Low Cost Voltage Reference Experiment - Page 1
Forum section: electronics / projects

author: paulie
author_rank: Frequent Contributor
timestamp: March 18, 2015, 03:54:10 pm
content: By "low cost" I mean pennies not dollars. "Voltage reference" refers to any device that might function as a transfer standard for calibrating 5 or 6 digit voltmeters or serve as reference for DIY meter or PSU with similar resolution. This was inspired by hours spent browsing volt-nut megathreads and excitement over my new 5 digit $5 Ebay meters. So the plan is to evaluate likely candidates including inexpensive bandgap references like LM4040a or TL431, DIY zener type built from back-to-back penny diodes, and maybe even properly biased LEDs and transistor base/emitter junctions.First step was to check shunt voltages to see what might be within useful range. The first piece of test equipment used here was my home grown combo super low noise PSU/5-digit voltmeter with current controlled test points (see 1st photo, DUT=NPN). Immediately devices divided into two major categories: 2.5v for the bandgap or LED and 7v for the transistors and zeners.Next step was to get a rough idea of TC which required a mid size temperature control oven like shown in next 2 photos (my Worlds Smallest was way too small and the big one takes too long). Commercial bandgap parts were very well behaved out of the box and after some optimization the DIY zeners were pretty stable too. Haven't checked TC for the LEDs or transistors yet.Final step is to characterize long term drift so dragged out one of the big ovens which were used couple years ago for a customer to test batteries. I had over 150 of these built for them and ended up with four units for myself. With a slight modification perfect to keep voltage references at a constant 25C (see next few pics). Also re-purposed was the 16 channel ADC multiplexer/datalogger shown below. Only mod there was to replace the ICL7135 ADC chip with one of the 5 digit $5 Ebay meters. An on-board 10x low noise amp provides for effective resolution of 6 digits (0.01mv). More than enough to track short and long term drift.In fact my first set of off-the-shelf bandgap references finished yesterday after cooking for 5 weeks. Conclusion is most of them will work fine as transfer calibrators for some friends 4 and 5 digit meters. And that was with junky generic 0805 SMD resistors which take some time to stabilize due to self heating. Next batch will use 10ppm Vishay resistors (see photo in the bandggap post below) which I also inherited from the day job. They actually test out at least 2-3ppm over 10deg range so performance should improve significantly.More to come and as usual comments and questions welcome.update: Someone asked for more info on the expander/amp seen in "meter_in_oven" photo. Unfortunately I don't have a schematic that has been updated since modifying for this project but attached is a pictorial diagram. I think the functions and labels haven't changed much so mostly still valid. I have a block diagram somewhere too but can't seem to locate it ATM. If anybody is really interested in building one it might be possible to dig up the channel switching and datalog code. In any case this diagram should serve for idle curiosity.

author: paulie
author_rank: Frequent Contributor
timestamp: March 18, 2015, 03:55:14 pm
content: Low cost bandgap devices. Most 20 cents or less each, 2 cents for the TL431.

author: paulie
author_rank: Frequent Contributor
timestamp: March 18, 2015, 03:56:50 pm
content: Back to back penny zeners. Less favorable voltages than the 2.5v bandgaps but potentially more stable. Not as good as "buried" type but we will see.

author: paulie
author_rank: Frequent Contributor
timestamp: March 18, 2015, 03:57:46 pm
content: LEDs and bipolar transistors. Not as promising but certainly inexpensive.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 18, 2015, 04:44:16 pm
content: Why not try LM385? Very low cost, and much better then zener or TL431(A)

author: paulie
author_rank: Frequent Contributor
timestamp: March 18, 2015, 05:15:18 pm
content: Yes, somewhat better than TL431 but not sure about zener. At about 20-30 cents they do qualify so I bought some from Polida2008. Lower cost than 4040a or LM336 but still 10x more than TL431. I had large qty of the others on hand so used them in the initial test. Now I'm planning to use zeners for the next run and just waiting for some buried type to arrive to start that. Because my 10x resolution expander has restricted range won't be able to include any of the 2.5v parts this time around but definitely next run after.

author: TimFox
author_rank: Super Contributor
timestamp: March 18, 2015, 06:07:33 pm
content: If you are making your own temperature-compensated Zeners with the series connection of a reverse-biased Zener (usually 6.8 V) and a forward-biased PN diode (which could be a second Zener), you can find a current for each set at which the tempcos cancel for a zero net tempco. With your ovens, you can put several such sets in the oven, stabilize the units at one of a discrete set of temperatures (the wider spaced the better), and measure the voltage-vs-current using your 5-digit meters. If your current values are repeatable, you can compare the voltages directly at different temperatures, or you will need to do some simple curve fitting.Details on compensated Zeners can be found in the old Motorola Zener diode handbook, which shows the combinations of Zener with one, two, or three diodes that gave the standard voltages of 7.5, 8.2, and 9.1 V nominal.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 18, 2015, 06:07:50 pm
content: Quote from: paulie on March 18, 2015, 05:15:18 pmI amUnless you got a selected zener of about 6.2 V, the LM385 (and also the TL431 and LM4040) have a way, way better tempco.QuoteI'd say ditch the zener, unless you can build it in a good oven, and a really good current source (with good tempco) to maintain a constant current through the zener. Also, current needs to be significant higher as the LM385 and LM4040. But also higher then the TL431.Probably the noise of the zener is better. Noise ( <10hz) for the LM385 is about 15µV pp and about 11µV pp for the TL431.But I don't think you're aiming for a 10µV resolution

author: paulie
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:45:25 am
content: There's no doubt even low end bandgaps perform better off-the-shelf than uncalibrated zeners in terms of both accuracy and stability. However I found that the diodes can be properly adjusted for almost zero TC. At least over the 10deg or so range I've tried. And that's without being directly concerned with complex or expensive current sources but just proper resistance with a simple voltage regulator which is much easier. Not possible with the bandgaps that are designed for stability over wide range of currents.The biggest issue so far is the extended warmup time due to my low mass (aka low cost) 10ppm SMD resistors. Also experienced with the bandgap references, just not so bad.But you might be right about wasting time with these glass penny diodes. Discussions in the volt-nut thread and technical papers indicate lots of noise and poor stability compared to buried. However note that before the advent of reference ICs such as LM399 and LTZ1000 companies like Solartron used surface devices, albeit selected, in high end 7 digit or more meters so there is hope. I'm just curious how mush variation there is between the penny zeners and how they compare with buried type. There is almost a thousand to one difference in cost.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:51:48 am
content: @Paulie: and how do you solve the constant current?

author: paulie
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:55:37 am
content: Constant current is not required directly. A simple resistor with fixed voltage takes care of that. So maybe more correctly referred to as "fixed current" since a constant current circuit is not really needed. This is a little troublesome with the low voltage zeners which required very low ohm resistors but I've pretty much given up on 2.5v zeners for other reasons.ps. Self heating of the resistor is an issue but handled with some warmup time.

author: tszaboo
author_rank: Super Contributor
timestamp: March 19, 2015, 04:02:47 pm
content: Quote from: paulie on March 19, 2015, 08:55:37 amAs far as I see you need more than 5mA for anything below a 5,6V zener, which is really big for this. I would try this at 5mA.http://www.digikey.com/product-detail/en/BZX84-A5V6,215/568-6787-1-ND/2676827Datasheet said -2 .. +2,5mV/K tempco for 5mA. Might even work with some tweaking. You can decrease the resistor self heating problem by just using a bigger resistor, which doesnt increase the price that much, or a more stable resistor, which does. Also, there is this old appnote:https://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http%3A%2F%2Fwww.microsemi.com%2Fdocument-portal%2Fdoc_download%2F14616-zero-tc-reference-diodes&ei=zvIKVcrGL8f2O_CdgbgN&usg=AFQjCNEj2okI2n5PGSzVBpoekY-17RsXpg&bvm=bv.88528373,d.ZWU

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 19, 2015, 04:21:45 pm
content: Quote from: paulie on March 19, 2015, 08:55:37 amWhow ... hey .. wait a minute! Fixed voltage? Why go in all the hassle with a zener and not use the fixed voltage?note1: a XC6206 is 1% accurate, has a way better tempco then a zener, can deliver more output current and probably costs less.note2: voltage references are often operated from a battery ..

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 19, 2015, 04:56:03 pm
content: I've had excellent results using the old circuit (Walt Jung's Opamp Cookbook, I think) where the zener is buffered by an opamp, and the stabilized output of the opamp supplies the zener current through a resistor. With a temp compensated zener, you can tweak both the tempco and the output voltage, though they do interact. Whatever you use as the reference, that scheme will usually make it better.

author: albert22
author_rank: Regular Contributor
timestamp: March 19, 2015, 07:32:54 pm
content: Some time ago I found this:QuoteAlthough I found no schematics or any other references. It seems worth to try. LM723 is cheap and you have the oven as a bonus.

author: SeanB
author_rank: Super Contributor
timestamp: March 19, 2015, 08:12:18 pm
content: I used one as a reference voltage to make an offset voltage meter, just using the internal reference to make a simple 7V voltage source. It is more accurate than the analogue meter attached to it could ever be. It runs nicely off a 33V rail derived from a 60VAC tap on the variac.

author: Andreas
author_rank: Super Contributor
timestamp: March 19, 2015, 08:53:06 pm
content: Hello,sorry that I do not take the thread as serious(otherwise the TO would simply use a LM385 as all other).So back to the roots:You can build a zener and a diode (for TC compensation)with just one double transistor.The BCV62 would be a perfect candidate with 2 matched transistors in one case.Simply use one transistor as zener (BE junction reverse biased gives something 5-7V)and the other as compensation diode. (collector + base connected).You only have to select the zener for around 5.6V to have minimum T.C.(Unfortunately I got around 8-9V for the whole cirquit when I tried this.With best regardsAndreas

author: paulie
author_rank: Frequent Contributor
timestamp: March 19, 2015, 11:20:58 pm
content: Yes, as mentioned in the first post and photo I've constructed several BJT base/emitter references but except for basic voltage check have yet to evaluate in depth. Certainly no long term drift tests yet. So I have a couple questions for you, Conrad, and the others with much more experience than me who have taken time to peek in here:1. How do these transistor junctions compare to buried zeners, Both in form and function. Would they be considered buried or surface?2. Same question about the LM723. I have a box full harvested from power supplies over the years. Chip schematics show the 6.2v component but nowhere can I find a description of the construction. Is it possibly buried type too?I ask because the cost (main point of this thread) is so low it would be quite a bonus if either of these actually were. If such luck then would absolutely be the real key to a "poor man's" precision reference. After many hours of searching I see lots of info on the net for high end references but pretty sparse for the penny parts. Hopefully somebody out there might have a hint.

author: Andreas
author_rank: Super Contributor
timestamp: March 20, 2015, 05:05:04 am
content: Hello,perhaps you should read a good book as overview:https://www.elsevier.com/books/current-sources-and-voltage-references/harrison/978-0-7506-7752-3but caution there are some mistakes in the book e.g. claiming the MAX6325 as precision band gap.(but this bug is/was also on the Maxim web page while the datasheet notes correctly a buried zener).All standard junctions are no buried zeners.Buried zeners need a ion implant process which is a cost adder.So your transistor is a surface zener.Also the original uA723 is a surface (temperature compensated) zener (according to TI datasheet).But be aware that there are "LM723" clones around with band gap references.Also the venerable 1N821-1N829 are surface zeners. So you will have to select for low noise.They were pennie devices in the past. But nowadays they are more several dollars.And I personally think that the devices nowadays are more noisy than they were in the past.The cheapest buried zener might be the LM329. (= LM399 without the heater).With best regardsAndreas

author: paulie
author_rank: Frequent Contributor
timestamp: March 20, 2015, 02:20:00 pm
content: Thank you for that information. It will save a lot of effort concentrating on the wrong technology. Space (and time) are very limited as I'm only able to keep one oven operational at a time.IIRC 1N829 are actually parts I referred to earlier used by Solartron in their high end meters so looks like there's hope for non-buried zeners.LM329 are definitely out of the price range as a low cost reference but janaf has graciously offered high end zener samples so I may get a chance to compare those with the low cost glass types too. I did purchase an LM339 because it was just under my self imposed budget limit and that has arrived so will be part of the testing. I can't decide right now whether to use it with heater as a high quality voltage standard to compare or just test the zener part along with the others.ps. Interestingly LM399 does not really have a simple zener available but instead part of a rather complex internal circuit as seen in the attached diagram. Looks like the "zener" in LTZ1000 is not available as a real diode either.

author: sync
author_rank: Frequent Contributor
timestamp: March 20, 2015, 02:25:03 pm
content: Quote from: paulie on March 20, 2015, 02:20:00 pm60 cents are too expensive???

author: paulie
author_rank: Frequent Contributor
timestamp: March 21, 2015, 09:36:55 am
content: I'm unable to find those for less than $2. Do you have a link? It should restrict to small quantity and include shipping because unfortunately I do have to pay that.A little explanation of my budget constraints, there are two: cost of parts as described in first sentence page one (pennies) and hobby expenditure of any one piece of equipment or bulk purchase ($5). That last is partly a self imposed limit from deciding not to pay more for a support item than the cost of the original 5 digit Ebay meter that started all this. Also some restrictions imposed by family obligations (aka WIFEY!).So for example if parts were available for 60 cents ea but only in 10 pc qty it would be just over the limit. But 300 pcs at one cent ea like with the zeners or 200 pcs TL431 for $2 no problemo. May be hard to believe but total parts cost for the PSU/voltmeter tester, the ovens, and the 10x/MUX logger were all under $5.

author: electrodacus
author_rank: Super Contributor
timestamp: March 21, 2015, 07:26:34 pm
content: I also need a voltage reference in my application and my option at the moment is TS6001b it is about 1$ maybe less in quantity but probably better than any alternative proposed here.Do you know anything better around 1$ ?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 21, 2015, 07:37:11 pm
content: Quote from: electrodacus on March 21, 2015, 07:26:34 pmMore equal, I think: LT1004 (mouser 0,71 euro)Better is more moneyWhy do you want to change, what I see spec is not bad? Brand?

author: electrodacus
author_rank: Super Contributor
timestamp: March 21, 2015, 08:03:08 pm
content: Quote from: JohnnyBerg on March 21, 2015, 07:37:11 pmThanks I will look at that one. This is a new project currently on Kickstarter Solar BMS and I did not had time to look in to voltage reference I just used TS6001B in the Bill off material to calculate the cost of the product I had no time to look in more details just used 30min on digikey for a good reference at around 1$. Cost is not so critical for me since is low volume and there are components way more expensive in that product.Also there are may other factors that contribute to errors so if the spec of the TS6001B is correct I do not realy need better.I was thinking some of you have use that or selected something else in the same price range.

author: sync
author_rank: Frequent Contributor
timestamp: March 23, 2015, 05:05:47 pm
content: Quote from: paulie on March 21, 2015, 09:36:55 am0.61$ at TME (Europe):http://www.tme.eu/en/details/lm329dz/reference-voltage-sources-circuits/texas-instruments/lm329dznopb/#1$ at Digikey.http://www.digikey.com/product-detail/en/LM329DZ%2FNOPB/LM329DZ%2FNOPB-ND/9546

author: paulie
author_rank: Frequent Contributor
timestamp: March 24, 2015, 01:48:54 pm
content: Thanks for the links but in small quantity and with shipping they don't seem to qualify as "pennies". Cheaper on Ebay at two for $4 and change but even they are just over the limit. Purchased in 100 or more these would be pennies each but unlikely I'll have need for that many.At this time bandgap references look more usefu, at least for the DMM transfer standard purpose. Zeners require quite a bit of support circuitry (op amps, precision resistors, etc) but the bandgaps work by themselves. Some without even a bypass cap it seems. Also 2.5v is more useful for modern ADC reference as opposed to 7v from the zeners. I do plan to test zeners though in case there is future need for the better long term stability and lower noise.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 24, 2015, 02:21:12 pm
content: I have a lot references in stock. If you want a LM385, I can send you one. Send me a PM if you want one.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: March 24, 2015, 07:17:40 pm
content: Quote from: paulie on March 20, 2015, 02:20:00 pmI thought pin 3 & 4 were the buried zener on a LTZ1000. What did you find to the contrary?

author: Miroslav
author_rank: Contributor
timestamp: March 24, 2015, 08:10:15 pm
content: While fully appreciating attempt to build something usefull for penniew or less, just wanted to add the "high end" solution that's for most cases still acceptable ($2 in quantities, up to $5 single unit orders).Check REF102. No external componets required (most cases)10V ±0.0025V OUTPUT (factory laser trimmed)VERY LOW DRIFT: 2.5ppm/°C maxEXCELLENT STABILITY: 5ppm/1000hr typEXCELLENT LINE REGULATION: 1ppm/V maxhackaday has a nice article posted recently

author: paulie
author_rank: Frequent Contributor
timestamp: March 25, 2015, 12:31:14 pm
content: Quote from: Miroslav on March 24, 2015, 08:10:15 pmYes, there is no doubt REF102 is a good part as evidenced by it's use in top performing commercial standards like D105 from Calibratory. Unlike some others mentioned here very good initial accuracy. This may be useful for laboratory standards and top end bench lab meters which need absolute 10v but not so much in my case where calibrating friends meters or building ADC circuits are the goal. Because many units will be required cost is more important in addition to temperature and time stability.ps. 2.5v is also more friendly for my application because most modern ADC chips can't handle the higher voltage.

author: paulie
author_rank: Frequent Contributor
timestamp: March 25, 2015, 12:49:21 pm
content: Quote from: Galaxyrise on March 24, 2015, 07:17:40 pmI'm not an expert but a semiconductor product engineer customer of mine thinks the LTZ die picture below hints at many more junctions than a simple zener diode. In addition he says there are other parasitic elements that are not visible. Probably not a huge amount of extra circuits like in the LM399 diagram I posted but not a simple diode like one might be led to believe by use of the term "zener".ps. I recall the reaction these comments created in the volt-nut thread. Probably chalked up to religious fervor sorta like in church mentioning archaeologic evidence Jesus might have been married or had a brother. LOL

author: paulie
author_rank: Frequent Contributor
timestamp: March 26, 2015, 12:12:10 am
content: Quote from: JohnnyBerg on March 24, 2015, 02:21:12 pmI really appreciate your offer but fortunately janaf informed me he already included some along with the buried zener samples. I also ordered a bunch from polida2008 but they probably won't arrive for a month or so. Thanks again. I'm constantly amazed at the generosity of fellow EEvblog members.ps. I'm am collecting another set of bandgap references but there's no rush because the next test run will be 7v zeners only. 2.5v devices will have to wait for next run after because my 10x expander don't have wide voltage range.

author: paulie
author_rank: Frequent Contributor
timestamp: March 27, 2015, 10:37:43 am
content: On request I've added another photo to 1st post with more details on the amplifier/multiplexer board. This design was actually for a customer who was investigating battery performance. They built over a hundred PCB versions but as usual I got to keep the prototype. One of two actually since I firmly believe in redundancy and backup in case things don't go so well.

author: Marco
author_rank: Super Contributor
timestamp: April 10, 2015, 06:13:35 pm
content: I found a scan someone did ofthe LM723 temperature controlled reference, it wasn't as smart as suggested ... it actually used a second transistor array for the voltage reference, the LM723 was a preregulator and oven controller.This seems wasteful to me ... the suggestion of using the internal LM723 transistors as sensor and heater for the internal reference seems better, although the way the transistors are connected makes it a bit non trivial.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 10, 2015, 07:14:48 pm
content: I wouldn't use a 723 as a reference. I just looked at thespecs, even a 2 cent TL431 is better

author: SeanB
author_rank: Super Contributor
timestamp: April 10, 2015, 07:31:40 pm
content: Quote from: JohnnyBerg on April 10, 2015, 07:14:48 pmIt was pretty good though when it came out. The competition was a zener diode, potential divider and pass transistor. Still is a good device though if used correctly.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 10, 2015, 07:44:31 pm
content: Quote from: SeanB on April 10, 2015, 07:31:40 pmI remember using it as a kid, however, that was decades ago.Quotehmm ..

author: Marco
author_rank: Super Contributor
timestamp: April 10, 2015, 08:10:33 pm
content: Quote from: JohnnyBerg on April 10, 2015, 07:14:48 pmThere are however no other cheap references with devices on the die you can use for temperature measurement AFAICS.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 10, 2015, 08:50:59 pm
content: Quote from: Marco on April 10, 2015, 08:10:33 pmTrue!But besides tempco, long term stability also sucks!It is very difficult to rebuild a Deux Chevaux to a Ferrari. It always will be a .. Deux Chevaux

author: rdl
author_rank: Super Contributor
timestamp: April 10, 2015, 09:37:00 pm
content: Quote from: The Art of Electronics, Third EditionOf course, they are describing it as a voltage regulator, not as a voltage reference.

author: EJE
author_rank: Newbie
timestamp: April 22, 2023, 12:21:59 am
content: Hello, PaulieLooking for discussions regarding voltage references (for DMM, for Power supplies, etc), I stumbled in this old thread and found your series of posts quite interesting, but had some difficulties to follow your experimental findings or to fully get your test method and steps you chose to do (but for the discussions held here this would be secondary, compared to your findings).From what I understood:(1)You measured several lower cost Voltage References, and of different types in an oven, after a given warmup time, using a kind of multiplexed ADC - as you pictured in that postQuote from: paulie on March 18, 2015, 03:54:10 pm(2)For several pictured devices, grouped as:(a)IC referencesQuote from: paulie on March 18, 2015, 03:55:14 pm(b)Zener+Diode assembliesQuote from: paulie on March 18, 2015, 03:56:50 pm(c)LED and Transistor samplesQuote from: paulie on March 18, 2015, 03:57:46 pmFor them, you typically used that “fixed current” through a given voltage:(as ~5V) and a known resistor to measure their “output” voltage.(3)There are data handwritten about the voltages you found, but(4)I could not identify the influence of Temperature (like Temp. coeff.) you might have identified and measured.(5)I also didn’t find if you measured/rechecked them for mid/long term stability at a given test condition (~ current & temperature).After such long time, you may have tabulated these findings, or somehow summarized them on an attachment, but I couldn’t find. So, if you have done so, could please point them where I could study your findings?Thank you,EuthymiosP.S.I’m an Mech.Engineer professionally and passionate about Electronics for more than 40 years, studying topics as a “serious hobbyist”.Sorry for the “inadequate indented” format, as this is an initial post (and done on an iPad, no mouse here); I had difficulties to do a nicer formatting and compromise this post.

author: magic
author_rank: Super Contributor
timestamp: April 22, 2023, 07:47:27 am
content: FYI Paulie hasn't been active here for many years.https://www.eevblog.com/forum/profile/?u=91790

author: iMo
author_rank: Super Contributor
timestamp: April 22, 2023, 09:59:46 am
content: @Euthymios: But there are answers, sure, better to ask in the Metrology section, there are people playing with various kinds of references..

author: EJE
author_rank: Newbie
timestamp: May 01, 2023, 04:19:07 am
content: Thank you for the info & tip how to check that.It is a pity if Paulie’s data might be now inaccessible.