parrallel ltz1000 thought experiment (infinitiy +1) - Page 1
Forum section: electronics / general

author: CopperCone
author_rank: Super Contributor
timestamp: July 05, 2017, 11:15:35 pm
content: assuming you want to spend big money, how many of these can you parallel up before it starts being redundant or parasitics take over?assume crazy manufacturing practices, deadbugging/busbars/whatever stuff is completely not feasible to do for a commercial design.

author: alm
author_rank: Super Contributor
timestamp: July 06, 2017, 12:57:06 am
content: I would say that depends on what your figure of merit is: noise or stability?For noise, I think the limit would be when the output impedance goes to the same order of magnitude as the wiring resistance. So doubling the number of references would barely lower the output impedance, because all the extra wiring / bus bars / whatever.For stability, I think it would be when the correlated, systemic parts dominate the uncorrelated parts of the drift. Drift due to ambient temperature, pressure, humidity, vibrations, drafts, etc will not improve significantly by increasing the number of references, since the temperature / pressure / etc coefficients will probably be quite similar.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: July 06, 2017, 01:23:18 am
content: This has been discussed before. Based on my experience, typically 4 or 5 LTZ units paralleled is a reasonable practical limit of profitability before you get to diminishing returns - the more you go after that the noise from the averager system itself begins to become evident. You'll also find that you get a batch of LTZ's that tend to all drift the same direction (and with time & temperature), and maybe you haven't improved short / long term drift like you thought it might.In other words reality begins to overpower the math noise theory somewhere about there. You're welcome to to stack on more LTZs, knock yourself out. That will mostly help reduce your bank account for no good reason.Same thing for '399's. Try paralleling about 25 together and see if you've got 1/5 the typical noise of a single '399...You probably won't. Should've used an LTZ from the start anyway.The other way to do it is to have multiple independent LTZ + ADCsystemsall reading the same signal exactly simultaneously and throw some averaging math + statistics at the resulting datastreams. That can work for some very critical applications and science experiments. But then again, who's going to pay for that, really? Probably not many customers.The very best way to lower noise is to not generate it in the first place...and the second best way is to be careful of the application requirements, and keep those expectations reasonable. Of course you need to keep the final system within the measuring capabilities at hand also. If you can't measure the final result accurately, you might be wasting your time. The LTZ is right at the top of the food chain for stable references, and not much else better to compare it to.

author: cellularmitosis
author_rank: Unknown
timestamp: July 06, 2017, 06:15:49 am
content: Quote from: MisterDiodes on July 06, 2017, 01:23:18 amExcuse a bit of a thread-jack, but this gave me an idea for a related metrology problem.I've been mulling over the problem that once you have an LTZ, you can no longer measure it in isolation. If you measure an LTZ using a 3458A, you are just comparing two LTZ's against each other. When the measurement changes, you don't know which LTZ moved (and indeed, if the measurement doesn't change, they might have both moved in the same direction!)(see attachment 1)However, if you had three LTZ's and you arranged them in descending order of voltage, you could use three ADC's (say, LTC2400's) and take three measurements. Now you can infer more about how much each moved.(see attachment 2)You can extend this to more LTZ's and ADC's...(see attachment 3).Using this scheme, you could monitor drift very precisely, and every so often you send one LTZ out to be calibrated, giving your relative data a single fixed point to anchor to.(Of course, there's the unfortunate problem of scaling the LTZ into the LTC2400's input range. When oh when will Linear or ADI release a 24-bit ADC which can natively read an LTZ?)I think that's about as much certainty as you can get without owning a JJA.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2017, 03:49:16 pm
content: When it comes to noise, one often has the option to repeat measurements and average. So for really low level measurements it can be worth it, to have an automatic setup, that can run for a long time (e.g. over night or even a weekend).Averaging over a longer time is much more affordable than several 3458 or LTZ1000.With several LTZ1000 one comes to a point where other parts (e.g. amplifiers, ADC) of the system will usually limit the noise.Usually there is not even a need to exactly synchronize several meters if they are used in parallel.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: July 06, 2017, 04:21:32 pm
content: Once again, this discussion belongs in LTZ thread I think.Celluar: I don't think you can infer anything about the multi-step strategy. You still depend on one cal'd reference (never a good idea). Maybe. Think about what happens when all the Vrefs drift down about the same amount. I.e. you wake up one morning and they all drifted down by some similar amount, say 3 ppm...How would you know? Don't depend on one Vref drifting while you assume the others stay stable...it's funny how that doesn't happen a lot; you'll see that effect when you actually build a bunch of these.You can try averaging data all you want for as long as you want on a single system - and that does really nothing for true absolute accuracy.PLUS: as you hinted at - it is possible (but not trivial) to boost or attenuate the voltage of an LTZ without any (or very low) drift. That task in itself will always add drift & noise to the system - and that's a big problem.There isn't any good low-ppm way to derive good drift detection / drift-less self-calibration without sending outmultiplereferences for cal to one landmark reference, and keeping accurate records of past drift rate and estimating current absolute value, and predicting the next cal adjustment. Otherwise you're in your own world trying to play math with the data, and you're just "playing catch with a handful of sand" while you drift around the endless desert of "unknown accurate value".In fact, if you have a good past history of drift rate trend of an older Vref, and you send it in for cal and your prediction was way, way off: that's usually the first early sign that you might have a Vref problem. That's why you keep all your Z540 cal records intact for every cal'd Vref you own, and pay attention to where the trouble spots are.A third precision measurement approach and related to "Multiple Independent Systems" is you expand that to "Multiple Independent Systems & Techniques". For instance you have a farm of LTZ's, 732a, 732b, etc. and you measure with not only with several 3458a's but with KVD / Null meter methods to make intercomparisons. And we're talking at least one real null meter in the mix, not a noisy DVM or ADC - and that will help reveal the truth and put a fence around the real actual absolute voltage values with higher accuracy. In other words you can reduce uncertainty / increase confidence by looking at the measured signal value from several different points of view. You have to do this when chasing after low PPM's...no free ride I'm afraid.