thought experiment - self-controlled voltage reference - Page 1
Forum section: electronics / general

author: branadic
author_rank: Super Contributor
timestamp: July 04, 2013, 09:52:46 pm
content: I just want to share and discuss an idea with you.Lets assume we have:- a calibrated adc, we use a calibrated LTC2400, means, compensated for linearity etc.- a calibrated high resolution dac with >20bit, means, compensated for linearity etc.- three references (ref1, ref2 and ref3) with good short-term stability, we use three off-the-shelf LT1236* in LS8 for the first assumption, because they output 5V nominal- an opto isolated µC that controls the adc, dac and the required switches- in front of the adc input is a 2:1 divider (e.g. LTC1043) to measure at half of the full scale (2.5V nominal)Ref1 is connected to the dac that outputs let's say for the moment 1V, that is measured against a Josephson standard and adjusted to the exact value of 1.000000V. The correction factor is saved (initial calibration and adjustment).We now connect ref1 directly to the ref input of the adc and measure ref2, afterwards ref3 and save the difference ref2 - ref3.We connect ref2 directly to the ref input of the adc and we measure ref1, afterwards ref3 and save the difference ref1 - ref3.We connect ref3 directly to the ref input of the adc and measure ref1 and afterwards ref2 and save the difference ref1 - ref2.Now the drift value for each reference is calulated and saved and the ouput of the dac with ref1 connected to it is corrected to the new value for 1.000000V based on the drift info.We expect, all three references are uncorrelated, so their noise and drift is different. After the initial calibration the system is monitoring itself. Any difference in trace length is static and therefore systematic, means compensated when calculating the three differences. If one of the three references dies, you will notice by an extrem change in the calculated difference, it needs to be replaced and the system to be calibrated and adjusted.Beside the additional power for driving ref2 and ref3, is this arrangement able to output a very, a high (not to say ultra) stabile output voltage of 1V? What are the traps I've overlooked in this is experiment?Remember the magical number3, suggesting ultra high stability (three legs are more stable than two or four --> see tripod, you need three sheets of granite grinding against each other to get high precision surface...). I'm curios about your opinion.* The reference can be replaced by any other reference of you choise, I used it because it outputs 5V nominal.

author: ddavidebor
author_rank: Super Contributor
timestamp: July 04, 2013, 10:20:26 pm
content: Mmmh it's late here but i don't know how and why it will improve something.

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2013, 06:35:11 am
content: Improved stability because you can compensate for the drift?

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 05, 2013, 08:03:07 pm
content: I would question the need for the extra complication and error of the DAC. If you have N references, N > or = 3, then you can do all the tracking and adjusting in software, presumably you will be doing that anyway.

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2013, 08:16:52 pm
content: I'm not sure if I got you right, how do you want to adjust without having a dac? What do you mean by saying:QuoteSure I use three references, but after an initial calibrate and adjust cycle the system is monitoring it's own drift and able to adjust its reference output itself.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 05, 2013, 08:56:41 pm
content: Quote from: branadic on July 05, 2013, 08:16:52 pmYes I should have been more explicit, I mean I question the approach of always tweaking or adjusting your references with a circuit element which is less accurate than the rest of the system. Consider Hoffman's approach and his three + references. He sends one out to the cal lab on a rotational basis, to have the output monitored but not tweaked, so that he builds a cumulative history. I like this approach and I think I will emulate it. If you want a separate output at some prescribed standard voltage then i suppose you could build a buffered circuit that you can trim for convenience. If I have multiple references then I would leave all the averaging of multiple measurement and compensation calculations in software that way I avoid worrying about what errors are being introduced by any kind DAC feedback.

author: madires
author_rank: Super Contributor
timestamp: July 05, 2013, 09:05:11 pm
content: Maybe I got it wrong but I think that you would just monitor the drift relations between all three references and the system will run away with time. It would be like placing three markers at random places around the world, measuring the distances between those and trying to figure which moves over time. You can tell how one moved in relation to the others but you can't tell if that one really moved or not, or the two other moved. To be able to do that you need a trustworthy observation point as a fixed reference.

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2013, 09:11:03 pm
content: Sending one ref periodical to a cal lab is one way, sure, but such a system is not self-monitoring. However, I don't like the idea to manually adjust the output to an ideal value, pots with tempco etc.Also mentioned that the dac needs to be calibrated, maybe you remember the 32bit dac approach? I concealed the requirement for monitoring the ambient temperature to keep the thought-experiment simple and also the 1V ouput was only an example, not a fixed specification.It's all about long-term stability, not absolut accuracy.

author: branadic
author_rank: Super Contributor
timestamp: July 05, 2013, 09:16:43 pm
content: QuoteUsing three references you can do, not absolute but relative. Therefore I do an initial calibration (inital absolut value) and from there on I monitore the relative movement. It is improbably that all three references move in the same direction, with the same velocity and have correlated noise, so there will always be a difference between them.

author: dfmischler
author_rank: Frequent Contributor
timestamp: July 05, 2013, 09:45:40 pm
content: Wouldn't you rather have a true, inherently accurateprimary standard?

author: Andreas
author_rank: Super Contributor
timestamp: July 05, 2013, 10:13:09 pm
content: Quote from: branadic on July 04, 2013, 09:52:46 pmThe LTC1043 is unnecessary in this experiment since the LTC2400 has +/-12.5% overrange. (Ok you can use at maximum some 10 mV before the input protection diodes create non-linearities.The noise of the ADC is nearly independant of the input voltage. But 10uVpp of a single measurement is 10ppm at 1V and only 2 ppm at 5V. Of course you can reduce noise by averaging many measurements.As madires already said: the measurements are not independant. Its like 3 boats on the sea orientated in the same direction. You can measure the distance between them from every boat. But the only thing that you can tell without seeing the polar star as fix point is which one of the boats is the fastest in forward direction. But this speed can be negative because all are drifting in backward direction.So the only information that you really have is which one of the references is drifting fastest against the average. In reality by using equal devices from the same manufacturing process they will be more or less behave similar in direction.Similar experiment in attachment.3 MAX6250A and 1 AD586MNZ in plastic DIP measured with a ADC with a good aged AD586LQ over nearly 1 Year.Without looking at the light blue line near 0 ppm which is the drift of the ADC against a LTZ1000A:Could you really tell whether the ADC or the references drift?Implicitly you would average the drift and blame the average value on the ADC making the references better.And note: after a initial "independant" drift all references seem to drift into the same direction.This implies that the ADC and the LTZ1000A are drifting the most. Funny?With best regardsAndreas

author: madires
author_rank: Super Contributor
timestamp: July 05, 2013, 10:16:24 pm
content: Quote from: branadic on July 05, 2013, 09:16:43 pmThe initial adjustment is the absolute starting point for ref#1 but as all three refs drift you can't detect how they drift away from the initial absolute adjustment if just the relative drifts between the three refs are measured. For example, assume you and two friends meet at a specific location in your town. Then everyone walks around and you try to estimate your distance to the starting point by monitoring the distance between all three friends. Does that work?

author: branadic
author_rank: Super Contributor
timestamp: July 06, 2013, 06:25:29 pm
content: QuoteUp to know I'm still unconvinced that the approach will fail. In your example the AD586 is the observer, a ref that itself is not long-term stable and drifts away over time, so no good oberserver at all, because the drift of him can't be compensated as you never know if some common-mode drift is due to hinmor the observed three references.The way you measure the drift is not comparable with my approach, because ref1 measures the difference between 2 and 3, it's actual absolute value is not of interest in this moment. I don't need a fourth high stable observer, you do.A drift of all three references in the same direction must have it's reason in some change in ambient condition, therefor a temperature and a humidity sensor is requiered.I think I will setup a matlab emulation to check this.

author: Andreas
author_rank: Super Contributor
timestamp: July 06, 2013, 09:16:48 pm
content: Quote from: branadic on July 06, 2013, 06:25:29 pmIn my experiment the temperature is controlled to 50+/- 0.2 degrees centigrade.Room humidity is similar after 1 year. winter 40-55 % summer 55-70%.By the way: the relative accuracy of differences from 2 similar sources is better when you directly measure the difference and not the absolute value and then calculate the difference.Example: your instrument has a uncertainity of 10ppm (reference voltage) + 6uV (thermocouples of setup)At 5 V this would give +/-56uV for each measurement resulting in a statistical error of up to 79 uV.If you measure the up to 5mV difference directly you will have only about 6.05 uV error.With best regardsAndreas

author: Unknown
author_rank: Unknown
timestamp: July 06, 2013, 09:49:14 pm
content: You can measure the difference very accurately, this is often done when comparing references, but what's the point if you don't know either value? In general one of the references you would be measuring would be a more accurate or recently calibrated one. You could average the references, although you might as well connect them in parallel instead of going through the elaborate scheme with an ADC. If drift was random, independent and identically distributed, then the drift of the mean would be a factor sqrt(3) better. It's not likely to be random, for example the LM399 will usually drift up and the LTZ1000 will often drift down. It that case, the improvement may be even less.If abs(ref1-ref2) and abs(ref1-ref3) is smaller than abs(ref2-ref3), are ref2 and ref3 drifting less, or is their drift more closely correlated? Three references is way to little for any meaningful application of averaging.

author: branadic
author_rank: Super Contributor
timestamp: July 07, 2013, 10:00:52 am
content: Okay, let's sum up a few things.Three references aresimliar but never equal, they will have different noise and drift rate, so they will never behave exactly the same. This given but not yet faced fact is required for this experiement.A shift of all three references by the same magnitude in the same direction is, in almost any case, caused by a change in ambient condition and not by mystical force. That means if I monitore the ambient conditions such as temperature, humidity and pressure I can compansate for that.For the upper given example of the boats this means, they were able to compensate for the drift by the sea if they would monitor the direction of the force of it. They don't have such a sensor, so they must use an absolut point (the polar star). It's also the same with the three friends. The don't need a fixed point unless they were able to monitor theirrelativeforced movement.Theoretical the approach is still possible, I haven't heard any argument that made it collapsing.

author: Andreas
author_rank: Super Contributor
timestamp: July 07, 2013, 11:37:02 am
content: Quote from: branadic on July 06, 2013, 06:25:29 pmYou will need it also:To determine the drift of 3 references over time you will need 3 independent pairs of measurement values.(3 unknown values can be determined with a set of 3 linear independent equations)In your case you have your initial calibration value. (3 individual independent measurement values can be generated even when 2 of them are measured as difference).Even when you are measuring in future 6 differences only 2 of the measurements are really independent.The others you can calculate (except of the noise of the ADC-converter) from each other together with the absolute values of day one.So you will need at least one measurement from a independent high stable observer ....With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: July 07, 2013, 12:16:57 pm
content: Don't mistake the initial calibration and adjustment for an independent high stable observer. All you do with this step is to set your system origin, so that it equals the origin of the rest of the world. You could also abdicate this step and define your value to be the new definition, your system will than be parallel translated to the rest of the world, that's it.

author: madires
author_rank: Super Contributor
timestamp: July 07, 2013, 12:24:13 pm
content: Quote from: branadic on July 07, 2013, 10:00:52 amIf somehow the voltage of all three refs is doubled your method woudn't be able to detect that, because the relations would stay the same. I think you should simulate or build your idea if you don't get the arguments. But don't complain later on if it doesn't work

author: branadic
author_rank: Super Contributor
timestamp: July 07, 2013, 12:37:29 pm
content: QuoteAn example that is far away from reality! And I claim that I would, because adc and dac would die by overvoltage. I could also observe the power rails and compare it to the ref-difference.And by the way, you wouldn't even notice if for an observer watching our world the same happens to all JJ standard and references too.

author: Unknown
author_rank: Unknown
timestamp: July 07, 2013, 01:42:00 pm
content: My argument is that the best you're going to get is an average of three independent random variables, and even then you only get an sqrt(3) reduction of the drift. If you have three balls move in random directions at random velocities, will the center of mass stay the same?How about you post an example where it gives a more substantial improvement? You could try modeling the three references just as a 1D random walk, calculate the 'true' value via the method you're proposing, and compare the standard deviation of this true value to that of the individual references after say 1000 steps.Something like:Code:nsteps=1000;ref1 = zeros(nsteps,1);ref2 = zeros(nsteps,1);ref3 = zeros(nsteps,1);true_value = zeros(nsteps,1);for step = 2:nstepsref1(step) = ref1(step-1) + randi(2)*2-3;ref2(step) = ref2(step-1) + randi(2)*2-3;ref3(step) = ref3(step-1) + randi(2)*2-3;true_value(step) = mean([ref1(step) ref2(step) ref3(step)]);endstd(ref1)std(ref2)std(ref3)std(true_value)This ignores any systematic component to the drift (eg. a time-related aging process within the silicon), but at least gives us an impression of whether your method could work under ideal circumstances.

author: ddavidebor
author_rank: Super Contributor
timestamp: July 07, 2013, 02:18:21 pm
content: Quote from: alm on July 07, 2013, 01:42:00 pmI think there is no more a lot to discuss

author: madires
author_rank: Super Contributor
timestamp: July 07, 2013, 02:44:13 pm
content: Quote from: branadic on July 07, 2013, 12:37:29 pmA doubled voltage is extreme, I admit. I tried to point out the problem. If you like to have it more realistic let's take an increase or drop by 10mV which may be caused by radioactivity or whatever. Since all three refs are in the same box all will be effected at the same time. Yes, you could put everything in lead and add tons of sensors to mitigate things but the basic problem of the method is still the same. Monitoring the relations between all refs gives you a hint about they drift in relation to each other. It won't give you any hint how they drift away from the initial adjustment. If ref #1 drifts while you're measuring ref #2 with ref #1 as reference for the ADC how can you detect if ref #1 or ref #2 drifted. In the mean time ref #3 drifted also a little bit. Or maybe ref #1 and ref #2 drifted at the same time. What happens if the refs drift with varying speeds? Would your method be able to detect very slow drifts? Is it just a ADC glitch or did the voltage changed? Or didn't it change but the ref currently used as ADC ref did?QuoteIf the observer wouldn't have his own uneffected reference he wouldn't notice it :-) The probabilty of three refs inside the same box being effected by something is much higher than the probabilty that all standards and references around the earth are effected at the same time, don't you agree?

author: branadic
author_rank: Super Contributor
timestamp: July 28, 2013, 11:41:43 am
content: I'm still not confident that my approach won't work, because I was reading some more about correlation in context with noise measurements and an approach to compensate for the noise of the gear by correlation.Because each reference can seperately be connected to adc_ref or adc_in it is also possible to connect two references to adc_ref and measure the third, what is close to the named correlation approach.So we have the following measurements:ADC-Ref | ADC-InRef1 | Ref2Ref1 | Ref3Ref2 | Ref1Ref2 | Ref3Ref3 | Ref1Ref3 | Ref2Ref1+2 | Ref3Ref1+3 | Ref2Ref2+3 | Ref1How ever, up to now I haven't heard any argument that was able to stop me giving it a try. A real world simulation in my opinion isn't possible because you give them a behavior and a correlation they won't have, the references are uncorrelated. As long as the "world police" avoids starting nuclear w. w. and all people remember that we live in 21th century with no need for "cold war" and all that paranoia shown up in the last weeks I don't care about radioactivity. And if otherwise expected we get in such a situation I'm sure that I don't need this experiment anymore

author: branadic
author_rank: Super Contributor
timestamp: July 28, 2013, 02:12:26 pm
content: Maybe lets get more practical. What relay would you suggest? A classical relay with gold plated contacts or one of those reed relais?I prefer a modular assembly,one board with the relais,the LTC1043 divider,the LTC2400 and the reference boards hucked up. For the first test I would use some LT1236LS8.

author: Unknown
author_rank: Unknown
timestamp: July 28, 2013, 02:41:09 pm
content: Quote from: branadic on July 28, 2013, 11:41:43 amYes, averaging helps against uncorrelated noise: the sqrt(n) best case improvement I have repeatedly stated.Quote from: branadic on July 28, 2013, 11:41:43 amI have no interest in keeping you from experimenting. I encourage you to convince us with results.Quote from: branadic on July 28, 2013, 11:41:43 amThe suggested simulation was for three uncorrelated random processes. Unless you question the quality of MATLAB's random generator for this purpose.Quote from: branadic on July 28, 2013, 11:41:43 amAnd as long as the world police keeps from enforcing the laws of thermodynamics, we might even expect drift to be completely uncorrelated.I would probably use a reed relay with gold plated contacts for these kinds of very low currents.

author: madires
author_rank: Super Contributor
timestamp: July 28, 2013, 03:25:34 pm
content: Quote from: branadic on July 28, 2013, 02:12:26 pmIMHO the reed ralay would be the right choice. It's more reliable, extremely good for switching low power signals, sealed and should have gold plated contacts anyway. Also the contact performance is more consistent.

author: branadic
author_rank: Super Contributor
timestamp: July 28, 2013, 04:38:47 pm
content: Your opinion agrees with my thoughts, there are this Meder SMD-Reed-Relais available. They are everything but cheap but they will save space on the board. I have some samples of the rf version, pretty nice but not useful for real rf application when isolation is needed up to some GHz.QuoteI don't doubt the quality of Matlab (couldn't check your code because I use Octave and wasn't able to get it run) but your assumptions. The three references are uncorrelated be means uncorrelated drift, noise and aging rates. So together with the devide by two at the input I'll be able to also see the shift by 10mV of the reference in the same direction (upper example), wouldn't I?

author: Unknown
author_rank: Unknown
timestamp: July 28, 2013, 05:04:55 pm
content: Quote from: branadic on July 28, 2013, 04:38:47 pmChanges to get it to run in Octave should be trivial. Uncorrelated does not mean the drift has to be different, just that they are as likely to drift in the same direction as they are to drift in the opposite direction. If all three references go up by 10 mV, then you don't detect anything. If two of them go up by 10 mV, then you can't tell whether two went up by 10 mV or the third went down by 10 mV (this is the same from the ADCs point of view). Note that time averaging works just as well to get rid of random noise. It's the not so random processes (eg. drift) that cause trouble.

author: Andreas
author_rank: Super Contributor
timestamp: July 28, 2013, 05:31:08 pm
content: Quote from: branadic on July 28, 2013, 02:12:26 pmHello branadic,For my LTZ + LM399 references I am using 2-pole bistable signal relays with bifurcated gold contacts of type TQ2L2 (Matsushita / Panasonic).The intention is that the thermal heating of the coil does not influence the thermocouples of the contacts.The coils are only powered during switching, so that a 9V 200mAh block will power the multiplexer including processor for about one week switching.With reed relays you will have a reliable contact over time but the wires will be most probably be of Kovar havinga 40uV/K thermocouple voltage against copper. So a good thermal design of the solder junction areas will be necessary.Another approach would be a semiconductor multiplexer that I use for the 5V-ageing experiment. It consists of four simple CD4067 CMOS multiplexers switching either the 5V output or the GND signal of the Reference to the ADC.The GND-signal is necessary since I have up to 60uV GND-shifts between the 32 references. I tried a differential multiplexer scheme but this was too noisy since I cannot use a battery supply with 32 references + heater for constant 50 degrees centigrade.With the LTC1043 you have to notice that you cannot load the output directly by the ADC input. The input current of the LTC2400 will affect accuracy + linearity of the LTC1043. So you will have to use a buffer amplifier after the LTC1043. I currently use a LTC1050 with +14/-1.2V supply for this purpose. The LTC2400 has a linearity error nearly centered on mid-scale. If measuring the own 5V reference you will miss around 40uV reading something around 2.499960 instead of 2.500000V. So you will have to compensate for this according to AN86. And yes: the typical 4ppm linearity error of the LTC2400 seems to be in reality measured by the box method (+/-4 ppm around the best fit straight line and not the end points 0 + 5V) giving actually 40uV error at mid-scale.The LTC1043 has a additional gain error of about +/-2-3 ppm depending whether you exchange input and output of the LTC1043 against the datasheet or not. So you should do a overall calibration of LTC2400 together with the LTC1043 for gain + linearity.good luck + with best regardsAndreas

author: madires
author_rank: Super Contributor
timestamp: July 28, 2013, 05:42:55 pm
content: Quote from: Andreas on July 28, 2013, 05:31:08 pmMeder offers some reed relays with a thermocoupling voltage <1 µV/K.

author: branadic
author_rank: Super Contributor
timestamp: July 28, 2013, 06:40:33 pm
content: Forgot to mention that, yes, the LTC1043 will get his buffer, both parts LTC1043 and LTC1050 are already here at the bench.QuoteReally? I have to search for that. Thanks for this tip.

author: Andreas
author_rank: Super Contributor
timestamp: July 28, 2013, 06:48:59 pm
content: Quote from: madires on July 28, 2013, 05:42:55 pmIf you think of the BT-type: It seems to be so that the circuit only works if using both contacts in a manner that the thermocouples are compensated against each other. (this is what I meant with carefully thermal design).This assumes that the voltage references are all galvanically isolated against each other and the 2:1 divider.And you will always have to use the whole contact pair for signal + return line.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: July 28, 2013, 08:59:51 pm
content: QuoteSomething that won't be easy to realize when switching the references between input and reference pin of the adc.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 30, 2013, 08:03:53 am
content: Hello branadic,you've constructed a very (too) complicated case, but it's a kind of Perpetuum Mobile only (Ultra Precision out of Thin Air), i.e. it does not work.Such Perpetuum Mobile constructions are always difficult to unmask, but I will try to do it in a handy way.To point out your error in reasoning, I have to sort your statements a little bit, and simply write down the calculation of the output of the ADC:Quote from: branadic on July 04, 2013, 09:52:46 pmThe ADC will output digital readings, e.g. Mx=Refx/Ref1*K, and the calculation of the 3 differences Y1=M2-M3, Y2=.. and Y3=.. will be:Y1 = (Ref2-Ref3)/Ref1 *K, Y2 = (Ref1-Ref3)/Ref2 *K, Y3 = (Ref1-Ref2)/Ref3 *KCalibration Constant K is determined by the exact value of the 2:1 divider and the digtal length (~2^24) of the ADC, and can be measured / eliminated by applying one of the Refs to Input and to Ref_in : M=Ref1/Ref1*KThis equation system:Y1*Ref1/K = (Ref2-Ref3)Y2*Ref2/K = (Ref1-Ref3)Y3*Ref3/K = (Ref1-Ref2)cannot be solved at all, because there are no independant variable on the left sides.It is not even possible to determine the absolute values, not to mention any drifts.If you try to elminiate two unknowns, you always end up with differences of them.Simply speaking: you are already missing one independant,knownreference at this point.If you would relate your measurements to a fourth Ref4, which absolute value you know in first instance, you could resolve this equation system, and would get the absolute values of Ref1, Ref2, Ref3, but the drift of this forth reference would always go into the drift determinations.To say it short: To measureabsolutedrift, which is needed to compensate for that, you always need to compare against one fixed (ultrastable) reference, e.g. an JJ standard.If you measurerelativedrifts, it is necessary only to measure against one reference, i.e. always the same known reference.If the differences between Ref1, Ref2, Ref3 are low, the uncertainty and stability of the ADC refence is not important, if you measure their differences directly.(E.g: you can measure small differences with 3 1/2 or 4 1/2 digits panels)In this case, you get the relative drifts of Ref1 compared to Ref2 to Ref3, and this gives you an idea, how stable those are, as an ensemble.The more different references of one kind you have , the smaller is the possibility that they all drift in the same direction.The max. spread of drift of the differences is then a measure of their individual stabilities, i.e. you can reallyestimatetheirabsolutedrifts.Of course it's possible to determine outliers, i.e. references with much bigger drift rates than the others..That's the classical metrological problem, to determine the stability of standards, if there is nothing "better" than those, or:"A man with one clock always knows the Time. A man with two clocks is never sure. A man with three clocks is able to decide."The following I also do not understand, how it will give results, or improve your constructionQuote from: branadic on July 04, 2013, 09:52:46 pmYou could only determine the absolute value of the refernces once, if you apply those 1.00000V to the ADCs input.Later measurements or corrections are always done on a drifty reference, therefore the correction drifts also, and there is again no fixpointQuote from: branadic on July 04, 2013, 09:52:46 pmNo that's not true! (What do you mean with trace length??)The differences are NOT static, and are unsystematic, as the drifts are independant and not determined.Anyhow, all measurements are self-related, and therefore you cannot eliminate the absolute values, nor the drift.Conclusion: There is no free lunch.Otherwise: If your perpetuum idea would work, others would have realized it already, wouldn't they?Frank

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 30, 2013, 10:14:56 am
content: Quote from: Dr. Frank on July 30, 2013, 08:03:53 amThe criticism that branadic's approach is a perpetuum mobile because of its circularity is a bit to simple.QuoteYes! this iskey.QuoteBut othershaverealized it!I have recently added metal working to my list of hobbies, and one of the precision wonders required is a surface plate, or even better- optical flats. As you have vastly more education in physics than myself I won't presume to tell you how they are made, I just give my understanding which is that you start with three crude articles (blanks) and grind them one on the other, flipping and exchanging the blanks in rotation. With patience and ever finer grinding media you can derive ever finer planarity and level. This is how amateur astronomers routinely gain optical quality surfaces with cave man equipment. Either implicitly or explicitly I believe this process to be the inspiration for branadic's contraption.Now I still see problems with Branadics attempt, as the high points (outlier references) do not necessarily stay high points as they would with surface plates (between rubbings), also there are many more points of potential contact with surface plates, which means you would need a huge sea of reference zeners. I also find the DAC suspect as it has its own error characteristic and it does not average like grinding media.The idea that you might be able to bootstrap into the next higher level of precision fascinates me, for the intrinsic possibility of getting something for nothing, and because unlike the other volt-nutters who have access to 3458's I am existing in precision poverty.

author: Unknown
author_rank: Unknown
timestamp: July 30, 2013, 10:58:04 am
content: The crucial difference in my opinion between the construction of an optical flat in the way you describe and branadic's approach is that drift and noise of voltage references is a stochastic (random) process, unlike the flatness and bumps of a solid surface which is for most intents and purposes constant. The difference between two random processes is just another random process. Its variance might be slightly lower, but only if they have a known and perfect correlation will subtraction result in substantial cancellation of errors. A substantial and unknown correlation is even worse, however, since you might actually be amplifying errors.I share Dr. Frank's criticism that it seems overcomplicated: a bunch of subtraction, switching etc that to me seem to serve no purpose, which is why I suggested simulations to reduce it to the simplest system possible. The problem with experimental validation is that to quantify drift, you need a reference with a substantially lower drift than a single reference. So you might actually need that 3458a to design the low cost reference. And the 3458a is not particularly great for long term drift (maybe a factor of two better than a single LM399), which is why the real volt nut will use a voltage transfer standard -- or even a JJ -- as reference for the 3458a.Starting with crappy references to prove the concept might actually be the simplest solution: if you can get 0.01% stability (something that you should be able to measure with a decent 4.5+ digit DMM) from a crappy reference that shows 1% drift over some reasonable time period (you don't want to measure for a year), then it shows that your method can substantially improve the stability. Of course more effects (like the noise of the DAC) come into play as you go to even higher stability, but something crappy like zeners might be a good way to show a proof of concept. It may for example turn out that drift in those crappy reference is dominated by the highly correlated change in ambient temperature, something that will play much less of a role in better references.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 30, 2013, 11:29:09 am
content: Quote from: chickenHeadKnob on July 30, 2013, 10:14:56 amSorry, but it IS REALLY as simple as this... it was my intention to clarify this as easy as possible.Alternatively, if it's still too complicated to see it clearly, one could write down the complete set of six equations M1 ... M6 for the ADC output, and then try to solve it, also with no result:3 of those 6 equations are inverse versions, and the fourth equation can be calculated from the other two.So you have 2 independant equations for 3 variables only.That's sufficient for determining all ratios between those 3 references, but neither their absolute values, nor their absolute drifts.Quote from: chickenHeadKnob on July 30, 2013, 10:14:56 amSorry: WHO has created an ultrastable Artefact Standard from 3 drifty ones, as described by branadic? (E.g. Zener Refs, Weston cells, kilogram standard of Sèvres, etc...)I don't know anybody. Please show me where I can find that.Quote from: chickenHeadKnob on July 30, 2013, 10:14:56 amA near perfect flat surface has nothing to do with such artefact standards for SI units.You have used a very, very inappropriate comparison.The analogon to your flat mirror in volt metrology is perhaps the Hammon type divider or the Kelvin-Varley divider, which create ultra - precise volt RATIOs by self-calibration only, without the aid of another hyper-standard (and currently there is nothing better in metrology, if you do not use a quantum standard, i.e. the JJ array.)Quote from: chickenHeadKnob on July 30, 2013, 10:14:56 amThis "bootstrapping" really exists, but in a different meaning!This is routinely used for stabilitydetermination, i.e. it's a common "metrological practice" to compare two or more stable standards against each other (the more, the better), and then from their relative drifts to ESTIMATE their individual absolute drifts.How do you think the Cesium beam clocks have been qualified, i.e. the statement " A Cs clock is precise to 1 sec in 30 million years" ??Very easy, there are two ways:Theoretically, you calculate all known external influences on the stability, try to mitigate them and calculate the worst case deviation / stability figures theoretically.Practically, necessary as a proof of the theory: you compare all Cs clocks of all state institutes (NIST, PTB, BIPM, etc.) against each other via dual view satellite connection.You will get the relative drift rates, which are very low, on the order of 10e-16/yr, and then do some statistics to estimate their individual drifts.Here again, you cannot measure the absolute drift rates directly, because up to recently, there did not exist more stable clocks.I have explained that technique in my article about the LTZ1000 reference, where I compare 4 independant ultra-refs, and estimate their individual drifts to < 1ppm/yr.I cannot improve their individual stabilities either, but it was solely a great thing (call it a metrological trick) to measure their relative drifts and estimate their absolute drifts WITHOUT using a JJ array standard.And indeed, that's possible with branadic's attempt also (if he removes all that unnecessary complications), not less, but also not more.Frank

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 30, 2013, 10:43:28 pm
content: Quote from: Dr. Frank on July 30, 2013, 11:29:09 amI hope we don't get into a misunderstanding with each other as I learn from your posts and enjoy them. I was almost in full agreement with your previous post but reacted to the phrase " perpetual mobile" as it invokes the laws of thermodynamics and would lead the discussion into a cul de sac. Which I worsened by using the phrase "toosimple" when I meant to convey the meaning: the mere presence of circularity was insufficient to be a flaw.And while I do have I simple mind I did follow your original argument with the equations and agree with it. You can't gettherefromhere, ifthereis an accepted standard value andhereis three drifty unknowns. My small disagreement was with the notion that any circular (self referential) process axiomatically meant you were stuck with the originating level of precision.The mental model I was operating under was that of a mathematician who needs to prove an infinite series set before him converges to a finite result. Just like in metrology there are dual burdens, first prove the series converges to a (any) finite result, that is, is it increasinglyprecise. Second if it converges, does it converge to a known value, that is, is itaccurate. Please correct me if I have reversed the definitions of Accuracy and precision. For what it is worth I never thought Branadic's putative machine achieves improvement in either accuracy or precision. But what if someone comes up with a system which iteratively evolves greater precision in its intermediate results, up to some physical limit. That is in my mathematical analogy does it converge to some value + epsilon? This is why I gave the example of amateur lens grinders. I think the laws of nature do not a priori prohibit such a process they just may make it difficult, again please correct me if I am wrong.Quote from: Dr. Frank on July 30, 2013, 11:29:09 amIt was not my claim or intention to convey that you could achieve a standard kilogram by rubbing three rocks together. Hammon, KV dividers, null meter, torsion balances and other instruments of precision do have many useful applications even if you do not have access to a known standard. Here in my wilderness, I consider it a partial victory if I have a machine or process which can make precise measurements even if I can not trace the result back to a known standard.best regards

author: branadic
author_rank: Super Contributor
timestamp: July 31, 2013, 09:45:29 am
content: 1. Comparing a thought experiment about a self-controlled reference that is powered by mains or battery with a perpetual motion machine harvesting free energy out of nothing only be applying a portion of energy to it for one time is farcical. I really tried to manage a factual discussion, but the perpetual motion machine statement is far beyond that.2. Believing that a few linear equations are adequate to explain a complex nonlinear construct is farcical too.3. Telling people that they are wrong because others would have already realized a soultion for a given problem is ignorant and nonscientific. If everything would have been realized I were unemployed in my job as r&d engineer.Nothing personal against you Frank, but you don't know it all.BTW.: Your holy JJ is everything but absolut stable too.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 31, 2013, 04:21:42 pm
content: Quote from: branadic on July 31, 2013, 09:45:29 am1. Yes it's right, I used this comparison with Perpetual Mobiles (more energy from less energy) because I really doubt that your construction is capable of creating more stability out of less stability.Sorry, it was not intended to insult you, I just wanted to pin point the problem, I have with your construction. And I am not the only one, obviously.2. As long as your relatively simple construction is a linear system, I think it's adequate to use linear equations only.I did not find nonlinear relationships in your description. Also, their existence would not prove your statement automatically right, I think.You are an engineer?Fine, then you should be able to express your theoretical construct in mathematical expressions, let them be linear or non-linear either. I'm really curios, where in your construct the non linear behavior is buried, which I have obviously overlooked.I really would appreciate, if you prove me wrong, and yourself right by presenting your calculations.3. You are right, I do not know everything, and I did never claim that.Honestly, I tried really hard to understand your puzzled description, and I failed, obviously.As far as I have understood it, your construct was far too simple, that nobody else should have had this idea already - if it really works.So, please help me, and explain your idea in another, more understandable way.Best would be to do it mathematically, because that's the most scientific way, and what engineers and scientists understand best."Btw.:" What do you want to tell me with that sentence?I did not state that either, and no, it is not "my holy JJ", what do you think?Stay objective, please.Frank

author: Andreas
author_rank: Super Contributor
timestamp: August 02, 2013, 05:35:47 pm
content: Hello,to come back closer to the original theme some of my thoughts:- the minimum quality what can be expected when bringing 3 references together is that the resulting drift of the whole system can be brought to that what the average value of the 3 references is.- further you can do some statistic functions (standard deviation, x-r card...) to find out if one of the references behaves unusual or has the largest drift with respect to the average value. So this unusual reference can be excluded for further measurements.Of course for statistics it would be better to have many references and not only 3. But if the system excludes the right of the references it may be more stable than just a system with 3 averaging devices.@branadic: what do you mean when writing Ref1+Ref2 do you want to add the voltages to around 14V or do you mean the average value of the both references Ref1+Ref2.When measuring small differences or drifts between 2 devices it may be better to measure directly the difference of the 2 voltage references instead of calculating the difference between the 2 absolute values.In my daily measurement of LTZ and LM399 references I do both. Measurement of the absolute value and the difference.If calculating the standard deviation in uV out of 50 succeding days for the measured difference and the calculated difference each I get the following values:meas. diff calc. diff1.04 1.36 first 50 days0.85 1.03 2nd 50 days0.53 0.851.12 1.301.05 1.191.15 1.270.90 1.230.70 0.800.66 0.80 last 50 daysSo you can see that the standard deviation of the calculated differences is regularly higher than the direct measurement.The reason is that with the direct measurement the noise of the adc counts only once. Whereas with the difference measurement the noise is included 2 times (or 4 times when regarding the offset nulling) into the result.With best regardsAndreas

author: Unknown
author_rank: Unknown
timestamp: August 02, 2013, 06:16:32 pm
content: Quote from: Andreas on August 02, 2013, 05:35:47 pmYou should be able to do better than the average. If you have ten references with a random drift that is identical in magnitude, then the average should be more stable than each individual reference. sqrt(10) times better in the ideal case.Quote from: Andreas on August 02, 2013, 05:35:47 pmOr it could just pick the two which drift in the same direction (there are only two directions to choose from), and make matters worse. Three is not enough for something as random as drift in my opinion, unless one reference is really an order of magnitude worse then the others. Plus I don't see the point of building such an elaborate setup for that, just do what Agilent et al. do and run them for a few thousand hours, measure the drift over time, and picking the best. Of course this brings you back to the question of how to measure drift.All this is a far from the (close to) 'absolute stability' that was the original claim.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: August 02, 2013, 06:37:18 pm
content: Quote from: alm on August 02, 2013, 06:16:32 pmDo better than the average? I am all ears. Please define the term and present me with a method that obtains this. Or do you just misunderstand "the resulting drift of the whole system can be brought to that what the average value of the 3 references is" which is exactly what you say. Whether it is for square root of 3, 10 or some other positive integer n is immaterial for the "standard error of the mean" principle.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: August 02, 2013, 06:54:56 pm
content: Quote from: alm on August 02, 2013, 06:16:32 pmIt is unnecessary for the references to have drift that is is "identical in magnitude" to obtain the effect. The stability of the average will be more stable than the worst reference.Another question is that one will normally only average references that are ofsimilarstability so that the worst one doesn't contribute negatively to the resultant stability ...

author: quantumvolt
author_rank: Frequent Contributor
timestamp: August 02, 2013, 08:52:14 pm
content: Quote from: alm on August 02, 2013, 07:00:30 pmIf you want other posters to increase the level of precision in their language, I suggest that you avoid formulations likeidentical drift. References with identical drift track each other and are meaningless to average. One might think that you mean "identical drift parameters" ...I am sorry I find your post difficult to read. If I need to consult formulas, I prefer a decent "wiki" or academic source. Finally, as I cannot see that you answer my question or bring anything new into the theory of "The Standard error of the Mean" based on "The Central Limit Theorem" (which everyone can search and find good resources for on the web), I prefer to not pursue the theme further. Over and out ...

author: branadic
author_rank: Super Contributor
timestamp: August 03, 2013, 02:51:50 pm
content: QuoteNo, what I mean by that is that the reference voltage for the adc is the average of both. Using cross-correlation algorithm it is possible to eleminate the influence of the test gear. The idea is based on the published articels:Two-channel amplifer for high-sensitivity voltage noise measurementsThree-channel amplifier for high-sensitivity voltage noise measurementsCross-correlation-based trans-impedance amplifier for current noise measurementsetc.

author: branadic
author_rank: Super Contributor
timestamp: August 03, 2013, 10:31:49 pm
content: I have started to type in some code in Octave (Matlab).Code:close all;clear all;% t=1000ht=0:1:1000;% Electrical Characteristics LT1236% Output Voltage Vout_min=4.9975;Vout_typ=5;Vout_max=5.0025;% Long-Term StabilityLTS=20e-6;% typ. Output Voltage Noise of LT1236LS8 Vpp=3e-6;% Random Initial Output Voltage for each referenceVout1_ini=(Vout_max-Vout_min)*rand(1,1)+Vout_min;Vout2_ini=(Vout_max-Vout_min)*rand(1,1)+Vout_min;Vout3_ini=(Vout_max-Vout_min)*rand(1,1)+Vout_min;% Random Long-Term Stability for each referenceLTS1=LTS*rand(1,1);a1=exp(log(length(t)-1)/(LTS1*1e6));LTSx=log(t)/log(a1)/1e6;LTS2=LTS*rand(1,1);a2=exp(log(length(t)-1)/(LTS2*1e6));LTSy=log(t)/log(a2)/1e6;LTS3=LTS*rand(1,1);a3=exp(log(length(t)-1)/(LTS3*1e6));LTSz=log(t)/log(a3)/1e6;% Noise for each referencenoise1=Vpp*(rand(1,length(t))-0.5);noise2=Vpp*(rand(1,length(t))-0.5);noise3=Vpp*(rand(1,length(t))-0.5);% Output Voltage over timefor i=2:length(t)Vdrift1(1,i)=Vout1_ini.*LTSx(1,i);Vdrift2(1,i)=Vout2_ini.*LTSy(1,i);Vdrift3(1,i)=Vout3_ini.*LTSz(1,i);endVout1=Vout1_ini+Vdrift1+noise1;Vout2=Vout2_ini+Vdrift2+noise2;Vout3=Vout3_ini+Vdrift3+noise3;% Data Acquisition with calibrated ADCin1_ref2=round(Vout2./(Vout1/2)*2^24);in1_ref3=round(Vout3./(Vout1/2)*2^24);in2_ref1=round(Vout1./(Vout2/2)*2^24);in2_ref3=round(Vout3./(Vout2/2)*2^24);in3_ref1=round(Vout1./(Vout3/2)*2^24);in3_ref2=round(Vout2./(Vout3/2)*2^24);in1_ref2_ref3=round(((Vout2+Vout3)/2)./(Vout1/2)*2^24);in2_ref1_ref3=round(((Vout1+Vout3)/2)./(Vout2/2)*2^24);in3_ref1_ref2=round(((Vout1+Vout2)/2)./(Vout3/2)*2^24);

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 11:19:34 am
content: A few quick remarks:- Long term drift can only have a positive sign, so references won't drift down?- The drift (LTSx) is very regular and predictable. Compare this to for example the long term stability graph from theLTZ1000datasheet (note that this is voltage over time, not dV/dt over time).- No ADC error or noise?

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 11:29:33 am
content: Quote from: alm on August 04, 2013, 11:19:34 amLet's assume the drift to be positive for all three references, I don't think this is a big deal.The drift is not regular, I use a random drift value in ppm after 1000h and then calcuate the logarithm drift behaviour over time. My considerations are based on the datasheet of the LT1236LS8.Noise is included, but the adc is assumed to be calibrated with a parabolic function, something I was shown live by Andreas.

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 12:02:18 pm
content: LSTx is a smooth function of time that is monotonous and predictable: if you give me the first few points, then I can calculate LTS1, and predict the rest of LSTx. This looks very different from the graph I referred to in the LTZ1000 datasheet. As far as I see, the LT1236 datasheet does not show drift as a function of time, only the magnitude after 1000 h.The simulation assumes that the ADC will perfectly measure the (noisy) reference voltage. Not sure how much impact it has on the results, but it will make measuring the average of two references kind of pointless: vin1_ref2_ref3 = (in1_ref2 + in1_ref3) / 2 (within one LSB).

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 12:19:36 pm
content: Okay, step-by-step:- LTS is given with typ. 20ppm @1000h- I generate a random value for each reference (LTS1, LTS2, LTS3) at the point t=1000h- I now generate a function with log characteristic for each reference (LTSx, LTSy, LTSz)See page 3 of the datasheet for long-term stability:Note 6: Long-term stability typically has a logarithmic characteristic and therefore, changes after 1000 hours tend to be much smaller than before that time. Total drift in the second thousand hours is normally less than one third that of the first thousand hours, with a continuing trend toward reduced drift with time. Significant improvement in long-term drift can be realized by preconditioning the IC with a 100-200 hour, 125°C burn in.Long term stability will also be affected by differential stresses between the IC and the board material created during board assembly. Temperature cycling and baking of completed boards is often used to reduce these stresses in critical applications.I don't consider Output Voltage Temperature Coefficient into calculation, because I assume that the complete circuit is ovenized.The simulation considers the 2:1 divider (LTC1043 * LTC1050) at the adc input into account. The adc is feet by on of the references, so the resolution of the adc is the actual reference voltage divided by 2^24.Furthermore the acquired value by the adc is rounded to full integer, so what you get is a set of integer values over time for the different setups. So it's wrong when you sayQuoteClear up to here?

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 01:40:42 pm
content: Drift will decrease over time, that's what you also see in the LT1000 datasheet. But it won't be the perfectly smooth process that (apart from the magnitude) is identical for all references. Inthe ref5025 datasheetis another set of curves showing real-world long-term stability data:If an ADC is perfect, then it will measure the input to one LSB. But ADCs have noise. Even if you short the input to ground (a very low noise, low impedance input) and take a bunch of readings, you get a distribution of output values, for example:Good caseBad case.Both figures come fromAnalog Devices: ADC Input Noise: The Good, The Bad, and The Ugly. Is No Noise Good Noise?.If you take one reading of say Vref/2, then the output might be anywhere within 2^23+/- 4 counts for this ADC with a good board layout.Plus you will get non-linearity errors:(From the LTC2400 datasheet)Although you can calibrate this out, as was done inLT AN86through a fairly elaborate procedure. No calibration will get rid of noise, however, although averaging helps.

author: robrenz
author_rank: Super Contributor
timestamp: August 04, 2013, 02:51:31 pm
content: I have personal experience in the three plate method of obtaining flat surfaces having hand scraped three 18"square surface plates to within 15µ" flatness over 35 years ago. Just like this electronic discussion, there are many things that limit the achievable results when you try to get below certain levels. The modulus of elasticity of the material (rigidity) affects how much the plates conform to each other elastically and are deformed by gravity. Vertical thermal gradient of the environment directly affects the curvature of the surface because most materials have some thermal coefficient of expansion (equivalent to a bimetallic strip curvature with delta temp). That means even in zero gravity just turning a plate over will change its curvature from vertical thermal gradient affects. Unless you take extreme measures all rooms have a vertical thermal gradient that varies with seasons. My point is, a method that on the surface appears foolproof and exact usually falls apart if you start to measure at low enough resolution and consider all the minute physics details.

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 03:14:32 pm
content: I think you didn't got my simulation model up to now.You are right if you say drift decreases and long-term stability improves over time. This is what I model for each of the three references, but with a random value for each of the three references after 1000h. I'm optimistic for the moment and say the value for long-term stability after 1000h is max. 20ppm, the typical given value of the datasheet for the LT1236LS8. The behaviour is a log expression and you can see that on page 8 of the LT1236LS8 datasheet.So what you see is a random start value for each reference between 4.9975V and 5.0025V plus a drift with random value after 1000h for each reference and log behaviour plus a random noise (I assume the integration time is 1s so I take the noise voltage of max. 625nVpp).This together gives a different walk for each reference.Got it up to here?

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 03:26:39 pm
content: The three plate methode is a limited example compared to the example we discuss in here.We don't won't to talk about the point where you have reached the "perfect surface" and than use the plates in real world apllication, but we talk about the way of manufacturing such a plate.It is known that you have to exchange the plates against each other while manufacturing them. You will get down to a certain deviation compared to an ideal flat surface, depending on several conditions. From this point on the deviation will again increase. You would stop at this point and sale the plates or bring them into application.Your plates are and this is different, correlated against each other, so this is the point where the examples reaches its limits.

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 03:53:42 pm
content: Attached is a set of data that contains time and the random inital value, random noise and random drift for three references.

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 05:25:12 pm
content: Over what time was that noise figure measured? In long term drift data, you see fluctuations in output voltage on the time scale of hours. You don't capture this in either your noise or long term drift data.Let's compare your results to the datasheet graph:Not a good fit to the supposed log expression.

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 05:59:32 pm
content: I have the feeling I repeat myself over and over and you don't understand. You will agree that you can't model absolut real world data, right?Don't deal with wrong notations, noise figure is something completely different! As visible and written I assumed to acquire one value per hour with an integration time of 1s, so the noise is within 625nVpp for each value.The fluctuations you mentioned in themeasureddata are common mode fluctuations, we are not able to find out what happened there.QuoteOkay, but then the additional text in the datasheet is wrong, wouldn't you agree? And notice that the shown graph is up to 2000h, while I do my calculations only up to 1000h at the moment, because specification for long-term stability is given at that time stamp.

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 06:37:15 pm
content: If there is real world data, then that is what you're trying to match in a simulation. Not individual fluctuations, but at least the power spectra should look similar. You first try to match real world data, and then simulate something you did not measure (or wouldn't be able to measure) and make claims based on that. If the simulations don't match any real world data, why would I believe that they are a good representation of reality?Some of the fluctuations are common mode, some are not (clearly visible in the ref5025 data). Some of the fluctuations may be due to environmental conditions, although the datasheet states that they were performed in an oven. Are you arguing that LT and TI did a worse job on their setup than you would, and no common mode fluctuations would exist in your setup?The way I interpret the datasheet is that the drift has a logarithmic component, i.e. it may be bounded by a logarithmic function. It does not mean there are no other components. In general you are only interested that your reference will be within X ppm if you calibrate it every Y hours (or you never calibrate it and want to know how bad it gets at the end of the service life). Their note indicates that you shouldn't just multiply the 1 kh drift figure by two to get the 2 kh drift figure: it will likely be much better.Now you can argue that this does not matter for your results, and maybe it doesn't. I would first try to get it working with a very simple model, and once you got that working worry about improving the results. I think that as long as you have some common and some normal mode changes (whether you call it noise or drift), you should have the essentials, and you can quantify in simulations how much your method reduces common mode and normal mode fluctuations.

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 06:52:44 pm
content: Sorry, but please don't come up with your Ref5025 when I want to make the experiment with LT1236 and all my ratings are related to its datasheet. And in this data I clearly see common mode fluctuations, where ever they are coming from. You really want to blame me for that?Seems you didn't understand the formulas. If you want to calculate the drift and assume a log behaviour as the datasheet of the LT1236LS8 states you would use the following expression:20ppm = log x (1000h) = ln(1000h) / ln(x)You than calculate x and are able to calculate how drift goes on beyond 1000h. This is simple math, logarithm laws. Got that?However, what expression the drift is about is as you already mentioned irrelavant for the experiment and so for the simulation, so don't stay at that single point, we can improve the model later, but we have to start from a certain point.

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 07:27:13 pm
content: Quote from: branadic on August 04, 2013, 06:52:44 pmNo, but I argue that whatever is causing that (in datasheets from multiple companies) is likely also going to be present in the setup you'll eventually use.Quote from: branadic on August 04, 2013, 06:52:44 pmIf you think being condescending is productive and helps you win arguments, then I wish you good luck with your project.

author: branadic
author_rank: Super Contributor
timestamp: August 04, 2013, 07:34:09 pm
content: QuoteAnd I doubt that you can compare measurements made by different companies. It's dubious that there is such a common mode behaviour in all their data (after all 80 parts), don't you agree?QuoteNo, that is not my intension, but I felt you didn't or didn't want to understand, so I was explaning the background.

author: Unknown
author_rank: Unknown
timestamp: August 04, 2013, 08:31:20 pm
content: Quote from: branadic on August 04, 2013, 07:34:09 pmIf multiple companies, who do essentially the same experiment (put a bunch of references in a board in an oven, connect to a DMM via a mux) get (qualitatively) similar results, then I would argue that the outcome of your experiment (put three references on a board in an oven, measure with mux and ADC) are likely to be similar. If it was only one company, then I might believe something being off in the setup, although I find it hard to believe that LT would publish data that would make their reference look worse in the datasheet.About your equation: I don't disagree with your equation, but with what the equation means. Does it model the exact output voltage (minus noise) of an individual reference, or an upper bound? If they quote a drift of 4 ppm / kH, that means the part will be within 4 ppm after 1 kH, not necessarily at V(0) * 1.000004. So if this 4 ppm/kH figure decreases as a function of time, it is the rate of change of the upper (and lower) bound of the output voltage that decreases.

author: Andreas
author_rank: Super Contributor
timestamp: August 04, 2013, 09:34:18 pm
content: Hello together,for the first: you cannot compare different references like the REF5025 in SO-8 or MSOP-8 packagewith a hermetically tight reference like the LT1236AILS8. They simply play in different worlds.The REF5025 is a bandgap reference whereas the LT1236 is a buried zener device.So the reference element behaves different. (also with respect to ageing).Ok ageing does not only depend on the reference elemet. The output voltage resistor divider and package related stress to the die also play a role. The most stress to the package can be assumed by the soldering process during production of the board.The common mode behavior on ageing curves of plastic packages I would blame on either humidity effects (swelling of the packages) or some other (mechanical) stress applied to all references on one pcb board.The ageing with x ppm / sqrt(khr) I could measure up to now only on a hermetically tight buried zener device. And it seems to be from my measurements that it is also only valid for large ageing drift values. Below a certain drift limit my measurements show more or less a "random walk" which seem to consist of some seasonal effects over one year and in my measurements additional thermal gradient effects of the ADC reference + perhaps some hysteresis effects.Attached some measurement data of 5 LT1236AILS8-5 buried zener references.First I have to explain what happens.I have several references running 24/7 within a thermal box. Each day the pcb is heated from around 35-40 °C (self heating temperature of the references at 20-25 degrees room temperature) to 50 +/- 0.2 degrees. After heating up all Gnd-Pins of the references are measured with my best ADC (there is a up to 60uV shift between them due to the supply currents) and after this all output pins are measured and the difference between output and gnd pin is recorded and the drift is calculated.The ADC#13 which is used together with a 2:1 LTC1043 based voltage divider finally measures his own offset and a high stable LTZ1000 reference. And also this drift value is recorded on the sheet. So if you would want to exclude the drift of the adc and the voltage divider you would have to subtract the pink ADC13 curve from the reference curves.Measurement starts at DAY89 when I put the 5 references into the temperature box.Reference #1 had already 2 temperature cycles from around 10 - 45 °C for TC measurement at this day. So #1 was already running for 1 week.For the other references I did the TC measurements after DAY89. Which can be seen as "shift" in their ageing curve.As I mentioned relatively large hysteresis shifts I tried if they could be removed by some "thermal handling" so at day 100 I put all 5 references into the freezer (-18 °C) and warmed them up with a hair dryer several times. At day 130 I repeated thermal handling with #1-#4 and #5 had its TC measurement. Some TC measurements where repeated. Last one was on #2 at day 137. Except on day 192 where I repeated TC measurement for reference #2.So "shi(f)t" happens as you can see on every "event" in life of the references. Every event starts a new ageing cycle. Either with a lower or a higher drift rate. The large drift rates after the events can be approximated by some sqrt-function. Whereas the final drifts after some thousand hours seem to drift randomly. (or perhaps my measurement system is not good engough).The 2nd picture shows the same beginning from day 138 (approximately after 1200 hours from the begin).Compared to other devices the LT1236 have larger hysteresis so it would be better not to heat them up every day but leave them at constant temperature. But since I'm still looking for the perfect non heated reference for battery supply with minimum power requirements I will not change my measurement conditions.With best regardsAndreas

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 05, 2013, 10:48:20 am
content: Quote from: Andreas on August 04, 2013, 09:34:18 pmI couldn't find an explicit description in your post of the values on your graph axes. Not being clairvoyant, I thought I would ask.It seems clear that the X-axis values in your graphs represent days. Are you taking one sample per day? Multiple samples per day? If multiple samples, how are you combining them?Are the Y-axis values in ppm? ppm / sqrt(khr)? Or what?

author: branadic
author_rank: Super Contributor
timestamp: August 05, 2013, 11:02:00 am
content: The question is related to Andreas, but I answer right nowx-axis represents time in hours.y-axis is the normalized value in ppm, normalized because they all start with 0ppm deviation at "day 0".

author: Unknown
author_rank: Unknown
timestamp: August 05, 2013, 11:57:14 am
content: What I think is nice in the LT1236LS8 datasheet graph is that they normalized the curves at t = 100 h. This skips the large initial drift, so the curves are closer together and it's easier to compare small changes between the curves.

author: Andreas
author_rank: Super Contributor
timestamp: August 05, 2013, 07:07:40 pm
content: Quote from: dfmischler on August 05, 2013, 10:48:20 amAs you can interpret from the description (DAY89) and from the general description on the plot ADC13 (ppm) X-axis is in days and y-axis is in ppm normalized to start of the measurement. So branadic got it right from the description.Usually I record 1 sample per day (integration time is 1 minute) the values are all as dot on the measurement connected with lines.Quote from: alm on August 05, 2013, 11:57:14 amIn the current data sheet it is normalized to t=10 h. In which version did you see the 100 h?And 10 h from what time after soldering. And what happened to the board after soldering before start of measurement? Perhaps seveal temperature cycles with decreasing amplitude around 35 degrees?What means the Therm: "Typical long-term drift is illustrated in Figure 1" in the datasheet? Really measured values or only drawn from mind?With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: August 06, 2013, 06:05:04 pm
content: I have asked Linear Technology for the data of their long-term stability test. Will be interesting what they will answer.It's sad that there's no standardized test condition and they don't say more than that the test was made using an 8.5 digit DVM in a "constant?" oven with T=35°C. What about the real temperature and humidity trend, what about the supply voltage while measurement? If everything was really constant than all the common mode behaviour could be due to the DVM and/or the scanner. I doubt that 80 exemplars have the same common mode behaviour. Everything is a bit nebulous.

author: Andreas
author_rank: Super Contributor
timestamp: August 30, 2013, 08:11:01 am
content: Quote from: branadic on August 06, 2013, 06:05:04 pmDid you get already a answer.Usually they reply within a few days.With best regardsAndreas

author: branadic
author_rank: Super Contributor
timestamp: September 02, 2013, 08:46:13 am
content: QuoteNo, I didn't, obviously they didn't care about my request.

author: babysitter
author_rank: Frequent Contributor
timestamp: September 03, 2013, 07:52:37 am
content: I guess they figured out that you are one of those precision voltage hobbyists from the eevblog, just with little value for their sales

author: branadic
author_rank: Super Contributor
timestamp: September 03, 2013, 04:08:44 pm
content: I can't believe that's the reason, as I settled my request with my offical account from work. And yes, I order LT parts by that account too.

author: rs20
author_rank: Super Contributor
timestamp: September 04, 2013, 01:43:58 am
content: Just a simple question, because we're getting massively sidetracked by trying to get data for specific parts. The point is, it's absurd to say that a group of three voltage references in close proximity will mystically communicate with each other and ensure that no common-mode-looking drift ever occurs. It's like saying that if you flip three coins, you'llneverget three heads because there's no systematic reason why theywouldall align.Put in more concrete terms: how are you going to tell the difference between:Case 1: refs A & B drift up by 10 ppm, ref C drifts down by 5 ppmandCase 2: refs A & B drift up by 5 ppm, ref C drifts down by 10 ppmIn both of these cases, A-B=0, B-C=15, C-A=-15, (A+B)/2-C=15, any relative computation you perform will be the same.Also, how are we five pages into this discussion without a working matlab/octave/excel/python/whatever model? I don't care how you model the drift of the voltage references. No need for part datasheets. The only way you'll get it to work, even theoretically, is if youcompute the drift of ref C by calculating it based on ref A and ref B. I.e., cheat. If all three refs are modelled with drift calculated from independent random drift, and you keep all your calculations relative, I suspect you're going to have a great deal of difficulty demonstrating this working even theoretically, beyond the sqrt(3) gain that you can get by just averaging the three references.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: May 09, 2017, 03:14:58 am
content: Found that I have been working on this idea independently and have started another thread on simulation of voltage references. I simulated the case of four references and measuring the differences.Using least squares equation solver and adding one or two sum constraints to the system of equations allows the system to be solved on each iteration and actual voltages to be precisely tracked (e.g within noise and measurement uncertainties).Java simulation here:https://github.com/RMcRee/ModelVoltageReferenceNor is my least squares idea as original as I had imagined. Here is a 1974 NIST paper talking about the same thing with gage blocks:http://emtoolbox.nist.gov/Publications/NBSIR74-587.asp

author: rs20
author_rank: Super Contributor
timestamp: May 09, 2017, 03:59:49 am
content: What do you mean by "is viable", "can be precisely tracked", etc? Where are your numbers, comparing the performance of your algorithm to, e.g.:a) A single uncorrected referenceb) Simply averaging the four references (i.e., a sqrt(4) = 2 attenuation)

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: May 10, 2017, 02:57:14 pm
content: Good questions....I am working on updating the simulation with answers. It may be a few days as life and work intervene.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: May 15, 2017, 05:11:09 am
content: I updated github with my latest simulations.It would seem that only if the range of drift and temperature is restricted can the method be useful.Long-term drift (I believe in the range of real voltage references) cause the LSQ solution to wander away from the actual values.So it is of limited utility.I certainly learned a lot. If for example, you have four accurate initial measurements and one reference (only) drifts it is possible to use differential measurements and LSQ to quantify that. But it is not realistic to have only one reference at a time drift.It is also easy to embed least squares solutions in a microcontroller see, for example, file QRDecomp44VRefs.