693f

author: calzap
author_rank: Frequent Contributor
timestamp: November 08, 2014, 09:23:17 pm
content: If you want something with a little more traceability than the Chinese ref on eBay and ability to have it recalibrated, look atwww.voltagestandard.com. He also sells on eBay. Voltage refs for as little as US$25 plus S&H. I have the DMMCheck (US$40) and have been happy with it. There can be a lot of satisfaction and learning in building your own, but when it's done, you need to have a standard to compare it with.Mike in California

author: babysitter
author_rank: Frequent Contributor
timestamp: November 08, 2014, 09:25:04 pm
content: First, try to get your needs together. How many digits, your temperature range come to mind as first factors.Do you really want it to be stable for years and years?Look around if you can leech the precision and accuracy of somebody else - a university lab, a volt-nut living not too far away, or the workplace, maybe. That way you can first compare your reference to a "golden device", immediately before use, and use it in a reasonable time after that, at a reasonable close temperature.This releases your long-term and temperature specs a lot. My first "personal" reference was a pocket-sized REF195-based thingy and a few button cells. Good for short time between tests at the same room, but not enough for the round trip workplace-home lab-workplace with the "same" result (YMMV) on the workplace 34401A on both occasions. LTZ1000A got me there. But it was like going for Rolls royce after bicycle was considered too uncomfortable.

author: nuno
author_rank: Frequent Contributor
timestamp: November 08, 2014, 09:53:37 pm
content: I'm not too worried with precision (at least not for my uses) so I "designed" and built myself a few of these.

author: rob77
author_rank: Super Contributor
timestamp: November 08, 2014, 09:57:59 pm
content: thanks Guys for all the suggestions.@ Alex Eisenhutthat chinese reference looks good, but i will not trust that onei doubt it'll have a genuine AD584 in it.. and the AD584 is almost identical to the LT1021 in terms of accuracy and drift (the LT1021 is a tiny bit better on accuracy and load regulation)@calzapthanks for the link ! those modules look great ! and actually the product "VREF10-003" uses exactly the LT1021 i was consideringbut i think i'll rather build my own. the prices are good there, but the shipping to europe would be more than the reference itself.@babysittermy needs are exactly in the same ballpark as the LT1021 or AD584 (and probably others i'm not aware of ), that's the reason why i rejected the idea of LM399long time stability is not a issue - i can check the reference once in a while against a calibrated meter, but even that's not needed for me...my best meter is a 6000 count 0,5% (i'm a hobbyist) so the 0.05% reference is really good enough for meeven if a buy a 50000 count meter (planning to buy a Brymen BM857) , i'll be still fine with that reference@nunobasically i'm in the same situationmillivolt accuracy is good enough for me, and if it will not drift away because of few degree celsius difference , then it's even better

author: dannyf
author_rank: Super Contributor
timestamp: November 08, 2014, 10:22:46 pm
content: Quotesame here, ghetto style. Those LM/TI parts are nice.

author: nuno
author_rank: Frequent Contributor
timestamp: November 08, 2014, 10:32:02 pm
content: My prototype was almost ghetto styleI measured most of them before (just let the chip over the footprint, press very very lightly) and after soldering (and a good cool down time, of course). Measured differences between -0.7mV and +0.15mV, on 6 units.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: November 08, 2014, 10:35:26 pm
content: I wonder if a 5V reference would be better suited for a 50000 count meter. It would put it at FS on the 5V range vs @10V would be 20% of 50V range.

author: Andreas
author_rank: Super Contributor
timestamp: November 08, 2014, 10:47:24 pm
content: Quote from: rob77 on November 08, 2014, 08:53:32 pmHello,I would go for a buried zener in hermetically housing (CERDIP or TO-99).But I doubt that you will get the LT1021 in TO-99.AD586LQ (5V) or AD587UQ (10V) are good candidates.Other Possibility is the LT1236AILS8-5 or LT1236AILS8-10 (ceramic SMD).In all cases I would do a run in phase after soldering of 5000-10000 hrs to pre age the devices before use.With best regardsAndreas

author: Yansi
author_rank: Super Contributor
timestamp: November 08, 2014, 10:48:16 pm
content: rob77, I see you are from Slovakia... try to search for MAB399, Denkl has them, funny low prices.//oh, I've just looked to denkl's web, the price is now five times bigger, than it was last time i had visited the web. But still pretty low. Maybe he recently was told what the thing is

author: nuno
author_rank: Frequent Contributor
timestamp: November 08, 2014, 10:54:52 pm
content: I think I got 2 or 3 LM4132AMF-1.8 for the prototype as samples from National.

author: rob77
author_rank: Super Contributor
timestamp: November 08, 2014, 11:28:29 pm
content: Quote from: Yansi on November 08, 2014, 10:48:16 pmthanks for the hint !i didn't knew Tesla did clone thosethat price is simply stupid if the performance is the same

author: JSnyder
author_rank: Contributor
timestamp: November 08, 2014, 11:49:14 pm
content: Here are my ref's based on the lt6654 a 5v version and a 1.250v version, good enough accuracy and tempco for my purposes.

author: Yansi
author_rank: Super Contributor
timestamp: November 09, 2014, 12:02:56 am
content: Quote from: rob77 on November 08, 2014, 11:28:29 pmYea, performance same. Was tested by my colleague.Tesla did clone big bunch of things. There was also some 10V reference... I think MAC series, metal can package. But cannot remember the type exactly.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: November 09, 2014, 12:31:18 am
content: I have an embarrassingly delayed project which I started more than a year ago - Weston Cell (http://www.ebay.com/itm/Cadmium-sulfate-octahydrate-98-0-Analytical-Reagent-ACS-30g-/271298422512?pt=LH_DefaultDomain_0&hash=item3f2aa566f0). I am missing the glass tube with a bridge and mercurous sulfate (need to go pick it up at lab supplier, the want $70 for shipment across town and also I am not sure the purity of cadmium bars that I got.). My understanding that Weston Cell is still used as Voltage standard. Up till 1990 it in fact was THE Standard - cannot get better than that! And it is an definitely an exciting project for DIY physics nuts.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: November 09, 2014, 12:47:11 am
content: Chemical cells do not require calibration. Here is a good example of 30 y old cell tested by student ( I suppose):http://www.drkfs.net/standardcell.htm- Still very very accurate.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: November 09, 2014, 03:36:20 am
content: Huh, I am not sure you can calibrate it - there are no adjustable parts as far as I know and it is not exactly 1V but 1.018638. I agree that modern superconductor based standard is more accurate, but I have seen Weston Cells sold for about $100 or less so it is still very very good deal in terms of price/accuracy. I have seen new one for USD$1500.If I look at specs of 8 1/2 multimiter (Agilent 3458A) for one volt I see its accuracy is 1.5+0.3 PPM Of reading + PPM of range which is 153 microvolt for 1V. Weston cell's max deviation over time is 83 microvolts. So what are you going to calibrate it with? I have attached a capture of Fluke voltage standard specs - notice that output is 1.018V not 1V. and +-0.8ppm (or 1.6 ppm?) of 1V in 30 days is same as 19.5456 microvolts over a year.So here is my understanding - if cell is saturated - that is solution inside is at its maximum capacity it gives accurate result which is very stable. Unfortunately to keep some solution at its saturation level you need to keep it at constant temp which is a problem because once temperature drops down it will start to crystallize in an instant. I have not seen Weston Cells with oven, so I assume that those that do not have temp stabilization are not saturated, therefore they may loose 80 microvolts which is 0.000080 V / year as reference say.Another problem with Weston cell is you cannot drive ANY current - so how good is that if you want to calibrate your multimiter for example? I think 1V over 10Mohm input should be Ok, but I have not experimented with it.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: November 09, 2014, 03:54:39 am
content: attachment

author: Vgkid
author_rank: Super Contributor
timestamp: November 09, 2014, 08:06:22 am
content: www.imeko2009.it.pt/Papers/FP_670.pdfHere is a paper on the 732's driftFor the ovenized weston look up Guildline 9152/4.

author: Andreas
author_rank: Super Contributor
timestamp: November 09, 2014, 09:07:47 am
content: Quote from: Alexei.Polkhanov on November 09, 2014, 03:36:20 amAlthough its getting now more and more offtopic:You are mixing up several things.1.5+0.3ppm in the 1 V (1.2V) Range are 1.8 uV (not 153).Saturated cells are long lasting but have -39ppm/K tempco so they need a temperature stabilisation.http://www.amplifier.cd/Test_Equipment/other/standard_9152.htmIf good treated, saturated cells can have a drift below 1uV/year (e.g. 0.5uV/year).And of course you will need to calibrate (compare against standard) the cells since they are ageing. (but you cannot adjust them)With 500 Ohms internal resistance of the cell you will have a 50 ppm error when reading with a 10 Meg instrument.Further you will have a permanent ageing by even this current.The only way to measure a weston cell is a comparison to a other stable voltage with a high ohmic zero voltmeter.To go on topic:ADR425BRZ will be fine for about 12 Bit A/D conversions. (or a 0.1% current source).I have tested a similar ADR4550B with a initial drift of about 80 ppm during the first weeks of operation.with best regardsAndreas

author: mikerj
author_rank: Super Contributor
timestamp: November 09, 2014, 10:48:25 am
content: I'm surprised no-one has mentioned theREF50xxseries of references.One important point to bear in mind is that mechanical stress on any package must be minimised to maintain accuracy long term.

author: nuno
author_rank: Frequent Contributor
timestamp: November 09, 2014, 11:45:54 am
content: That's why I have mine in a PCB corner.

author: Andreas
author_rank: Super Contributor
timestamp: November 09, 2014, 12:12:03 pm
content: Quote from: mikerj on November 09, 2014, 10:48:25 amIts simple: the REF5050 is no buried zener but a bandgap reference(thus the noise is a factor 5-10 higher for the same supply current)And there is no hermetically package.To package: the effect of pcb stress is larger the smaller (and not stiff) the package is.A SO-8 package has around factor 3 worse values (e.g. hysteresis) than a DIP (plastic) package.with best regardsAndreas

author: mzzj
author_rank: Super Contributor
timestamp: November 09, 2014, 12:21:25 pm
content: Quote from: Alexei.Polkhanov on November 09, 2014, 12:47:11 amYou get maybe 20 ppm initial absolute accuracy and several ppm drift during the first year if you follow thepainstaking proceduresoutlined in the old standards literature.IE.http://www.nist.gov/calibrations/upload/mn84.pdfHigh quality mercury is washed with nitric acid, followed by a wash with distilled water. After that the mercury is distilled under partial vacuum with 5mm oxygen partial pressure to oxidize some of the other impurity metals. After distillation the mercury is filtered.Electrolytic pure cadmium is sublimed at 400 degrees C and so on...Preparation of rest of the chemicals is not any betterYou might as well make your standards based on concentrated hydrogen fluoride, fluoroantimonic acid and a hint of chlorine trifluoride

author: kxenos
author_rank: Frequent Contributor
timestamp: November 09, 2014, 12:35:56 pm
content: Has anybody used LTC6655 devices? What do people think about them? I think it was Jim Williams' last design, wasn't it? I checked the datasheet and I think the LS8 package has pretty good chances of being a good candidate for a precision general purpose voltage reference design.

author: rob77
author_rank: Super Contributor
timestamp: November 09, 2014, 12:57:01 pm
content: Quote from: kxenos on November 09, 2014, 12:35:56 pmthat's again a band-gap reference, so it's a different story.Quote from the LT1021 datasheet:Quoteif you consider that the best reference around (which is btw used in the 8,5 digit meters) is also a buried zener reference (the all mighty LTZ1000), then the logical conclusion is that for long-term stability and low noise the buried zener is a better choice. the ltz1000 is just silly for a hobbyist (it's like plowing with a luxury sports car) and lm399 (also a buried zener) is still a overkill (lm399 is used in some 7,5 digit meters).

author: G0HZU
author_rank: Super Contributor
timestamp: November 09, 2014, 01:19:10 pm
content: QuoteThe problem here is that you can find yourself getting obsessed with voltage references.For about 25 years my voltage reference was my cheap 3.5 digit Maplin Gold DMM that I bought as a student in the 1980s. So basically, in my house, 5.00V was defined by my meter. This was good enough for checking my power supplies as well. I did a lot of design work based on this meter and nobody complained or died in all this time...In 2014 the old Maplin DMM is still accurate when checked against modern DMMs and I don't think I've ever adjusted it.However, in 2014 you can buy a cheap 5V reference like the AD REF195 for £2 here in the UK. I have one here but I haven't adjusted any of my DMMs because of it.

author: Andreas
author_rank: Super Contributor
timestamp: November 09, 2014, 04:49:54 pm
content: Quote from: kxenos on November 09, 2014, 12:35:56 pmThe only advantage is that you get a relative low noise (at a high current) with low drop out voltages.Compared to (good) buried zeners hysteresis is a horror.ADC20 is in LS8 package.ADC14 is in MSOP package.ADC8: Buried zener as comparison is LT1027CCN8-5 in DIP8 plastic package.Unfortunately there is no more a hermetically package with LT1027 available.So ADC8 suffers from humidity changes of 0.5ppm/% humidity.With best regardsAndreas

author: mikerj
author_rank: Super Contributor
timestamp: November 09, 2014, 05:18:09 pm
content: Quote from: Andreas on November 09, 2014, 12:12:03 pmIs that a significant issue for calibrating a DMM?

author: jay
author_rank: Regular Contributor
timestamp: November 09, 2014, 06:19:00 pm
content: Quote from: rob77 on November 08, 2014, 08:53:32 pmI had similar requirements and I ended up using LT1236. Simple to use and the price was reasonable. It's certainly enough accurate to for my low end measurement equipment.

author: AJ
author_rank: Newbie
timestamp: November 09, 2014, 10:41:39 pm
content: I use the Microchip MCP1525 in my projects.http://ww1.microchip.com/downloads/en/DeviceDoc/21653C.pdfScreenshot of Page 1 of the data sheet attached.Microchip has a decent samples policy.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: November 09, 2014, 11:20:35 pm
content: Quote from: Andreas on November 09, 2014, 09:07:47 amYou right it is 1.5 ppm of reading + 0.3 ppm of range. So 1.5 ppm of reading is 1.018 / 100 * 0.00015 = 1.82700 microvolts where 0.00015 is 1.5 ppm converted to %.or(((1.01800 volt) / 100) * 0.00015) + (((1 volt) / 100) * 0.00003) = 1.82700 microvolts.I have chosen the 24 hour column by mistake so for one year it will be(((1.01800 volt) / 100) * 0.0008) + (((1 volt) / 100) * 0.00003) =8.44400 microvoltsWhich is 10 times better than Weston cell. I guess that is the reason why you don't see many of them around anymore.(this is the datasheet I referred tohttp://literature.cdn.keysight.com/litweb/pdf/5965-4971E.pdf).

author: macboy
author_rank: Super Contributor
timestamp: November 10, 2014, 03:11:08 pm
content: Quote from: Alexei.Polkhanov on November 09, 2014, 03:36:20 amThe standard today is a Josephson Junction. A particular number of series Junctions, excited by a particular frequency, will give exactly 1 V. The physics behind the JJ mean that the voltage output is dependent only on the frequency of the exciting RF. Since we can measure and generate in time very accurately (better than parts-per-billion), then using JJ's, we can create voltages that are of comparable accuracy. This is the only way to calibrate super-meters like the 3458A; Agilent owns one of only a handful of JJ's in the world.Weston Cells are definitely calibrated. They are notadjusted, but are measured against a known standard. No two cells have the exact same voltage. So you measure the cell voltage, then you know thatthis particularcell is 1.01863789 V for example. Regular calibration is necessary in order to know the relative stability of the cell. Otherwise how can you trust a cell that has sat on the shelf untouched for 10 months? If you know that every year your cell drifts down 3 microvolts, then you can actually interpolate the voltage at any time.You must never connect a Weston cell to the 10 Mohm input of a multimeter. You can however get away with the high impedance inputs of most meters at low ranges, these are usually >10 Gohm and are acceptable. With the 'normal' 10 Mohm input, or an analog meter, the process is to use a null method. You provide a variable voltage source equal to the cell voltage (1.018... V) and connect a null meter across the two. The voltage is then close to nill and close to nill current will flow, even in a old school analog null meter. You adjust the variable source until the voltage is truly nulled, then the variable source is exactly the same as the cell voltage, and you can connect the device under test to the variable source, which can source enough current for the test.The fluke references are internally a zener based reference. They go so far as to have an internal oven powered by a rechargable battery that keeps the whole thing from undergoing a thermal cycle if the power fails, or during shipment for calibration. A thermal cycle can cause a thermal hysteresis effect, ruining the accuracy of the reference. They even have an "IN CAL" indicator that will warn if this thermal cycle has occured, and the reference can't be trusted until re-calibrated. The 1.018 V output is provided for convenience only, in order to use old calibration processes that had used a Weston Cell.

author: rob77
author_rank: Super Contributor
timestamp: November 10, 2014, 04:16:46 pm
content: Quote from: macboy on November 10, 2014, 03:11:08 pmthat's not true... there is a bunch of those around the world holding the SI voltage standard and many countries have those as national voltage standards. Even Slovakia has a JJ as a national voltage standard, and we're a really small country (5meg citizens).EDIT: added link to photohere's a photo of the beautyhttp://240.smu.sk/storage/root/pic/Josephson1.pngbtw.. the Slovak institute of metrology is the owner of it and they provide calibration services to the public (they calibrate 8 1/2 digit meters)

author: macboy
author_rank: Super Contributor
timestamp: November 10, 2014, 04:37:37 pm
content: Quote from: rob77 on November 10, 2014, 04:16:46 pmI guess we could define "handful".For many decades, the Weston cell (or rather, specific Weston cells) were the basis for the definition of the Volt. There must have been 10's if not 100's of thousands of Weston cells nearly identical to those ones in use in standards labs, cal labs, and industry labs around the world. The JJ is much less common, with about 50 worldwide in 2010. Since then the technology has become more commercialized and more affordable, so there will be more today, but I have no idea how many.As for accuracy, the NIST (USA) and NRC (Canada)compared theirsin 2007 to 0.207 parts per billion.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 10, 2014, 06:15:53 pm
content: Remember that it's been illegal (at least here) to produce Weston cells for many years. The saturated cells last almost forever, but can't be inverted and have a significant tempco. They need to be calibrated against another reference. The unsaturated cells, like the common Eppley found on eBay in spite of their rules about selling anything containing mercury, arealljunk. They are all old enough that the stability is shot. The only worth of cells on eBay is for the nice thermally lagged enclosures that you can build a nice period looking reference inside of using solid state components!

author: wiss
author_rank: Frequent Contributor
timestamp: November 10, 2014, 08:10:55 pm
content: Quote from: Conrad Hoffman on November 10, 2014, 06:15:53 pmI have a "bunch" at work, there's only one (a dual version) that is even close to the correct value. A new 34461 is most likely more trustworthy.QuoteThat is a brilliant idea! I will have a go with the hacksaw tomorrow!Regarding JJAs, there is at least one in Sweden (national standard institute), on their web-page they state that they fire it up twice per year to check their zeners, the zeners are then used to cal costumers equipment. They will probably fire it up on request, for lots of cash...

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: November 10, 2014, 08:54:48 pm
content: The LT1236 and LT1021 datasheets are so similar, but the 1236 doesn't claim to be a better LT1021... are there functional differences?A LT1236LS8 may be easier to get a hold of than a hermetic 1021 if 5V is ok.

author: Andreas
author_rank: Super Contributor
timestamp: November 10, 2014, 09:58:52 pm
content: Hello,in fact the LT1021 and LT1236 are very similar.The main difference is:The (elder) LT1021 you can get either trimmed for low initial error (C-Grade) or for low TC (B-Grade) but not both.The (more modern) LT1236A has both low TC and low initial error.On the other side from LT1021 the metal CAN package is still available (when ordering large quantities) and alsofor military and space applications you can get hirel and radiation-hard parts (RH1021).http://www.linear.com/product/RH1021-5with best regardsAndreas

author: rob77
author_rank: Super Contributor
timestamp: November 10, 2014, 11:39:22 pm
content: @Andreasthanks !finally i know the difference between LT1021 and LT1236. the LT1236 looks better, but i'll stick to the LT1021CCN8 because a local reseller have those on stocki'll also try to source a few of those MAB399 references from Czech Republic for later experiments. Those are NOS and much much cheaper than LM399, and the only differences are (at first sight) a taller package (by few mm) and a higher zener voltage 7.25V instead of 6.95V.

author: MK
author_rank: Regular Contributor
timestamp: November 11, 2014, 05:07:13 pm
content: Quote from: Andreas on November 10, 2014, 09:58:52 pmDoes anyone have a copy of an older version of the lt1021 datasheet that explains the d version differences?

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: November 11, 2014, 05:49:35 pm
content: Quote from: MK on November 11, 2014, 05:07:13 pmhttp://www.digchip.com/datasheets/parts/datasheet/262/LT1021D-5-pdf.phpworked for me

author: dannyf
author_rank: Super Contributor
timestamp: November 11, 2014, 08:11:09 pm
content: QuoteQuoteQuoteI would think "a handful" is quite different from "50" or "more" (than 50).QuoteNow, we really don't know what to make out of your statements.