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.
How 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
Maybe lets get more practical. What relay would you suggest? A classical relay with gold plated contacts or one of those reed relais?
The suggested simulation was for three uncorrelated random processes. Unless you question the quality of MATLAB's random generator for this purpose.
I 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?
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.
With reed relays you will have a reliable contact over time but the wires will be most probably be of Kovar having
a 40uV/K thermocouple voltage against copper. So a good thermal design of the solder junction areas will be necessary.
Meder offers some reed relays with a thermocoupling voltage <1 µV/K.
With reed relays you will have a reliable contact over time but the wires will be most probably be of Kovar having
a 40uV/K thermocouple voltage against copper. So a good thermal design of the solder junction areas will be necessary.
Meder offers some reed relays with a thermocoupling voltage <1 µV/K.
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.
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.
I just want to share and discuss an idea with you.
Lets assume we have:
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).
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,
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.
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 really estimate their absolute drifts.
Of course it's possible to determine outliers, i.e. references with much bigger drift rates than the others..
Dr. Frank goes on to say:
That's the classical metrological problem, to determine the stability of standards, if there is nothing "better" than those, or:
Conclusion: There is no free lunch.
Otherwise: If your perpetuum idea would work, others would have realized it already, wouldn't they?
Frank
The criticism that branadic's approach is a perpetuum mobile because of its circularity is a bit to simple.
But others have realized 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.
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.
The criticism that branadic's approach is a perpetuum mobile because of its circularity is a bit to simple.
Sorry, 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.
Sorry: 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.
A 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.)
Frank
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.
- 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.
- 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.You 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.
...
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.
...
...
That sentence is ambiguous in my opinion.
...
(for identical drift, hence my ideal case statement)
...
@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.
close all;
clear all;
% t=1000h
t=0:1:1000;
% Electrical Characteristics LT1236
% Output Voltage [V]
Vout_min=4.9975;
Vout_typ=5;
Vout_max=5.0025;
% Long-Term Stability
LTS=20e-6;
% typ. Output Voltage Noise of LT1236LS8 [V]
Vpp=3e-6;
% Random Initial Output Voltage for each reference
Vout1_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 reference
LTS1=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 reference
noise1=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 time
for 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);
end
Vout1=Vout1_ini+Vdrift1+noise1;
Vout2=Vout2_ini+Vdrift2+noise2;
Vout3=Vout3_ini+Vdrift3+noise3;
% Data Acquisition with calibrated ADC
in1_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);
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 the LTZ1000 datasheet (note that this is voltage over time, not dV/dt over time).
- No ADC error or noise?