Absolute Divider Concept - Page 1
Forum section: electronics / general

author: Lesolee
author_rank: Regular Contributor
timestamp: February 17, 2020, 07:42:12 pm
content: I have been recently disappointed by the technical merits of the Fluke 752A reference Divider, as highlighted by the excellent analysis work done by e61_phil (https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/).Then there is Conrad Hoffman’s budget Kelvin-Varley divider (http://conradhoffman.com/mini_metro_lab.html) at the other end of the financial scale (from 1996)It got me thinking, could I do good Absolute Divider at a pretty low cost? I have no use for such a beast, and don’t have any kit to use it with, but just as a design exercise, and secondarily as a tutorial for others. There are lots of skilled specialists on this site, many of whom are not reticent about pointing out errors, so anything said here is automatically checked --- which is good for the purposes of a peer-reviewed tutorial!Conrad selected his resistors from 1% metal film resistors. I suspect that the relative cost of tighter tolerance components has come down considerably in the last 24 years. And a 0.1% resistor that you buy is not the same as a 0.1% resistor selected from a whole bunch of 1% resistors. The load life stabilities of the two beasts are not the same thing.My first choice is the prime divider resistors, picked from a local distributer’s range.https://uk.farnell.com/vishay-foil-resistors/y145310k0000t9l/res-10k-0-01-600mw-radial/dp/2820505They are not “cheap” at £12.74 (10 off price) but we have to make a box, and put on binding posts, and so forth, so it is a reasonable expense for the most critical part of the whole design.And the specs are just AWESOME. Voltage coefficient < 0.1 ppm/volt. Power coefficient of resistance less than 5 ppm at full rated power. Those numbers just scream “make an attenuator out of me”. The 0.2 ppm/°C spec quoted by the distributer is pretty dodgy. The data sheet says 0.2 ppm/°C typical, but 1.8 ppm/°C max. But when you dig into the curves, the TC is second order so the equivalent linear TC drops according to the ambient range. 0.2 ppm/°C apparently drops to 0.05 ppm/°C over the restricted range 25°C ±25°C. It makes me think that over the ±5°C required lab range, 0.2 ppm/°C is a pretty realistic expectation.The power coefficient of resistance is a number you don’t usually see. What it means is power heats up the resistive element, and the resistance changes due to its own TC. Normally you have to work this out for yourself. They have done the work for you. At 25°C the rated power is 250mW, which for a 10K resistor is 50V. I want to run the attenuator at up to 100V, so that is 10V per stage.(10/50)² = 0.04. It means the self-heating error is expected to be 0.04 x 5 ppm = 0.2 ppm. The voltage coefficient is a different effect and at 10V per resistor we get 1ppm. But this effect should match between near identical resistors run at the same voltage, so it should not be much of a worry.I have 0.01% resistors and I need to adjust them all up to a common value. There is no point in putting in a series resistor at 0.01% of 10K (10 ohms) because we don’t need the 10K to be an absolute value. 0.02% is a bit tight on adjustment range, but should be ok. It means we can use a shunt 1M resistor. Again a 0.1% has been chosen for better long term stability, but also a 5ppm/°C spec was needed. The shunt resistor is 100x larger so it contributes TC to the overall resistance by a factor of 1/100. It means a 15ppm/°C part would contribute 0.15ppm/°C, which is pretty high compared to our 0.2ppm/°C expected value. I would therefore prefer a 5ppm/°C part.https://uk.farnell.com/vishay/ptf651m0000bzek/res-1m-0-10-250mw-axial-metal/dp/1703752.Next we have a trimmer. The sensitivity to the trimmer is a factor of 5000 down, so the 100ppm/°C becomes 0.02 pm/°C and can be neglected. The stability and setability are another matter. The 0.02% adjustment range is 200 ppm. We would like to set the attenuator to better than 1ppm, and preferably better than 0.1 ppm. This is too tight for a single turn pot. To get better stability, instead of using a track that is the same size as a single turn pot, the huge linear track has been chosen.https://uk.farnell.com/bourns/3006p-1-203lf/trimmer-15-turn-20k/dp/9352341The binding posts are a key feature of the design, but I don’t have any practical experience of choosing them for such an application. Farnell has 167 to choose from! Some of the voltage ratings are really low, but we want lots of insulation on these posts, so I would pick a voltage rating above 1kV to get good insulation. That’s down to 46 types. Now we pick gold plated contacts for 18 results.For this distributor the choice would seem to be a POMONA 3750, since it is available in several colours. At £7.35 (with no price breaks) that is quite expensive, but the last thing you need is a broken terminal on a standard you want to use for 20 years.https://uk.farnell.com/pomona/3750-2/binding-post-15a-turret-red/dp/2406404. Probably worth shopping around though. (EDIT: Use gold plated copper for lower thermal EMF. See post bysplinbelow.)The panel on which the binding posts are mounted is potentially the most expensive part of the whole design. It should not be metal because then it cannot be guarded for leakage (Although the metal is good as a shield of course). The problem here is that you can clean the panel, and measure the insulation resistance when nothing is fitted. As soon as you fit the parts you can no longer measure the leakage current. As dust and grime build-up over the years, you have no idea how badly they are affecting the performance. Hence guarding the posts reduces the error by a factor of 500, hopefully reducing the worry.One idea is to use a plain plastic panel and paint on guard tracks on both sides with conductive silver paint. It’s just a bit – nasty. Another idea is to use a double sided copper clad laminate board and use a Dremel to mill out gaps so you can guard large copper lands. But now we have exposed laminate which will allow moisture ingress, and it looks a bit nasty again. Finally we have a custom double sided pcb with guard tracks built in. Clean and nice, with solder resist all over, except on the guard tracks. But the cost is pretty high. Any other ideas?EDIT:It took a couple of goes for the comments from Conrad and Kleinstein to sink in. The adjustment of one resistor set affects the voltage across all other resistor sets horribly. For example a 0.01% resistance change in one, changes the current in the chain by 0.001%, and therefore all other voltage stages by the same amount. Given that each null-to-1V step takes some significant time, this "design" is pretty nasty. I really ought to delete it to "save face". On the other hand one does need to admit errors occasionally.EDIT: Up-issued PDF to highlight the nasty errors and delete the author.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 17, 2020, 09:01:00 pm
content: Interesting, but not cheap! I have to wonder if all the attention to low leakage is warranted. Not because it isn't needed, but because of the small size of the resistor bodies. I'd think they're the weak link in the chain, not the generous amount of insulation material on the binding posts. A metal panel could be used, though I've seen contamination over the years that has to be avoided. I like a metal chassis with a floating metal guard chassis inside. I don't fully understand the calibration method and how the interaction gets removed, though you've obviously got it figured out.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 17, 2020, 10:47:39 pm
content: Quote from: Conrad Hoffman on February 17, 2020, 09:01:00 pmSo a small resistor body can leak across itself, changing the resistance of the resistor. But leakage to ground is quite different. The resistors are inside the box and therefore not as subject to the debris floating around in the air. In principle we can calibrate out the leakage across the resistor body. The leakage to ground is more problematic as it affects the attenuator accuracy after we think it is all set up correctly.Quote from: Conrad Hoffman on February 17, 2020, 09:01:00 pmNot necessarily.I may have missed something obvious which you have spotted.My simplistic notion is that you measure and calibrate each output pair individually to 1V (for example). The floating null-to-reference box is entirely isolated from ground so theoretically calibrating one pair has no effect on the others. Unlike you, I am an "armchair metrologist" these days, and I have never built such a device. My Datron 1051 (which I made from manufacturing reject parts when I worked there) died many years agoIt is therefore entirely possible that I have made some rookie mistake in my thinking! I await correction.BTW: I enjoyed reading your Mini Metrology Series the other day, although I admit to downloading them as JPGs, then converting the whole lot to a PDF before reading, as that makes it easier scroll on the screen.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 18, 2020, 12:05:13 am
content: If there's a calibration issue, it probably goes away when you do multiple passes. How many for perfection? No idea. Those resistors are about $17 each, depending on the exchange rate. No idea from local vendors but probably not much different.It's been a long time since I got permission to post the Mini-Metrology article but I think there was some requirement that I keep it in the form of jpgs for each page. Every time I count the years back to that article I go into shock. It was originally proofed by Jim Williams. It seems whatever I do in electronics, I find his footsteps ahead of mine. He's one of the people in this business I miss the most.

author: splin
author_rank: Frequent Contributor
timestamp: February 18, 2020, 01:21:51 am
content: Quote from: Lesolee on February 17, 2020, 07:42:12 pmA pretty expensive piece of kit you have no use for - are you intending to actually build it or is this just a thought exercise? As a design exercise it would be seriously compromised if you don't have any means to test its performance, otherwise it could just as well be an exercise in how not to go about designing a high precision piece of test kit. Without reliable test results, I'm afraid neither you, or us, would learn a great deal.QuoteWhy not? If they have the same part number I'd suggest that it is extremely likely that 1% and .1% parts are made on the same production line using the same processes and equipment. After 100% testing the 1% parts may be those that fail to meet the .1% tolerance.Of course it is possible that, when producing higher grade parts, the same production process may be operated slightly differently with more rigorous control - eg. operating more slowly, controlling temperature more closely etc. I'd think that unlikely however. Testing time is an expensive commodity so I'd expect a 1% manufacturing run (if such exists) would simply test for 1% limits. The tester would be capable of rather better accuracy but to do so would likely have to operate more slowly to allow more settling time.If they are made the same way their load life stability should not be any different.QuoteThe starting place for selecting the resistors is surely to specify the requirments? Eg. What short stability (time and temperature) is required of each resistor, to enable the overall specification (not stated) to be met? From that you can search for suitable resistors that would enable you to meet your goal - assuming they actually exist that is.Choosing decent components, at prices that you believe to be acceptable is really the wrong way of going about it. Having said that, as you haven't stated a target spec you can of course attempt to calculate the performance of the kit using the selected parts and decide then if it is acceptable.It has been stated many times here that you have to treat Vishay foil resistor specs with a certain amount of skepticism, especially with respect to the temperature coefficient specs. They are good resistors, but I suggest you search these forums thoroughly before spending a lot of money on foil resistors. Andreas has done a lot of resistor testing so do look for and read his thread.QuoteThat is a gold plated brass part which will have rather high thermal EMFs which could contribute significant errors. Much better would be the 3770 gold plated copper parts; they aren't much more expensive.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 18, 2020, 08:07:01 am
content: Quote from: splin on February 18, 2020, 01:21:51 amIt's a thought exercise. The "interesting" thing about an absolute divider is that it is "too good" to calibrate. You will note in Hamon's original 1954 article it was better than the available standards. So you have to then compare against what is avaialble to build up confidence. Obviously you could just send it out for calibration, but that would cost quite a lot. Only practical for a commercial design.Quote from: splin on February 18, 2020, 01:21:51 amRight, but if you look at the load life stability of a typical 1% resistor it will probably be 1%. The load life stability of (good) 0.1% resistors should be better than 0.2% or there is little point. They should not be of the same design. That's the whole reason to buy precision resistors. Otherwise you could just calibrate out the error.Quote from: splin on February 18, 2020, 01:21:51 amI agree for a commerical design. For a hobby project the starting point can be "what can I afford". Clearly I have in mind the order of magnitude of acceptable error. It can be adjusted to the range around 1ppm. Would it reach such a spec when thermals are taken into account? That's more difficult to assert for a paper exercise.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 18, 2020, 08:15:31 am
content: Quote from: splin on February 18, 2020, 01:21:51 amThat's a good catch. Thanks.From National Instruments:

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 18, 2020, 01:14:01 pm
content: And that's the reason you wantcleancopper!

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2020, 01:36:11 pm
content: It takes a lot of effort to adjust the individual stages. One point is that for the initial run there is some interaction and it may need 2 or 3 adjustments each. But this is still the easy part. Even with good resistors the stability will not be much better than 1 ppm over a longer time. So it would need an adjustment before each use. This needs a stable 10 V and 1 V source. As the 1 V source is floating this would need a separate low noise, low drift reference (e.g. Fluke 734 class). Even than there is the noise of the 2 references that may limit the accuracy and speed of the adjustment.In comparison the Hamon type divider is far less critical. The 752 may not be best implementation, but the idea behind it is superior. It does not need 2 high grade reference sources for the adjustment, but just the null meter. In addition the adjustment before use is much faster (only 2 or 3 points instead of 10).With some extra current sharing resistors one could reduce the effect of the switch resistance quite a bit. I don't know for sure the 752 includes this, but between the lines it looks like it does. So the 752 may be better than the simplified picture. This may need some additional adjustment, but this is not as critical and could be a once at production point. One could argue this may need a check / calibration from time to time (e.g. every 10 years or so).There is no need for clean copper - a thin gold layer would not have much temperature difference across and thus not much thermal EMF. However something like brass (for the whole terminal) can be a bad idea.

author: splin
author_rank: Frequent Contributor
timestamp: February 18, 2020, 05:27:52 pm
content: Quote from: Lesolee on February 18, 2020, 08:07:01 amI think you are misunderstanding the resistor specs. For example, look at these NIC components NTR series precision Nicr thin film resistor specs:Yes each specification is tighter for Tol <= .05% than for higher tolerance, but note that the typical values, for tol > .05% parts all comfortably meet the <.05% specs. That's not surprising as the lower tolerance parts, being the same parts made using exactly the same processes will have much the same characteristics as the high tolerance parts.All it means (I believe) is that higher QC levels will be applied to the high spec parts to ensure they will meet the better specs. Obviously most of those specs are not for individual resistors but will be batch tested - eg. load stability.Of course the lower tol parts won't be guaranteed to meet all the specs of the high tolerance parts but I'd expect the vast majority will do so - unless someone has good experince to the contrary. It is of course quite possible that batches that don't meet one or more of the <=.05% tol specs may be sold as > .05% tol parts (even if they are actually <= .05%) but I doubt that happens often.The production process will, IMO, be set up to ensure they can produce parts such that the vast majority will meet the highest specifications, selecting only for the most variable characteristics which are the resistor tolerance and temperature coefficient. Note the most interesting aspect of the attached specs are how the typical specs vary for the different resistor sizes. This isn't something I've seen in a resistor datasheet datasheet before and some of the numbers are surprising (to me at least) - especially that the 0805 numbers are rarely the best and often worse than for 0402 and 0603.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 18, 2020, 08:57:38 pm
content: Quote from: Kleinstein on February 18, 2020, 01:36:11 pmConrad mentionedinteraction, but without specifics. I was too thick to catch-onI was thinking 0.01% resistance change would have almost no effect on the current. But each stage I increase by 0.01% increases the current by 0.001%, and that increased current directly affects all other stages without any desensitisation. It ideally needs to be calibrated from a constant current source!I think that constitutes a gotcha.Well spotted guys.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 18, 2020, 09:20:32 pm
content: I didn't work it out exactly, just gut feel. I still think that some finite number of calibration passes would take care of it.I once had a boss who didn't have a clue about technical details. In spite of that, he'd walk into the lab where we were having a problem and point to something and say, "what about that?" More than half the time he was pointing right at the problem.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2020, 09:36:25 pm
content: The interaction between the resistors in the dividers should not be that bad. A 1 % change in one resistor would require to change the others by some 0.1% . So after one run of adjustments expect the settings to be better by maybe a factor of 5 or 10. So it is a finite number of adjustments needed. So maybe change every pot 2 or 3 times if the initial error is not so large.With some extra math (e.g. only change the voltage by 90% of what is needed to be spot on, things could get even better).The real weak point is that one needs 2 very stable references. So the divider alone is only a small part: one also needs those 2 references (possibly better than an LTZ1000), including one with a divider down to 1 V. One would also need to adjust that ratio to work with a conventional null meter. So the shown divider would be only a small part of the solution.The more practical way would be to adjust the resistors to be the same as the neighbor, with a resistor bridge, that one would move around. One would still need to include those extra resistors to compensate for contact resistance.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 19, 2020, 04:57:04 pm
content: Interesting thread. I missed it until now.The question ist, what do you want to achieve? I'm not a fan of adjusting something, if measurement is enough. Therefore, I often used my SR1010-1k to verify my 1V against 10V. The resistors in the SR1010-1k are very equal. Equal enough to not suffer from INL issues of the DMM. To verify the 1V range of my DMM (34401A or 3458A for example) I applied 10V to 10 resistors in series and measured the voltages on each resistor. The sum of all voltages must be equal to the sourced 10V (sourced from the calibrator (Fluke 5440B) with 4 wire connection). As long as the DNL of the meter, the input current and the leakage to ground is ok, it is an easy method to make a 10:1 transfer. With 10x 10k resistors it should also work for a 100V to 10V transfer.It is still a lot of work and one have to be careful to not heat the connectors too much by touching it with the fingers, but nothing to adjust.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 19, 2020, 05:45:08 pm
content: Quote from: e61_phil on February 19, 2020, 04:57:04 pmAs compared to the rest of us, who (apparently) just like to fiddle with pots.Quote from: e61_phil on February 19, 2020, 04:57:04 pmI have attached the datasheet so people don't have to hunt around for it. A very nice piece of kit.I see they have not (leakage) guarded anything because they claim >1E12 leakage. But in this case it is nice because you can just apply 100V to the whole chain, and provided you measure less than 1mV into a 10M ohm input DVM that 1E12 is then proven at any point in the future. It's something you can't do with a complete instrument like the 752A, with all its switches and leakage paths."Each transfer standard contains twelve equal value precision resistors connected in series by specially designed true 4-terminal junctions. These special junctions assure that a 4-terminal measurement of a series of resistors agrees with the sums of the individual resistors in the series."The Guildline 9350 also mentions tetrahedral junctions, with a paper by Chester Page, but there are no detailed descriptions or pictures. I need the hobbyist's guide to making your own tetrahedral junction. Sounds like a job for Conrad.I like the SR1010-1K, but how much are they (rough estimate)? I am guessing £3k.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 19, 2020, 06:07:53 pm
content: I think you don't need a tetrajunction between the resistors. Just make sure you have a proper connection to the series connection of the resistors. I think it should be enough to build your box without all the trimmers. Better TC than the SR1010 would also be nice.There are also some papers about a step-method. In that method you use a (known) 10V source which is switchable (0V and 10V), a calibrator and a DMM with good DNL. In the end you add up your 10V source n times. The nice thing here is, you can run it fully automated.https://ieeexplore.ieee.org/document/6399596

author: Lesolee
author_rank: Regular Contributor
timestamp: February 19, 2020, 06:49:51 pm
content: Quote from: e61_phil on February 19, 2020, 06:07:53 pmThe basic resistors are 0.01% (100ppm). I notionally wanted 1ppm. Adjustment is essential, especially as the main resistors will drift by more than a few ppm over time.In theory, with a good enough bit of kit, each stage could be measured, and the values typed into a suitable (clever) piece of software so each stage only needed to be adjusted once. But that somewhat defeats the idea of having a simple easily-calibrated absolute divider.>I think you don't need a tetrajunction between the resistors.Agreed. That was for other applications. Such a divider as this, automatically has 4-wire sensing, since there is no current in the sense leads.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2020, 06:58:57 pm
content: A tra junction is not that complicated and can be be done DIY. There are several possible solutions, two simple ones use a equal sided triangle or a small disc of plate copper with contacts at the 3 corners or equally spaced at the edge. The 4 th contact would be in the center of the disc or triangle. If needed on can check the 4 wire resistance and do some trimming of required.Exactly equal resistors would be nice and easy to use, but in times of pocket-calculators it would not be so bad if if one would get an accurate divider that is slightly off integer ratio. An accurate 1:10 ration is nice, but a stable 1:9.9 is nearly as good.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 19, 2020, 09:19:55 pm
content: Quote from: Lesolee on February 19, 2020, 06:49:51 pmThe idea is to not adjust the resistors at all. They should be closely matched to give very close readings on the DVM to not suffer from INL issues (100ppm should be fine).Just do it like this:Apply your known 10V. Switch your meter into the 1V range. At this stage the calibration of the meter doesn't matter and the absolute values of the resistors also doesn't matter.Now, you start measuring the voltages above all resistor. All the readings are close to each other (~1V). In the end you sum up all the readings. The error of the sum regarding to your known 10V is the error of the meter. At this point you have derived everything what is interesting. You know the exact gain of the 1V range of the meter and you also know the ratio of your divider. That works without adjustement and a 6.5 digit meter is sufficient for roughly 1ppm of output. You don't need a special meter. Just DNL and isolation should be good.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 20, 2020, 07:58:03 am
content: Quote from: Lesolee on February 19, 2020, 05:45:08 pmQuoteebay 300$

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 20, 2020, 02:13:05 pm
content: Been there, done that. Just some copper plate and a hacksaw. There should be another thread here from way back with some simulations using FEMM for current flow. Ahhh, here...https://www.eevblog.com/forum/metrology/zero-ohm-diy-4-wire-standard/msg1079328/#msg1079328

author: splin
author_rank: Frequent Contributor
timestamp: February 20, 2020, 05:44:43 pm
content: You might like to take a look at this thread:https://www.eevblog.com/forum/testgear/dmm-linearity/msg698554/#msg698554Do read (or at least speed read) the paper linked in reply #3Then read this thread which links to the above, especially zlymex's responses starting at reply #18:https://www.eevblog.com/forum/metrology/which-is-the-next-best-ratio-measurement-device-after-the-3458a/?nowap

author: Lesolee
author_rank: Regular Contributor
timestamp: February 20, 2020, 08:41:00 pm
content: Phew! There is so much to read. I have been clicking on the links, that lead to links, ... It's hard to know where I've been.There's a wealth of expertise on this site, and lots of generous contributions of photos andmeasurements.Much appreciated

author: dietert1
author_rank: Super Contributor
timestamp: February 21, 2020, 08:34:41 am
content: The document of Jonas Wissting (Wiss) explains very well the mathematics that you need to arrive at better accuracy than each single resistor in the divider has. I think Andreas used a similar method with his ADCs.One thing that i would recommend, though: Add some extra resistors to have roughly the same source impedance on each node of the divider. Otherwise there may be minor errors due by the small current the DMM input takes. The middle of the divider chain determines the minimum source impedance.Regards, Dieter

author: Lesolee
author_rank: Regular Contributor
timestamp: February 21, 2020, 09:02:46 am
content: Quote from: dietert1 on February 21, 2020, 08:34:41 amMy divider already had impedance balancing resistors. The difficulty is that the SR1010 allows 4-wire measurements of each individual resistor, which makes setting it up as a divider much easier.I also note that there is an adjustable SR1010 with 100ppm adjustment, almost exactly like my effort.Source balancing resistors and 4-wire setup of the individual resistors are mutually exclusive, since the balance resistors are so large compared to the divider resistors. The required I+ compliance of a 4-wire ohms measurement would be prohibitive (and also another error source).I downloaded the Wiss paper, but I have (unusually) some pressing domestic issues to resolve.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 21, 2020, 09:25:15 am
content: I still don't understand why do want to go through all the effort and introduce many sources of errors only to have a round number in the end.What is the benefit from having a 1:10 divider instead of a 1:9.983478 divider?For a simple divider it should be sufficient to have 4W connectors on the ends. In between you only need voltage connections.Another problem is driving the divider. If you have a source with 4W output it is easy, but if you only have a reference with 2W output you have to compensate for the connections (if you want to compare the output of the divider directly against the reference).

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2020, 10:53:51 am
content: If one wants to do a high accuracy measurement of the resistant (and not the voltage), having 4 wire connection also for the single resistors is a good thing. Using the adjacent resistors to drive is likely to high an extra impedance.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 22, 2020, 12:28:57 pm
content: Quote from: e61_phil on February 21, 2020, 09:25:15 amThat's easy. If I don't have a well-calibrated long scale DVM to measure the individual stages (but only a null-to-1Vish transfer box) I can calibrate them all to a common value and guarantee the linearity and the division ratio. Sadly that idea doesn't work with my current setup, as the individual stages can't be independently adjusted, they have to be iteratively adjusted (which is tiresome).With a second hand SR1010 costing $300, that is a pretty hard act to follow in terms of getting something both cheaper and better!It is not necessary to have a calibrated 4-wire calibrator to check the linearity of a long scale DVM. The source merely needs to be stable (which is already a requirement to set up the stages.) Then we can do linearity checking at multiple fixed points. For example on the 10V range we can check at 1V, 2V, 3V, 4V, 5V, 6V, 7, 8V, 9V, 10V and any in between sets of points 1.2V, 2.4V, 3.6V, 4.8V, ... You obviously can't check it completely as that would take forever on a 7 digit DVM.Using a long scale uncalibrated DMM on ohms, each stage can be calibrated to some fixed value, provided each stage has a 4-wire link to the resistor (tetra junction). Verifying a DMM against itself is a nice concept. Since the ohms will be less accurate than the volts on a typical DMM, the linearity check will be limited.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 22, 2020, 03:34:50 pm
content: How about you calibrate the divider with a bridge. All you need are three resistors (the 4th is the DUT), a battery and a sufficiently sensitive meter for a null detector. No iteration, just set each section to a specific (identical) value. For the time it will take, stability shouldn't be a problem, even with 1% metals. If the divider resistors are of reasonably high value you really don't need to worry about 4-wire connections and tetrajunctionators, though an interocitor could be handy.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 22, 2020, 03:56:26 pm
content: Quote from: Conrad Hoffman on February 22, 2020, 03:34:50 pmEffectively the divider is 4-wire sensed in as much as the current is supplied via the chain and sensed via the terminals. Measuring through the sense wire gives a badly defined error because thefinesense leads will have significant resistance, even at 10K stage resistance. I also put impedance balancing resistors in the circuit, which would need to be shorted out for the balancing operation. (I would want to use fine sense leads so there is no mechanical motion on the main divider resistors when screwing or plugging in to the output terminals.Quote from: Conrad Hoffman on February 22, 2020, 03:34:50 pmI have heard bad things can happen when you build interociters. Just hearsay of course

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 22, 2020, 06:48:26 pm
content: If you build it physically the same for each step, the wire resistance will cancel out when you calibrate. Also, why use fine wire? Look inside old GR bridges- they interconnected with very heavy tinned copper bus wire.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 22, 2020, 11:04:53 pm
content: Quote from: Lesolee on February 22, 2020, 12:28:57 pmThere is no need for a calibrated DMM. You just need good DNL, nothing more (for voltage). Just a chain of resistors with taps between every resistor. That's it.You could also use such an divider (without trimming) for INL tests of the DMM. I did that in the past and it correlated very well with 3458A comparisions.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 24, 2020, 08:23:59 pm
content: Quote from: splin on February 20, 2020, 05:44:43 pmQuote from: dietert1 on February 21, 2020, 08:34:41 amSo two of you think this WISS paper is good. Probably Affine Operators and Moore-Penrose pseudo inverse matrices are easy for you. Not for me.After reading through the whole paper I had no idea what he was doing. He did a test with 6 resistors in series. 6 resistors with unknown nominal values, unknown TCs, unknown tolerances. All we are told is that they were “lab-grade”. Is that a thing? Don’t cal labs usually use huge 4-terminal resistors in oil baths?So having got to the end I had to go back and “reverse engineer” what it was he was actually doing. So he is measuring each resistor individually, then pairs, then triplets, and so on.The claim is made that the resistors do not need to be of high absolute accuracy, which he has ‘proved’ by using resistors accurate to 0.5ppm relative to the group. There is no evidence or mention of checking for drift during any particular voltage setting, say the 1-resistor voltage set.There is no mention of possible problems with the different source impedances as you move up the chain, or where the DVM guard shield is connected.I suspect that it has been expressed in an overcomplicated way in order to impress the examiners.Nevertheless he has achieved some impressive results, using some impressive test equipment.One can only hope that these results were not “cherry picked” out of less good sets of results.

author: Gyro
author_rank: Super Contributor
timestamp: February 24, 2020, 09:42:11 pm
content: Quote from: Lesolee on February 22, 2020, 03:56:26 pmI've often wondered about that bead capacitor that got delivered to him in 'This Island Earth'. An MLCC using an exotic dielectric, or some miraculously resilient Tantalum bead?(Sorry, ot).

author: Lesolee
author_rank: Regular Contributor
timestamp: February 25, 2020, 07:54:06 am
content: Quote from: e61_phil on February 19, 2020, 09:19:55 pmMeasurement Request:So this represents a good opportunity for a test by volt-nuts with kit. Put 10V on the divider chain, with the DVM reading around 1V half way up the chain (maximum source impedance).Report DVM type, resistor values in the chain, type of chain (SR1010-10K, etc),change of reading when switching from local guard to remote guard referenced to the bottom of the divider chain.It may be that the change is just more noise, but that is still a valuable measured result to report.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2020, 09:01:12 am
content: Quote from: Lesolee on February 24, 2020, 08:23:59 pmI had a look at the text from WISS: There are a few odd points in the math and way looking at the data. So I would not call this a really good one. For some reasons it does not include measurements with a short and also the effect of the source impedance that can possibly cause trouble. However there is some sens in the method. However a linearity test is a little different from making an absolute divider. The linearity test only needs a stack of voltages - this don't have to be a resistor chain: it could be a stack of references or a resistor chain with individual buffers. Buffer offset and bias would be just part of the voltage sources and thus not directly a problem.The usual DMMs have input currents of some 10-50pA, so with a 10 K source impedance this can be errors in the 100-500 nV. The common mode input current tends to be lower, so less effect is expected from where one set the guard level.

author: dietert1
author_rank: Super Contributor
timestamp: February 25, 2020, 09:06:33 am
content: Yes, the study Wiss made is not complete concerning the divider source impedances. He wrote that one can use any kind of voltage generator, even a chain of voltage references. I think if you add source impedance matching resistors and if you are using a lab grade DMM with input impedance in the hundreds of MegOhms, the deviations caused by DMM input current will be negligible.As far as i understand the idea behind the method is the multitude of measurements. If you use six resistors and feed 6 V, you can start by measuring the DMM zero and scale (full 6 V). These two won't have an unknown linearity error. Then you can 6 times measure 1V, 5 times the 2 V difference and so on. So you will get 20 measurements. You don't care whether you determined nonlinearity at 1V or at 1.00001 V, so you don't care for the precision of of each resistor. From the 20 measurements you want to extract- the 6 unknown resistor values- minus 1 since you don't care for the resistor scale/divider current- plus 5 nonlinearities at 1 to 5 V.If the resistors and the voltage reference and your DMM are sufficiently stable during the measurements, you will have 10 unknowns and 20 measurements.If you don't want to use the same calculus as Wiss, maybe you can try to get the best fit with EXCEL solver. I haven't done it, but i will try.Regards, Dieter

author: Lesolee
author_rank: Regular Contributor
timestamp: February 25, 2020, 09:35:47 am
content: Don't expect this method to work for all 2-element chains, however.See next post (work in progress): Corrected image and pdf to add a missing "R" in the sum of R's equation.[EDIT2:]: Corrected typo in the body of the post, and changed from "2-element chains" to "all 2-element chains" since non-overlapping 2-element chains are also minimum error configurations.

author: Lesolee
author_rank: Regular Contributor
timestamp: February 25, 2020, 11:13:04 am
content: : corrected JPG and PDF. Last resistor error is E10not E9.

author: Lesolee
author_rank: Regular Contributor
timestamp: March 17, 2020, 11:09:04 am
content: The original absolute divider design for this thread was weak since it required too much equipment, too much interactive calibration, and its absoluteness was questionable according to the calibration method.This new design is less capable, having only one mid-way tap, but is self-calibratable without additional calibrated equipment.You might reasonably ask what the purpose of such an incapable device is. Ideally we would like a stable chopped source that had guaranteed linearity by virtue of accurate timing. But such a device would first need to be calibrated by an absolute device in order to guarantee that it was linear within a certain range of duty cycles. This absolute divider allows us to calibrate the more complicated chopped divider at a few cardinal points, and the time switching could then be relied upon to be linear in between these points.We also have the ability to check a few points on our DVM to make sure it is not obviously broken. It is not a foolproof test, in the sense that the DVM could be non-linear elsewhere, but it can be checked at say 100% to 50%, then knowing 50%, down to 25% and so on. Also arbitrary pairs can be picked; 120% to 60%, and consistency verified. This makes quite a comprehensive linearity check.We start with the basic design, and suppose we are linearity calibrating a 5 digit DVM. The DVM is connected:LO to LO_1 Hi to HI_2 local guard. A stable power supply of around 8 V is connected across IN_A and IN_B. We zero the DVM.Next. LO to LO_1 Hi to HI_1. We adjust the power supply so the DVM under test reads 8.0000 V.Next. LO to LO_1 Hi to Mid. We record the DVM reading, say 4.0004 VNext. Reverse the input connections on IN_A and IN_B.LO to LO_2 Hi to HI_1. Verify zero is still zero.LO to LO_2 Hi to HI_2. Verify 8.0000 V is still correct.LO to LO_2 Hi to Mid. Verify 4.0004 V is still correct.Linearity error of DVM is 4 digits. This works because the two resistors R2 and R3 are perfectly matched, and the DVM sees the same impedance for each step. Shield currents are not changed because the circuit is symmetric.But resistors are not perfect, so we adjust the pair so they are as perfect as we can measure using the DVM under test. Rather than average the two different readings, we adjust the resistor ratio to give the same reading, before and after reversal. This is better as it eliminates unknown weirdness in the DVM under test. If it was known to be without linearity errors we would not need to calibrate it!The exact detail of how we wire the precision resistors together is left until a later post.Notice that we are balancing output resistances carefully so that we can directly calibrate a DVM of arbitrary input resistance. For example at 100 V we expect the DVM to have 1M or 10M input resistance. Nevertheless we calibrate it directly since the output impedance is matched for both the full range and half-range positions.R7 is not a hidden internal preset. We expect that for each new full range voltage we may need to adjust R7 slightly, due to mismatched voltage coefficients for example.The problem we now have is with the terminals. Rewiring between tests is slow, since the thermal EMFs will take time to settle, and we have at least 4 wires to swap around. We want to switch quickly so the ambient doesn’t have a chance of changing the resistor values from their set positions.The switching is most easily shown with switches. The input reversal switch needs to be a break-before-make two pole changeover switch with low thermal EMF. Ideally gold-plated beryllium copper contacts should be used.The LO reverse switch again needs to be a break-before-make changeover.Bistable relays could be used, so the 5 ms coil energisation requirement gives minimal battery consumption, and minimal thermal EMF generation.For switches, SW2 and SW3 would ideally be the same type, for optimum symmetry. If using relays it becomes more complicated as the HI would need two series relay contacts. (Wiring the relays in parallel would be risky as we might connect HI_1 and HI_2 momentarily, causing unpleasant smokiness.Note that SW2 can’t actually be marked ZERO/MID/FULL since the ZERO and FULL positions are reversed by the operation of the LO reverse switch.The trouble with rotary switches is that they seem to be a relic from the past in terms of availability from distributors.The big pot and precision resistors are in the post.[EDIT: tweaked the example to maintain the same number of digits]

author: grizewald
author_rank: Frequent Contributor
timestamp: March 17, 2020, 05:14:19 pm
content: One trap here is actually believing Vishay's tolerance values for their precision resistors. While I was getting my Solartron 7061 calibrated recently, I took my selection of Vishay precision resistors with me (bought from Mouser, not some shady eBay seller) to get them calibrated against the cal lab's Fluke 8508A.Much to my surprise, the 5K 0.005% 0.2ppm/C Z201T resistor was out of tolerance! Not by much - it was 4999.737Ω - but that's outside the lower limit of 4999.750Ω allowed by the tolerance specification. Admittedly, if I include the 8508A's 90 day uncertainty for the reading, then it could read as low as 4999.7275Ω and just squeeze in, but even so, I'd say it's more likely that the resistor is actually out of spec. I'm also talking about a resistor that has not been soldered, it's just the part, as supplied.So, if absolute accuracy is key and you are prepared to spend money on Vishay's Z201 series resistors to get it, you need to already have a calibrated, high precision multimeter to double-check that the resistors meet their specs when you receive them.For my part, it's too late to complain and get a replacement. It's no big deal for me as I bought the various resistors to have accurate test values for checking my own equipment and as long as I have a calibrated measurement to refer to, the fact that the resistor is out of spec doesn't invalidate its use for that purpose.

author: Lesolee
author_rank: Regular Contributor
timestamp: March 17, 2020, 05:33:55 pm
content: Quote from: grizewald on March 17, 2020, 05:14:19 pmIn this design, the absolute accuracy is only required to know how much the output resistance will vary. In this design the relative accuracy is only necessary to know how much adjustment range to allow for. My (hobby) budget doesn't extend to the absolute precision you have paid for.I just bought the Vishay matched pairs from Mouser, so I will have to check what other goodies they have! I didn't think that level of accuracy would be available from distribution.I just tried to look up the 8508A spec for resistance:Best 1 Year Absolute Specification: ± 7.5 ppm of reading[EDIT: I misread this absolute spec. It is simply ±7.5 ppm of reading, which is very straightforward, although it doesn't agree with the ful data sheet, which has 7.5 ppm of reading + 0.25 ppm of range]

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2020, 05:37:15 pm
content: Using a DMM to show a constant reading with the divider swapped over relies on the external reference and DMM reference to have low noise. So it does require either a larger number of repetitions or really low noise references.There is a reason why the very accurate bridges have 2 bridges and compare the two bridges. This takes out the references.The MPM dividers in SOT23 are only specified with a noise index of -30 dB. So there can also be significant excess noise (1/f type) from the resistors. This would be about at the level of an LM399 reference. So the accuracy of the circuit is limited to some 1 ppm at best.With higher voltage one also has to check the linearity of the resistors and self heating. So 50 V with only 50 K resistors is likely too much to get a stable value.

author: Lesolee
author_rank: Regular Contributor
timestamp: March 17, 2020, 06:17:01 pm
content: Quote from: Kleinstein on March 17, 2020, 05:37:15 pmYou can off course see if it is not stable. If the DMM is itself not stable then the linearity to that level is not stateable anyway.Quote from: Kleinstein on March 17, 2020, 05:37:15 pmFantastically insightful. That skipped my mind entirely.Quote from: Kleinstein on March 17, 2020, 05:37:15 pmNot so much. I expect that every time I adjust the input voltage I need to readjust the balance pot. Not a big deal. In any case the 1/f noise limit means that to get the best ppm accuracy we need better quality resistors. But we can effectively measure the quality of the result (in terms of the noise) so the absoluteness of the linearity is not in question.If my particular resistors only work to 10 ppm, due to noise, that will not be the end of the world. So long as I can see that it is noisy, and can’t detect better than a certain level of linearity due to noise, I still have an absolute standard.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 17, 2020, 08:56:00 pm
content: Quote from: Lesolee on March 17, 2020, 05:33:55 pmThe particular meter used to check my precision resistors had a calibration label on it which showed that it was calibrated less than three months ago, so therefore the 90 day specification applies which is +/- 3.5 ppm of reading + 0.25 ppm of range. For a 5K resistor, the 20K range applies. That works out to +/- 0.0225Ω Given that manufacturers quote "worst case" values in their specs, plus that this particular meter lives in a calibration lab, I very much doubt that the meter is even close to the worst case 90 day spec. That's a good basis to declare this particular resistor as "out of spec" and I was honestly surprised to discover this.I know this kind of accuracy doesn't influence your current design, but in your first post in this thread, you were waxing lyrical about the accuracy of Vishay metal film resistors. While they are quite special, my own experience indicates that you actually need to confirm that any you buy meet the quoted specs, even when bought from a trusted distributor.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: March 17, 2020, 09:06:05 pm
content: QuoteThere are several good sources of precision resistors kicking around. I've picked up many 2ppm Alpha Electronics precisions on sale at Newark.com. Since you are in the UK sometimes they have ones available from the UK (I don't buy these since they tack on a $20 shipping fee).I bought all of these for under $9 (each, normally $45):Another source is old differential voltmeters from Fluke. They consist of matched precision wire-wounds to create their voltage dividers. Presumably you could also upcycle the rotary switches.Yet another great resource is to buy new from Edwin Pettis.I agree with other posts regarding the (lack of) trust you place in the specifications, if you go this route you need to come up with a measurement scheme.

author: Lesolee
author_rank: Regular Contributor
timestamp: March 17, 2020, 10:45:02 pm
content: Quote from: Kleinstein on March 17, 2020, 05:37:15 pmOk, let's run the numbers and see what we get (and if we can agree on the calculation).Noise index of -30 dB = 0.032 µV/V.Let's apply 2 V to the chain with 1 V output. 1 V per resistor. The current noise per resistor is 0.032 µV / 50K = 0.64 pA rms per decade.Let's say 2 decades 0.01Hz to 1 Hz. 0.64 x sqrt(2) = 0.905 pA rmsWe have 10 resistors, so sqrt(10) x 0.905 pA = 2.86 pA rmsThe current flows in a 5K equivalent resistance, giving 0.0143 µV rmsUse a ptp to rms factor of 6giving 0.086 µV ptp in a 1 V value, so0.086 ppmOn paper it looks ok to me.It has to be said that they are quoting a noise index of -30 dB for all values from 250 ohms to 100K. It should be less at the lower resistances, so I just think they can't be bothered to specify it any more tightly.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: March 18, 2020, 04:28:45 am
content: Since I've seen the noise spec from Vishay bantered around quite a bit, you need to know that their noise spec/measurements are based on the 1960 MIL-STD-202 method 308 which is still in effect today (geeze guys fix it!). This noise spec is at 1Khz and does not cover any noise from most 1/f resistors except possibly carbon comp. That's right, they are using a narrow bandwidth filter centered at 1KHz so little if any 1/f noise is included in these noise specs. The Quan-Tech model 315 was based on that spec and even includes the bandpass filter response of the instrument. So if you're looking for 1/f noise in the DC - 10Hz bandwidth, it isn't there.

author: Lesolee
author_rank: Regular Contributor
timestamp: March 18, 2020, 08:51:25 am
content: Quote from: Edwin G. Pettis on March 18, 2020, 04:28:45 amIn effect you are saying that the 1/f noise corner for modern resistors is lower than 1 kHz.Do you have any data to back up that statement?

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 18, 2020, 09:36:24 am
content: Quote from: Lesolee on March 18, 2020, 08:51:25 amNo, Edwin G. Pettis is saying that the noise spec includes only a narrow bandwidth centered at 1 kHz:Quote from: Edwin G. Pettis on March 18, 2020, 04:28:45 amThat does not imply anything about the 1/f noise spectrum properties, except that a narrow bandwidth centered around 1 kHz has very little 1/f noise. Which is true if the spectrum does not have a spike at 1 kHz; so, it is true for 1/f spectrum regardless of where the knee is.Or, conversely, any narrow bandwidth centered aroundXkHz has very little 1/f noise, for anyX> 0.To properly account for 1/f noise, you need a low-pass filter. (If you know the noise spectrum properties, the knee frequency for roughly 1/f noise, you can estimate it from a bandpass filter; or, you can use several narrow bandpass filters and estimate the 1/f noise spectrum shape from their ratios, and then extrapolate the full 1/f noise to DC.)Of course, EEs know that in practice, there is a lower limit to the frequency at whichnoisebecomesdrift. So, instead of a low-pass filter, you can use a bandpass filter, if the lower edge of the band is somewhere around the limit where you start considering noise drift instead. For me, a hobbyist working with data acquisition from scientific experiments, it'd be somewhere between 1 and 10 Hz, not 1 kHz.

author: Lesolee
author_rank: Regular Contributor
timestamp: March 18, 2020, 10:23:48 am
content: I think these recent measurements from the Max Planck Institute should settle any questions about the validity of 1/f noise measurements in resistors.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 18, 2020, 11:48:39 am
content: If you look at Figure 6 on page 9, especially the noise curves for the resistors in various shades of cyan, you'll see that they are essentially flat, at around 10-9, above 102Hz = 100 Hz. This means that characterizing the noise by using a band centered at 103Hz = 1000 Hz can be very misleading; it does not really tell us anything about the lower-frequency noise, which varies between resistors.So, the point about how a noise report based on a band-pass measurement centered around 1000 Hz being insufficient, is correct.The report was useful to me personally. It clearly shows (in Figure 2 on page 5) that thermal orJohnson-Nyquist noisedominates for typical resistors; by an order of magnitude above 10 Hz. This means bumblers like me can estimate thetotalnoise based on only the thermal noise, to roughly an order of magnitude using Johnson-Nyquist (\$V_{RMS} = \sqrt{4 k_B T R f_\Delta}\$).(When doing ADC on slowly changing inputs, noise can actually be helpful: when the noise level is about 1 LSB, you can increase practical resolution through averaging. If the noise level is less than 1/2 LSB, the quantisation step is too large for averaging to work. If the noise is much greater than 1 LSB, you need averaging to reduce its effects on the measurement first. So, when you can estimate the magnitude of the noise, you can make quite reliable estimates on the effective number of bits (ENOB) your data acquisition can achieve. Of course, one needs to measure to be sure, but in the design/prototype phase, that sort of stuff is extremely useful.)

author: Lesolee
author_rank: Regular Contributor
timestamp: March 18, 2020, 02:04:35 pm
content: Quote from: Nominal Animal on March 18, 2020, 11:48:39 amFigure 6 on page 9Notice the dashed line relating to a noise index of -40dB. The Vishay devices are quoted at -30dB so they are not in any noise floor. It means the point about 1000 Hz centre frequency is not correct. It is true that for noise indexes below -60dB (the cyan lines) the old fashioned Quantech method is inapplicable. We knew that already as it has a stated limit around -60dB (from memory), which was from the "perfect" wirewound reistors used.That figure (figure 6) also shows how well modern resistors can now be made and tested.Figure 11 for 10K resistors is also interesting. The noise index at -20dB is getting close to the measurement limitfor this methodat 1 kHz.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 18, 2020, 02:22:16 pm
content: Good point about the 10k resistors. (I didn't realize before that at 10 Hz point, thermal noise still dominates for the lower-noise resistor types at 10kOhm. I think I'll have to extract the graph and examine it in Inkscape to see which resistor is which curve, though! Edited: Yep, it's the ≥100ppm resistors that are the noisy ones.)

author: Lesolee
author_rank: Regular Contributor
timestamp: March 18, 2020, 02:30:16 pm
content: Quote from: Nominal Animal on March 18, 2020, 11:48:39 amTo be honest, I have not read the report yet, I just looked at the pretty pictures:"In order to estimate the limitations due to the bridge excitation voltage, the output voltage noise of the voltage source for several DC output levels was measured. The noise induced by the voltage source must be well below the thermal noise of the bridge impedance." (Page 5, just above Figure 2)He is just showing that his voltage source is adequately quiet for the purpose (provided it is balanced to 40dB or better).

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 18, 2020, 02:57:09 pm
content: I do understand. In my case -- completely offtopic for an absolute divider -- this was helpful in estimating the noise contribution from a voltage divider (and differences between resistors for such), when sampling some source voltage. You're doing the opposite, generating precise voltage levels from a single source, and thus minimizing the noise instead of "optimizing" it like I am. However, in both cases the exact same resistor properties matter.Apologies if I accidentally derailed the thread!In general, I'm so stupid I don't even see the benefits of cascading the dividers instead of using a tree of voltage dividers. (The difference is that instead of the voltage passing through ~Ndividers to reach an output, it would pass only through ~logNdividers, thus less noise, and easier calibration; and given a large enough number of outputs, many dividers would have the same ratio.) So, I shall be quiet now.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 18, 2020, 03:08:20 pm
content: Binary Voltage Divider

author: splin
author_rank: Frequent Contributor
timestamp: March 18, 2020, 07:53:43 pm
content: Quote from: Nominal Animal on March 18, 2020, 09:36:24 amI assume you know well the following and the above could have been worded better, but I have seen this said a few times over the years so I think it's worth making clear.Noise *never* becomes drift no matter how low the frequency. Noise by definition is random; drift isn't. At very low frequencies it may become difficult to impossible to differentiate because there is a limit to the amount of time available for observation.Drift may be a short term event - eg. an instrument warming up, or may occur very slowy over very long time scales, such as tectonic plate movements. Drift is caused by some systemic physical, chemical, mechanical etc. process, such as a resistor slowly corroding/oxidising. Typically it will be a noisy process (eg. earthquakes as part of tectonic plate movements).Drift may be relatively easily predictable such as Fluke voltage standards drifting at < 1ppm/year. It may change direction as one drift mechanism overtakes another - eg. the change in resistance of a resistor due to relaxation of stresses in plastic encapsulation may be a short to medium term effect whereas oxidation of the resistive element may be a lifetime continuing mechanism. On top of that cyclic drift changes may occur due to seasonal humidity changes.QuoteIt very much depends on what you are observing. Many posters here are measuring and reporting drift of components, especially voltage references, over periods of many years. Perhaps it would be better to word it as a low frequency limit, below which drift is not of material interest for the purpose of the particular experiment.

author: Castorp
author_rank: Regular Contributor
timestamp: March 18, 2020, 08:58:45 pm
content: Here's a little plot I generated in MATLAB. I hope it's useful, although it's not super pretty. Sometimes you get a better idea about things when they are visualized.It's pretty straightforward to interpret, when you remember that by definition the Noise Index of 0 dB is 1 uV/V/decade. So if your resistor is biased with 1 V, the tilted lines directly correspond to its 1/f noise density. Then, depending on the actual value of the resistor, the 1/f line intersects the flat line (Johnson noise) at the 1/f corner frequency.Also, I agree with splin - noise is noise, drift is drift. Their physical origins are different. 1/f noise may look like drift for a while, but eventually it always tends back towards zero. It's an ensemble of relaxation processes with vastly different time constants. In other words, it's a lossy process. Drift (random walk or steeper) implies some sort of lossless memory - usually some physical process.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 18, 2020, 09:18:27 pm
content: Quote from: splin on March 18, 2020, 07:53:43 pmThat's exactly what I was trying to express, yes.Mathematically, if you observe a signal for timeT, the spectrum you can obtain for the signal contains only frequencies ~ 1/Tand higher. Any contribution from lower-frequency components will appear as a constant component (DC component) and harmonics (in the rest of the spectrum); they cannot be recovered or distinguished without additional information about the signal. This is also why Nyquist-Shannon sampling theorem involves a band-limited signal, and not a low-pass one.Quote from: splin on March 18, 2020, 07:53:43 pmExcellent point; noted. Thanks!Quote from: Castorp on March 18, 2020, 08:58:45 pmYes, I most definitely used the term "drift" incorrectly. Apologies. (I would edit my post, except that it would break the continuity of the thread for others reading the thread afterwards.)It is also useful to remember that \$1/f\$ noise is an approximation that only holds above some lower limit frequency \$f_0\$, because if it did hold for arbitrarily small frequencies, then the amplitude would tend towards infinity when the frequency approaches zero. It is just that there is an often unstated and often completely arbitrary lower frequency limit, below which the contributions are neglible or irrelevant. The figures in the aforementioned article typically uses 1 Hz (100) as the lower cutoff limit, for example.

author: splin
author_rank: Frequent Contributor
timestamp: March 18, 2020, 10:58:48 pm
content: Not many resistor datasheets quote excess noise but here are some more that do. Presumably all are measured to IEC 60195, as some of the datasheets state. Of particular interest is how the physical size and the resistance value affects the noise and non-linearity levels.From IEC 60195:2016:QuoteSo presumably the same as the 1960 MIL-STD-202 method 308 Edwin Pettis mentioned.It is interesting to see the non-linearity graphs included in a few datasheets. This is defined in:https://www.ieee.li/pdf/essay/basics_of_linear_fixed_resistors.pdfQuoteVishay TNPW Precision thin film SMD:Vishay TNPU Precision thin film SMD:Vishay Beyschlag UXA0204, UXB0207 UXE0414 Precision thin film through hole:Vishay Precision MELF MMU 0102, MMA 0204, MMB 0207:

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 19, 2020, 07:55:01 am
content: Quote from: Castorp on March 18, 2020, 08:58:45 pmTell me how did you get the red lines?

author: Castorp
author_rank: Regular Contributor
timestamp: March 19, 2020, 08:25:33 am
content: Quote from: MegaVolt on March 19, 2020, 07:55:01 amThey are the solutions of a simple equation, which relates spot noise density to RMS noise voltage in a given bandwidth for 1/f noise. It's derived from a simple integral. You can find it here:https://www.edn.com/1-f-noise-the-flickering-candle/You just need the solution for f2/f1 = 10, i.e. a frequency decade. Doesn't matter where the decade is - the resulting equation will give you noise density as a function of frequency. The RMS per decade is then a parameter. The resulting curve is a straight line in a log-log plot.The MATLAB code is trivial:Code:f = 0.001:0.01:10000; % frequency vector - doesn't have to be so finely steppedplot(f, 1e-6./sqrt(f.*log(10))), '--r', 'LineWidth', 1.5); % line for 0dB = 1 uV/V/decset(gca, 'XScale', 'log');set(gca, 'YScale', 'log');

author: Kleinstein
author_rank: Super Contributor
timestamp: March 19, 2020, 08:53:25 am
content: The excess noise lines are calculated for 1 V at the resistor. So if the voltage is higher there would also be more noise.Especially when staying very close to the MIL202 standard, the level to which the excess noise can be measured is limited. So in the resistor data sheets one can also have just limits to which the resistors can be measured at the manufacturer or are actually tested. Because of the long time and effort it take this would be only limited samples.

author: Castorp
author_rank: Regular Contributor
timestamp: March 19, 2020, 09:09:07 am
content: Quote from: Kleinstein on March 19, 2020, 08:53:25 amYes, in Seifert's report (figures 6-7-8) you can see the -40 dB line is one decade higher, because he gives it for 10 V excitation. In my plot you can just jump to the next line upwards to get from 1 to 10 V.It's true that ATE time is an expensive resource. It's the same with op amp manufacturers and LF noise in datasheets, given as p-p from 0.01 or 0.1 to 10 Hz. It takes forever to produce a nice and smooth plot from averaged FFTs when you get down to sub-Hz. In any case, it's good to know the actual numbers, rather than some conservative limit. If anything, it leaves less room for random speculations and propagated rumours.

author: KK6IL
author_rank: Regular Contributor
timestamp: April 20, 2020, 11:33:51 am
content: 50 years ago, I was an engineering tech at Wavetek when they were trying to compete with Fluke with a differential voltmeter featuring KVD switches coupled to automatically roll over when switching 9 to 0 or back. At that time, I built a divider with 0.01% 1K resistors matched as closely as I could at the time, an sprayed with Humiseal to protect against moisture. Measuring with my HP 3456, I find they have drifted a bit over the years, but the chain is still useful to check my 3456 linearity, and to see the effect of the guard terminal on the measurement.Measuring with the 3456 Shield terminal not connected, connected to the low end of the divider chain, or connected to the LO meter terminal with the front panel switch showed only an occasional +/- 1 digit difference.Wanted to also post a picture of the setup and divider, but I keep getting a file too large, over 5000 KB for my 667 KB jpg.

author: KK6IL
author_rank: Regular Contributor
timestamp: April 21, 2020, 06:48:59 am
content: Many years ago, I was told that if I took 6 equal value resistors, put 3 in series and 3 in parallel, to make a 10:1 divider, the errors would average out to be much more accurate. So I took the spreadsheet I posted yesterday and added calculations to see what accuracy taking different sets of 6 in a row from the 12 resistor string. Even with a 63 PPM error resistor in the string, the divider error was only 1.4 PPM. I guess even with the individual errors, my resistor string isn't worthless.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 21, 2020, 07:41:30 am
content: Quote from: KK6IL on April 21, 2020, 06:48:59 amThat is true,statistically, but not guaranteed. Since 1 ppm is 1 milli-ohm for an individual resistor, the interconnection resistance is the big problem. Given a series chain already, with 4-wire pickoffs for each resistor, the series chain part is well defined. The parallel part is problematic though.Even a tetrahedral junction has a 2-wire resistance, It means that joining the top three resistors in parallel is non-trivial and must add some extra resistance to some of the resistors. That would seem to be the limiting condition for accuracy.

author: branadic
author_rank: Super Contributor
timestamp: April 21, 2020, 08:13:43 am
content: You still can use NOMCA or TDP resistor networks to get good results.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 21, 2020, 09:58:26 am
content: Quote from: Lesolee on April 21, 2020, 07:41:30 amFor the Hamon divider one also needs the adjustment step, that the 2 groups of 3 are the same vales. Than it is more than statistical luck to get an accurate 1:9 ratio.The switch resistance can be a problem and so can be isolation / leakage. Another point is having a different power level to the resistors and thus a different level of self heating.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 21, 2020, 11:01:32 am
content: Quote from: Kleinstein on April 21, 2020, 09:58:26 amThat is a very important point.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 21, 2020, 11:08:37 am
content: Quote from: KK6IL on April 20, 2020, 11:33:51 amResistor 12 is a bit out of line. If you buy a cheap 15M or 18M 5% 200 ppm/°C wire-ended resistor and just solder it across, R12 will then be back to a better value and your attenuator will be far more harmonious and beautiful.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 24, 2020, 06:35:46 pm
content: Here it is, my first absolute linearity calibrator ..The biggest physical problem with the design is the SOT-23 ratio resistors. Soldering 5 of them on wires, up in the air, is horrible. I really need some more substantial bodies.These test results won’t mean much without some explanation.I apply a +FULL signal, say +19.80 V, as read from the DVM under test. I reverse the polarity using the reversing switch and it should read -19.80 V. If it doesn’t that is an error which is recorded at the -19.80 V position on the graph. I then set the jig to +MID. The result should be +9.90V if the midpoint is balanced (which is checked by switching to +MID (reversed) ). The fact that both readings are the same shows that the resistors are indeed balanced, but the fact that it actually read +9.92 means the linearity is off by 2 digits, which is recorded at the +9.92 V position. Likewise the –MID read -9.93 V, so it is off by -3 digits.Effectively I have mixed polarity errors with linearity errors, which may not be a good thing, but the polarity deviation is still an error.These are cheap 3.5 digit hand-held multimeters. The voltage source was a bench power supply, and a pot was put in series to tweak the level a bit finer than could be achieved with the power supply setting control.

author: splin
author_rank: Frequent Contributor
timestamp: April 24, 2020, 09:57:20 pm
content: I don't know what accuracy you're hoping for but for low ppm measurements thermal EMFs will be an issue - the switches especially are unlikely to be low therrmal designs so could introduce several uVs or even tens of uVs of errors. The 4mm terminals similarly.Hopefully many of the thermal EMFs will cancel out but the only way to be sure is to measure them - without disturbing the thermal environment. You'll probably need wires from all the relevant nodes terminating somewhere convenient to attach your microvolt meter. Those terminals will of course need to be low thermal ones (eg. the ends of the copper wires connected to the nodes being measured) and close together to minimise temperature differences. Switches that aren't *very* expensive don't normally (if indeed you can find any that do) specify thermal EMF. You'd do better using low thermal latching relays. Panasonic SX relays are described as low thermal with thermal EMF specified at <= 3uV. Their TXS relays are better still with a 300nV (typical) spec.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 24, 2020, 10:40:14 pm
content: Quote from: splin on April 24, 2020, 09:57:20 pmI have actually been using Panasonic latching relays on the Dielectric Absorption test jig (in the Microvoltmeter thread). I bought them for this project, but when it came to wiring it up I didn't bother because I don't (currently) have any high resolution kit to use them with. If there is no heat in the box then the thermals should be pretty good.Quote from: splin on April 24, 2020, 09:57:20 pmSX relays are obsolete as at August 2011.The TXS relays are indeed quoted as you say, but for the non-latching relays. The coil heats the contacts and creates a temperature difference. An externally applied thermal gradient is a whole different (unspecified) state.The output binding posts are tellurium copper. The input ones are not because Farnell have run out and I didn't want to wait!

author: splin
author_rank: Frequent Contributor
timestamp: April 24, 2020, 11:15:24 pm
content: Quote from: Lesolee on April 24, 2020, 10:40:14 pmWell you'd hope so but standard switches aren't designed for low thermal EMFs so they could be absolutely horrible in this respect - you'd have to measure them to be sure.QuoteI mentioned them as another reference point for thermal EMFs for a relay described as low thermal. There would be no reason to use one when TXS2 relays are easily available.QuoteTrue, but I suspect you would be very hard pressed to create a temperature gradient, from an external source, across the contacts which exceeds that from the coil temperature rise of 5 to 10C specified in the datasheet. For a reasonable thermal design you should be looking at very much less than 300nV.See this thread for some measurements - particulary chuckb's posts #30 and #37:https://www.eevblog.com/forum/metrology/measurements-on-emf-error-of-switches/?allAlso see Andreas's post #38.Chuckb's tests of an Electroswitch C4 rotary switch in post #39 is also interesting with thermal EMFs < 40nV. They start from £15 at Mouser.

author: CDN_Torsten
author_rank: Regular Contributor
timestamp: April 25, 2020, 12:51:35 am
content: It's possible to minimize the thermal input of the non-latching relay coil by dropping the coil current to it's minimum hold current once the contacts have closed. The hold current tends to be much less than the required current to start the contacts moving.

author: chuckb
author_rank: Frequent Contributor
timestamp: April 25, 2020, 02:07:06 am
content: This week I happened to be testing the leakage resistance of some C&K red 7201 series switches. They look like whats in your setup. C&K specifies over 1 Gohm insulation resistance and the switches meet that. With a Keithley 617 Electrometer I measured 400 Gohm from the common contact to case. From common to common of the DPDT switch I had 200 Gohm. An open switch contact measured 700,000 Gohms. You may want to evaluate if this will cause you any issues.I have not noticed DIFFERENTIAL Temf over 20nV with the DPDT switches inside a metal box.I also looked at some small NKK M-2022 DPDT switches and they were over 2,000 Gohms from common to case and between common terminals. The open contact was over 1,000,000 Gohms.ITT Pamona Low Thermal EMF 3770 Binding posts have greater than 14,000,000 Gohms leakage resistance to the mounting plate at 100V excitation.Hope this helps.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 25, 2020, 06:37:55 am
content: The elephant in the room is the resistors wired to the unused switch contacts. It was soooooo tempting to use the unused switch contacts as insulated standoffs.PTFE standoff posts seem to be rare and expensive. However, it is far from ideal to couple the heat from the dividers into the switches!100²/20K = 0.5W, which is a lot of heat to get rid of.I would think the thermals would balance out due to the reversing action of the jig, or at least be visible as drift or offset in the readings. (There are 4 zero states, for example). More resolution needed to see the problem.

author: quarks
author_rank: Frequent Contributor
timestamp: April 25, 2020, 08:10:42 am
content: very interestingunfortunately missed this completely, but now it is bookmarkedthanks for sharing

author: Lesolee
author_rank: Regular Contributor
timestamp: April 25, 2020, 10:17:30 am
content: Quote from: splin on April 24, 2020, 11:15:24 pmI am not as confident in that idea as you are. The relay design could have a nicely balanced thermal path to dissimilar metals so that it gave a low thermal EMF to its own heat, whereas an external temperature gradient could just heat one brass/copper junction and give a big problem.Just to get a feel for the thermals here I used my home-made micro-voltmeter (https://www.eevblog.com/forum/metrology/project-micro-voltmeter-design/45/,post 45 onwards) to measure the voltage across SW3 closed contacts. I didn’t use screened leads, but the result is pretty clean. Clearly there is not a HUGE thermal problem.The µVM was wired up and on for 50 minutes before turning the PSU output up to 60 V. It is hard to say there was any actual thermal change. There is an ‘interesting’ jump at 9.5 minutes for no obvious reason. The power supply was turned back down to 0 V at around 12.3 minutes, and I suspect the dip is due to the common-mode change.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 25, 2020, 03:35:58 pm
content: I used the Panasonic relays for my low thermal multiplexer. I tested a couple of cables (short).I touched the cable for 30s with my fingers to see the thermal effects of the cable.The voltage was measured with a Keithley 182. <300nV is achievable with the relays.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 25, 2020, 03:56:03 pm
content: Quote from: e61_phil on April 25, 2020, 03:35:58 pmCan you spell out exactly what you are doing here please. It looks interesting, but I just can't follow the plot.You touched the outside of an insulated cable?Is the cable 4mm long?What has "Fluke" got to do with anything?You touched the cable for 30 seconds and the effect lasted for 10 minutes?

author: Lesolee
author_rank: Regular Contributor
timestamp: April 25, 2020, 04:34:32 pm
content: This is a test of a 5 digit DVM module I got off ebay. It is the sister of the module used in the micro-voltmeter project. The discontinuity around 10 V is because the DVM auto-ranges at that point, so the half-value point suddenly gets an extra digit at that point. 60 digits translates to 6 digits which would have carried on the previous trend.This other way at looking at the same data says the high/low ratio should be 2, but it deviates from that fraction by a certain number of ppm. If there was no linearity error then the curve would be a straight line along the horizontal axis. I created the red dotted line as a quadratic, where the x² multiplier was -0.0000313It means we have an absolute INL reference.[EDIT: This testing was done with a 100R pot in series with the linearity jig +IN connection, to fine tune the voltage to a nice value. This was an exceedingly bad idea! The source impedance creates a ratio error of up to 75 ppm. This probably explains some of the wiggles in the response (but not the overall shape).]

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2020, 05:09:39 pm
content: Getting some error even near zero suggests that there is some offset error. The error is also really large.That the error does continue even after the range switch, can have two reasons: The range switching can be more or less a software thing, not really changing the gain before the ADC. The likely used MCP3421 has some internal "Gain", using a faster input sampling for the SD ADC. The actual source of INL can be the divider before the ADC. In this case the main expected error would be of U³ type from a self heating effect and the linear TC of the divider.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 25, 2020, 05:54:15 pm
content: Quote from: Lesolee on April 25, 2020, 03:56:03 pmSorry, I just wanted to show that these relays can achieve way less than 300nV.The setup was the following:The muliplexer has three channel. Two Inputs and a third one which is internally shorted. Which means one can switch Ch1, Ch2 or a short (Ch3) to the DMM.I tested different low thermal cables (in the graph it was the Fluke 5440A-7002, which is one of the best). I used the "black" cable to short Ch1 and the "red" cable to short input 2. After some stabilization I touched one red and one black plug, to create a thermal gradient between the the plugs of the same color. This was done to simulate handling with the plugs. And yes it took many minutes to become stable again. But 30s ist quite long touching.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 25, 2020, 07:22:34 pm
content: Quote from: e61_phil on April 25, 2020, 05:54:15 pmhttps://eu.flukecal.com/products/accessories/test-leads-probes-and-clips/5440a-7002Ok, so the plugs are rated at 1.3µV/°C relative to tellurium copper. Assuming that is what they were plugged into, you heated each by 0.6°C and they took 10 minutes to settle down again. That seems fair. I thought you were saying you touched the middle of the cable.So what are the relays doing? Are you switching channels during the plot? Is it like 1 second per relay, then switch to the next? Are these latching or non-latching relays? Probably you have a good idea of what you are doing. I am old and slow, and it is difficult to figure out what exactly you are doing.(but it is clearly relevant and interesting)

author: Lesolee
author_rank: Regular Contributor
timestamp: April 25, 2020, 07:37:26 pm
content: Quote from: Kleinstein on April 25, 2020, 05:09:39 pmIf I apply linear voltage, 1 V, 2 V, 3 V to an inverting active attenuator (so all the power goes into that resistor)I get powers of 1, 4, and 9 power unitsI then get temperature rises of 1, 4, and 9 thermal units due to the thermal resistance (°C/W)This converts to shifts of 1, 4, and 9 ppm units due to the TC (ppm/°C).Could you explain how you derive a cube law from this?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2020, 08:06:22 pm
content: The power at the resistors is proportional to the voltage square. This can shift the gain proportional to the voltage square. For the voltage error it is applied voltage times gain error this the Voltage as an additional factor. So a total of U³ * TC of the resistors * 1/R / thermal resistance.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 26, 2020, 11:00:24 am
content: I should say a little about how this plot was measured. The voltage source was an ordinary bench power supply, ISO-TECH IPS603 (as it has been for all previous tests as well). This is quiet enough to get no least significant digit noise. The voltage output is not a pot, but a digital rotary thing, so it is nice to set, but doesn’t have quite enough resolution to get the last digit spot on. I therefore put a 100R pot in series with the power lead to the jig to tweak in the last digit.I should also mention something about the presentation of these results. For a 58.00 V input I got readings of +28.98 V and -28.98 V. The correct values are of course +29.00 V and -29.00 V. The positive reading is 2 digits low. The negative reading is arguably 2 digits high (more positive than it should be). I initially drew it that way, and it looks nice, with a linear sort of growth in the error. On reflection I changed it to make the negative error read as negative. In other words the magnitude of the reading is lower than it should be. I think this is more reasonable in the sense that it is the voltage magnitude which creates the power, which creates the temperature rise, which creates the TC error. This is symmetric. Making the curve look symmetric therefore seems more correct to me.No doubt this will offend the technical sensibilities of at least half of the intended audience. I don’t claim that this is the “correct” way of presenting the data, I am merely stating what I have done. YMMV.

author: KK6IL
author_rank: Regular Contributor
timestamp: April 27, 2020, 03:05:01 am
content: In the distant past I've used a insulated terminal which was a teflon bushing which a metal, probably brass, pin was inserted. Result looked about like the Keystone 11320, in stock at Mouser <https://www.mouser.com/ProductDetail/Keystone-Electronics/11320?qs=%2Fha2pyFadugenAX7QbWVFFz%252BtNpkTySy2A752Cfwr1QsAp2L0KjgLQ%3D%3D>I really miss the days of printed catalogs where one could turn pages to find pictures of the item desired and see similiar items and prices.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 27, 2020, 06:57:32 am
content: Quote from: KK6IL on April 27, 2020, 03:05:01 amI looked for the same sort of thing and didn't find this one, so thanks.(It is indeed PTFE bushing)https://www.mouser.co.uk/ProductDetail/Keystone-Electronics/11320?qs=sGAEpiMZZMuHIszJOUpTOs%2FDGXBqE7ETWe used to use PTFE "cloverleafs", a tinned brass cup in a PTFE bush that you could insert wire leads into in order to solder them together without electrically interacting with the pcb, but still having mechanical support.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 28, 2020, 05:00:53 pm
content: Quote from: chuckb on April 25, 2020, 02:07:06 amThe switches I am using here are from RS components (rswww.com). They are own-brand parts, RS part number 734-7022.They quote insulation resistance at 1000 M min and dielectric strength at 1000 V. I measured 1.2 T from a centre contact to the body using the micro-voltmeter from the micro-voltmeter thread (100M // 10nF NP0 across the input)Open contact leakage 120 T.Common to common is an interesting one. I was measuring around 3.8 T, but then I realised that this is astupidmeasurement if the body is left floating. Grounding the body (naturally) killed the common to common leakage. I didn’t measure it properly, but at least a factor of 10 increase (ie > 40 T).What I was most interested in measuring was thewholelinearity jig. That is 4 switches, a pot, and 4 binding posts. Any connection to any point in the circuit connects to all points as far as leakage is concerned. The total leakage was around 800 G. That is adequate since my circuit diagram claims that 0.1 ppm linearity can be achieved with 200 G mismatched leakage from the mid point to the case.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 28, 2020, 05:27:51 pm
content: Quote from: Lesolee on April 25, 2020, 07:22:34 pmYes, exactly. The relays are switching between the three channels. One complete scan takes 12s. It switches the relays, wait 1s for settling and take 10 readings to average them from the Keithley 182.The relays are latching ones and I routed both signals (HI and LO) through the relays to cancel out thermal EMFs.The box is very ugly, but it works fine. (I had to use another red connector, because all the black ones from digikey were broken, but they send new ones immediately). On the pictures is another test with other very expensive Fluke cables. The "red" cable is used to short one of the inputs and the "black" is used to short the other one. Due to the wrong color on the connectors it is a bit harder to see what is really happening.

author: Lesolee
author_rank: Regular Contributor
timestamp: April 29, 2020, 09:18:22 pm
content: Quote from: e61_phil on April 28, 2020, 05:27:51 pmNot ugly at all; mine look the same!I forgot to upload the photo of the test. I applied the voltage to the case of the linearity box because the smaller terminals-and-electronics has less pickup area -- it is going to the current-sense node. It was not dangerous as I could only apply ±3V to the case since the micro-voltmeter is so sensitive. (I managed to get up to ±60 V doing the open-contact leakage on a single switch, since that leakage was up at 120 T).

author: KK6IL
author_rank: Regular Contributor
timestamp: April 30, 2020, 12:52:02 am
content: PTFE double turret terminals available at Electronic Goldmine, 20 for US$ 5.00.<https://www.goldmine-elec-products.com/prodinfo.asp?number=G24775&utm_source=Goldmine&utm_campaign=dcbac0a3e7-MAY23-2014_COPY_01&utm_medium=email&utm_term=0_15cb8e0368-dcbac0a3e7-60329037>

author: Lesolee
author_rank: Regular Contributor
timestamp: April 30, 2020, 06:51:17 am
content: So we have a straightforward buy from Farnellhttps://uk.farnell.com/itt-cannon/011-2049-040fb9/terminal-ptfe-test/dp/1347795at £3.21 each (+20% VAT)Then we havehttps://www.goldmine-elec-products.com/prodinfo.asp?number=G2477520 off for $5.00 (plus unknown international shipping cost)The price difference is staggering, and may justify the international shipping.

author: Lesolee
author_rank: Regular Contributor
timestamp: May 03, 2020, 12:29:18 pm
content: Quote from: e61_phil on April 25, 2020, 03:35:58 pmI just came across this Fluke short-circuit tool in an article "Watch out for those thermoelectric voltages!"They want a short (brief) settling time so they are going for low thermal mass. I would suppose that this means it is more susceptible to drafts.It is an interesting idea.

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 03, 2020, 12:39:54 pm
content: A couple of these shorts came with the 8508A and also with the 8588A.

author: wiss
author_rank: Frequent Contributor
timestamp: January 13, 2023, 01:23:45 pm
content: Quote from: Lesolee on February 24, 2020, 08:23:59 pmHiI _only_ test "linearity" (in mathematics speak "affine") of the multimeter, if there is thermal drift, it will add to the non-linearity, as any other problem such as loading.Data was cirtainly not cherry-picked, it was very tedious to read 7 digits of the multimeter, switch, wait for it to stabilize, repeatI found the resistors in a drawer at my old job, I do not have real data for them(I could post a picture of the device)You only have to understand what a pseudo inverse do, not how to reach it, there is a python implementation. It gives the least square solution to a problem with many solutions.BTW, the 2 examiners were very confused, one did _not_ understand the mathematics (experimental physicist), the other (theoretical physicist, general relativity etc) did not understand the application, it was fun