Mini Metrology Lab Build (update) - Page 1
Forum section: electronics / general

author: Jebnor
author_rank: Regular Contributor
timestamp: July 10, 2013, 10:59:54 pm
content: I decided to build Mr. Hoffman's Mini Metrology Lab  as per the article. Except for two things: I'm using a MAX6350CPA+  instead of the LT1027BCH in the voltage reference and using the homebuilt rail splittler on pg 78.I'm laying out and etching my own boards to fit in my cases, a Hammond 1455J1201  for the Voltage Ref, and 1591DTBU  for the Null detector. I have yet to determine what I'm going to use for the KVdivider.I attached a picture of my parts that arrived today from Digikey.http://conradhoffman.com/mini_metro_lab.htmlhttp://datasheets.maximintegrated.com/en/ds/MAX6325-MAX6350.pdfhttp://www.hammondmfg.com/pdf/1455J1201.pdfhttp://www.hammondmfg.com/pdf/1591D.pdf

author: Jebnor
author_rank: Regular Contributor
timestamp: July 15, 2013, 10:32:44 pm
content: I have finished the test build of my Voltage Reference before I build a second one and use the more expensive chips. I am testing my PCB etching process and the physical fit in the case.I used a 1N5231B Zener diode as a voltage "Reference" in series with a 499ohm resistor across the 15V rail. Once tested, this Zener gave me 5.19V. Then I selected resistors for R4 and R5 to give me the proper feedback ratio to yield 10.0V. As you can see from the attachments, I didn't do too bad. I used a LM741 for the test build as it has the same pinout as the LTC1050.Before I build with 'real' parts I'm going to squeeze the IC's, R4 and R5 together horizontally. Everything else will stay the same.Enjoy

author: JP16A60
author_rank: Unknown
timestamp: August 13, 2016, 08:03:42 pm
content: Hi, Jebnor.I know that this thread is pretty old, but wondering if you made any further progress.I'm planning on building the same "Mini Metrology Lab" suite sometime soon, and would welcome any thoughts or tips on this.JP

author: Gyro
author_rank: Super Contributor
timestamp: August 14, 2016, 11:45:38 am
content: Conrad Hoffman is actually a contributor to this forum. If he doesn't notice your post then you might want to PM him to see if he has any new tips.

author: Andreas
author_rank: Super Contributor
timestamp: August 14, 2016, 05:15:29 pm
content: Quote from: JP16A60 on August 13, 2016, 08:03:42 pmHello,if you want to have a 10V output I would use a AD587UQ (CERDIP-package) as reference.Any additional resistor divider adds unnecessary temperature errors.If you have high demands you could use a temperature compensation like on the Geller SVR-T references.With best regardsAndreas

author: Gyro
author_rank: Super Contributor
timestamp: August 14, 2016, 06:14:49 pm
content: EDIT: Probably more appropriate in the Buy / Sell / Wanted category so have moved it there...https://www.eevblog.com/forum/buysellwanted/spotted-on-ebay-us-muirhead-10k-voltage-dividing-resistance/

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 22, 2016, 07:56:26 pm
content: That article series is long of tooth now. Learn from some of the techniques, but I'd use different chips and topology these days. The best info is right here at EEV if you read the voltage standard threads and the standard resistor threads. Whatever you do is going to rely on those two things. FWIW, the null detector circuit worked well to get people into the game with only a DVM, but I still prefer an old fashioned center-zero meter! Since then I did the quick write-up on the Hamon divider and find that to be a very handy device.

author: Gyro
author_rank: Super Contributor
timestamp: August 22, 2016, 09:35:46 pm
content: Quote...and a very helpful write-up it was too!A short thread on mine and link to Dr Frank's here:https://www.eevblog.com/forum/testgear/anyone-else-built-a-hamon-divider/msg776035/Two decades is good for 'leapfrogging' up and down meter ranges to compare calibrations.

author: JP16A60
author_rank: Unknown
timestamp: August 24, 2016, 08:02:00 pm
content: Quote from: Conrad Hoffman on August 22, 2016, 07:56:26 pmHi, Conrad--thanks for commenting.I'm planning to build not because these would be the most modern instruments, but rather because it looks like it would be a fun challenge, and a way to get myself started actually building things from other than a kit or "hold-your-hand" textbook-based project.I also find quite a bit of satisfaction in learning and building "old school".Thanks again for your comments and for this excellent three-article series.Regards,JP

author: Dbldutch
author_rank: Regular Contributor
timestamp: September 13, 2016, 08:15:30 pm
content: I have just finished the build of the Null Detector, I wanted to use that with my "normal" 3.5 digit DMM's, but mostly I can use my bench DMM which has 1 uV resolution. This works fine for me using the original design.However, I was mostly intrigued with the KVD, because I believe (never used one before) that it can do some things I can't do any other way. Unfortunately, for one particular task, it seems that I'm not getting the precision I want, to use the KDV to transfer a reference voltage. (it calibrates precisely, that's not the problem, but since that process goes from decade 6(output) to 1(input), it looks to me that the base it starts from is not "aligned" to get a true /10.I have PM'd Conrad if he would be so kind to elaborate on the design constraints he made at the time, to deviate from the standard KVD decades, especially in the lower 4 ones, and what the consequences are. I suspect that is why I can't do what I want.Here is a short description. I have a calibrated voltage reference, that at the 10 V setting actually produces 10.00673 V. My plan was/is to use the KVD to adjust the appropriate decades to lower the excess voltage to get 10.00000. So set decade 4 from 9-6 to 3, decade 5 from 9-7 to 3 and decade 6 from 10-3 to 7. You can then use a Null detector between de KVD output with the 10.00000 V and the output of the new reference and adjust that to have a null resulting in a 10.00000 V "transfer" without using 6+digit meters. I get close, but not close enough, and I suspect it has to do with the deviation Conrad used at the time from the 10K, 2K, 400, 80, 16 and 3.2 Ohm standard.I'm planning to modify my KVD to the standard values later, but could use some more input on the applicability of my proposed method. I will provide a description of my KVD project with pictures etc. at a later date. Stay tuned.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 14, 2016, 08:23:53 am
content: Quote from: Dbldutch on September 13, 2016, 08:15:30 pmWith such a small difference it would be easier just to use a simple divider, 10K + 8.2 Ohm and a 100 Ohm Cermet pot in parallel to 8.2 Ohm, to trim to the exact value . Even using standard metal film resistors with 100ppm/C tempco, the resulting output voltage change from this divider would be below 0.1 ppm/C, and if you use something better, like RC55 with 5ppm/C, the tempco from the resistors value drift would be essentially unmeasurable.CheersAlex

author: Dbldutch
author_rank: Regular Contributor
timestamp: September 14, 2016, 09:27:01 am
content: Hi Alex,Thanks for the reply.You are right of course, but in this case I wanted to verify if mymethod, using my new KVD, was correct. The underlying idea is to transfer a reference with say 6 digit accuracy to a new one, by using (only) a 3.5 digit multimeter, a null detector and the KVD. It's for those that have no access to 4+ digit equipment. I have never used KVD's before so I'm on a fact finding mission for myself, and hopefully can share that with others.I did get a reply from Conrad, and what I want to do is indeed sound, providing there is no load towards the reference, and towards the KVD. Because I started to doubt my thinking and the absolute precision (not the relative one if you know what I mean), I already changed the first decade. Originally, I used a matched set of 10K resistors (<0.01%), but the actual value was 9.950 Ohms. I recently added a 49.9 Ohm resistor to get to 10K and redid the matching, but since I was busy, did not try the transfer method yet. After Conrad's reinforcement, I tried it again this morning and low and behold, this time it worked. Question now is if that was due to my changes to the first decade? Both times I calibrated the decades to within 10uV using the bridge method.More later.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 14, 2016, 02:14:57 pm
content: From our PMs, I just wanted to mention for others that each decade has to be large enough so the adjustable shunt can bring it down to match the previous decade. I don't know if the slightly low resistors on the first decade are enough to prevent accurate calibration, but it sounds like the 10k fix did the trick.FWIW, some KVDs used 11 steps on the first decade, giving 1.1 and 1.2 ratios. On Julie Research dividers they were the "H" models. It's trivial to add those steps when building one and it certainly increases the utility. I've got two Julies and one ESI, plus some Dekaviders (sp?) and none of them has that feature.Off on a tangent, if you do AC, it's easy to get ppb (no, that's not a typo) ratios using ratio transformers. Henry Hall did a write-up on that, probably in one of the GR Experimenters.

author: Dbldutch
author_rank: Regular Contributor
timestamp: September 14, 2016, 02:42:48 pm
content: True, if the total decade chain can be adjusted by shunting, the calibration will work, as in my case. The resistors need to be relatively close, because there is an obvious end to shunting.With some changes to the shunt combination to make the adjustments finer, I was able to get to a few micro volt decade to decade match using the bridge method.But, that only means that only one condition of the KVD is met. I'm still not at peace over what the deviation from the 5x rule to get to 10K 2K 400 80 16 3.2 actually means or does.Thoughts?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 14, 2016, 03:46:42 pm
content: Shouldn't mean a thing. Consider any one decade. As long as the total shunt value across the switch positions is correct, the divider works as it should, with the correct voltages at the switch points that will be the connection of the next decade.. How you arrive at that total shunt value doesn't matter. The calibration shunt to alter the total value of the next decade has no effect on that decade, as the voltages from the previous decade will be correct if the total shunt is correct. The "perfect" values were a target for designs that used no calibration shunts, but that's all. I'm not sure, and it would be a dumb design, but I think you could build a KVD with all the same resistor values, and shunt each decade.

author: Dbldutch
author_rank: Regular Contributor
timestamp: September 14, 2016, 08:54:33 pm
content: Thank you Conrad, another mystery explained.I just looked at the Fluke 720 schematic again and I can confirm that they used 1K resistors for each of the last 4 decades.Thank you for letting me sit on your lap. I learned a lot.Paul

author: Macbeth
author_rank: Super Contributor
timestamp: September 14, 2016, 09:56:32 pm
content: I can confirm that my esi RV622A which is a 6 digit divider uses shunts on the last decades.1st decade is 11 x 10k2nd decade is 11 x 2k3rd decade is 11 x 400 ohm4th decade is 11 x 400 ohm shunted with 1k5th decade is 11 x 400 ohm shunted with 1k6th decade is 10 x 400 ohm shunted with 1k

author: JS
author_rank: Frequent Contributor
timestamp: September 14, 2016, 10:00:56 pm
content: Quote from: Conrad Hoffman on September 14, 2016, 03:46:42 pmThis is done and is good for tempco tracking (having all equal resistors) but bad for output impedance. Even if it always should work with no load with a null detector, lower the output impedance minimize the error and noise. The load error being of course the output resistance times the output current, while, if both known, could be corrected.JS

author: zlymex
author_rank: Frequent Contributor
timestamp: September 15, 2016, 01:16:41 am
content: The very large(and variable) output impedance of 720A is mainly due to the 1st decade. It won't lessen much if all the resistance of subsequent decades are following the 5x rule. 720A was using 1k in all lower 4 decades because of another reason(to minimize the BOM).One easy way to access an KVD is to buy a used Fluke 887A(or 883A or 885A or similar) differential voltmeter such ashttp://www.ebay.com/itm/Fluke-883A-AC-DC-Differential-Voltmeter-/201294712925

author: JS
author_rank: Frequent Contributor
timestamp: September 15, 2016, 02:29:07 am
content: Quote from: zlymex on September 15, 2016, 01:16:41 amOf course the first decade already mess things up, but having all high valuer resistors would make it even worse. After you reduced enough the value (at the third decade in this case) there are diminishing returns in lowering the value even further and lower values bring their own problems. The first decade is also limited by the contact resistance in the switch, 6 decades is a lot to chew... 1ppm in the first is a whole step in the last, of course the last decade wont have the same importance/care.JS

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2016, 06:45:12 pm
content: There is nothing wrong with using the same resistance as in the higher decade and an extra shunt resistor. This even has 2 advantages: the resistors don't get too small to need special switches. Also the total resistance can be trimmed independent from the steps.Just for the 1st, 2nd and maybe 3rd decade one often prefers a higher impedance, not to load the input so much and to have less power dissipation. Also switch resistance is less critical at high impedance. So the values at the RV622A make perfect sense.

author: Dbldutch
author_rank: Regular Contributor
timestamp: October 17, 2016, 03:29:08 pm
content: Interesting comments! Thank you for taking the time to contribute.I am greener than green with this topic, but determined to learn more about the KVD and it's applications in the hands of a hobbyist, like me. There are not many tools that you can build that, potentially, let you sniff at ppm level precision with budgets that can be pretty minimal. I said potentially!So, while playing with this tool, help me to understand what can be done to increase the value of the instrument, without having to rob a bank.Having said that, it is now clear(er) to me that the accuracy and applicability of the KVD is greatly depending on the accuracy and the matching of the resistor Ohm values within the decades, especially the first three.However, it seems that a lot is also depending on the TC of the resistors as well. This is a grey area to me (as well), but it is for me under much less control than the pure Ohm-ic selection. I begin to understand the troubles Fluke and the likes went through to get it right.You can use a batch of less precision resistors to find higher precision individuals, and match them, quite easily. Obtaining a large enough batch of resistors with a basic low TC, like below say 15 PPM/C is quickly getting very expensive, let alone 5PPM/C or lower.Is there justifiable merit in combining inexpensive resistors with a second set of resistors with an opposite TC to reduce the total? Is that even feasible for a hobbyist? While searching, I came accross this article that seems to support that idea, but I really do not have the insight to figure out if this even makes sense.http://web.engr.oregonstate.edu/~moon/research/files/iscas07_res.pdfSuppose you only use the KVD occasionally, and only in a room with temperatures that do not differ more than +/_ 5 degrees C over the year. And before you make a measurement, you calibrate the KVD decade to decade shunts and you only use it in a bridge configuration (no self heating by currents). Does it then even matter, or if so, how much, if the TC of the resistors are not below say 50PPM/C?Looking forward to learn some more...

author: Dbldutch
author_rank: Regular Contributor
timestamp: October 17, 2016, 08:29:52 pm
content: Never heard of this technology, but I think I found them :http://www.vishay.com/docs/49905/_aorn_vmn-pt0449.pdfFour 10K, 2K or 1K resistors in an SMD package with .1% precision, 0.015% ratio, 25PPM/C and if I understand it, with a 5PPM/C ratio after a long burn in period. Seems made for a KVD.Digikey sells them for $4.00 1 off.http://www.digikey.com/product-detail/en/vishay-thin-film/AORN1002AT5/764-1149-1-ND/5138323This could be very interesting, but now I'll need to find out how to cook them at 155 degrees C for 42 days. Glad winter is coming soon.Has anybody tried to use these resistors for a KVD as far as you all know?If this is a possible option, and makes some sense with a reasonable rate of success, I'm willing to give this a try.BTW, I'm using the Conrad Hoffman method without expensive switches at the moment.

author: JS
author_rank: Frequent Contributor
timestamp: October 18, 2016, 11:14:26 pm
content: Vishay does have networks with more than 4 resistors, with similar specs as the one you said. Would make more sense using one package with all you need than using 4 resistors in a package?Depending on the goal some trimming may be needed for the individual resistors, starting with 11x 10k resistors in series, within 0.1% of each other would allow trimming them with 100k+2k trimpot in parallel, having just enough to play with and needing more relaxed tempco as the main network. The MML from Hoffman asks for 40ppm matched resistors for the first decade, so you are not going to find that matching for 10 resistors in a single package in any cheap available product. Also, long term drift would probably take out your resistors from those 40ppm so it does makes sense having some trimming there rather than having your KVD useless (not precise anymore) after some time. Fluke 720 uses a special switch to allow the self calibration of the 2 higher decades resistors and shunts without opening the device, the third decade needs an extra screw driver to do the job. If using the pin strip instead of the switch makes the test points available, you need to have the null amp and the bridge anyway, you can take the extra step and calibrate it before critical tasks instead of relaying in the long term stability of the resistors.JS

author: Dbldutch
author_rank: Regular Contributor
timestamp: October 23, 2016, 09:31:14 am
content: JS,I think you refer to the Vishay NOMCA series, with 7 or 8 resistors in an SMD package. I could not find others that looked suitable. The NOMCA series have similar specs as the AORN series, and cost about the same (about 5 Euro's) for twice the resistors. Go figure.What really surprises me is the fact that these (pretty) high precision resistors with a (pretty) good TCR are so inexpensive, compared to through-hole resistors. The only downside that I see is that the power rating per resistor is low with only 100mW. I will not use my KVD with inputs higher than 30V. Correct me if I'm wrong, but I think that self heating by the current is no real issue then, certainly not on my more typical 5 to 10V inputs, let alone if the KVD is used as a bridge component.So it still looks to me a viable alternative for a hobby application for the KVD.Going down that path a bit further, I found that both Mouser and Digikey do not have the 2K resistor version in stock for both the AORN or the NOMCA series. You would have to use 2 x 1K in series.As you alluded to, the 10K resistors of the first decade need to have a method for trimming each individual one, to get them within the matching specification of +/-0.0037%.If I'm going for the NOMCA series, the absolute TCR is +/-25 PPM/C, and a tracking (I think they mean from resistor to resistor within the package) of 5 PPM/C. Putting a 100K trimpot in parallel, with a typical 100 PPM/C, as you propose, would degrade the total PPM I think.In my opinion, it is better to use a small resistor in series (can be 50PPM/C) and a 100 Ohm 1 turn trimmer (100 PPM/C) in parallel to the small resistor. The maximum PPM influence with that combination should be less than 2 PPM/C on the 10K, if my calculation is correct. With a 1 Ohm/100 Ohm combo, you can adjust the 10K with an additional 0-0.9 Ohm, and stay close to the absolute 10K value. The value of the 1 Ohm depends on the absolute variations of the 12 resistors (for a 1.1 KVD input) that can be selected from the total of 16 that are available with 2 NOMCA16031002AT5 packages. If they vary too much, you can increase the value of the 1 Ohm.By using the NOMCA16031001AT5 1K packages for all the other resistors, you could potentially select enough to match for 11 2K resistors and avoid the trimming. What I mean is that by using 1K resistors for decade 2, you need 22, and you need 11 for decades 3, 4, and 5 each, and 10 for decade 6, for a total of 65 resistors. This is one resistor short for 8 x 8 resistor packs, so you can add a normal 1K resistor for the last decade, or add another package. In that case you have 9 packages x 8 = 72 resistors to select from, to get a matching set (+/- 0.037%) for the second decade. Should be possible, I was able to select better matching than spec with a set of 100 1% 2K05 resistors. In this case you can also combine two 1K resistors in series to help matching.The expenditure budget would be 11 x NOMCA1603 for 5 Euro's each, 11 trimmers for say 1 Euro each and 12 1 Ohm resistors for say 0,10 Euro's, or a total of about 66 Euro's. Not bad. I spent more on my first prototype with batches that totaled about 600 resistors of 1% metal film through-holes to match them to specifications. Actually, I was able to match them to much better than spec.One of the advantages I think are that it will be easier to control the TC of the individual decades and the total board, compared to using individual resistors, especially when mounted Manhattan style, which is what I did.Are there any objections to following this route or do any of you see pitfalls?Tks!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 23, 2016, 11:37:04 am
content: The TCR matching applies to a single package. Using separate packages can work but matching is not guarantied. However there are also chips with 10 and 12 resistors in one package. Anyway one still needs reasonable matching between the decades. So the total TCR is still important, not only matching.If at all self heating is a problem for the first decade, the second decade sees only 1/5 the voltage and half the current, so 1/10 the power. It also depends on the resistance chosen.For trimming it is not such a good idea to use low value pots, as the wiper contact is one point that can change over time. For the combined TC there is no principle difference in using a series or parallel path. The parallel way is usually easier, unless the resistor value is very large.The more usual way would be a 1 K main resistor in parallel with a 100 K resistor (of reasonable quality, e.g. <= 100 ppm/K) and pot of e.g. 10 K in series - this gives about an 0.1% trimming range. depending on the precision needed / given the trimming range might need to be smaller for the first decade. In this example the TC of the 100 K resistor contributes with about 1/100 and that of the pot with about 1/2000 of its value, so they are far less critical.With parallel trimming the resistor value is reduced - so one will need an adjustable resistor shunting the second decade as well (should be something like 200 K + 20 K pot for the example above).With 1 K resistors in decade 2, one would be at 5 K resistors for decade 1. This would be more like using two 10 K resistors in parallel.

author: Dbldutch
author_rank: Regular Contributor
timestamp: October 24, 2016, 12:06:40 pm
content: OK JS and Kleinstein, I humbly stand corrected!I obviously made a mistake with my PPM calculation. I was influenced by the trim construction that Fluke used in the 720 on the first decades.OK, another little step closer, thank you for taking the time to point this out.Kleinstein, or anybody, I could only find one more candidate, the Vishay OSOP series with 10 resistors that seem to have the right specifications. Could you please point me in the right direction for a suitable resistor network with a low TC that packs 12 resistors with adequate precision?After thinking some more about the practicality of resistor matching for the second decade, it dawned on me that it is quite impossible, or as a minimum really messy, to combine resistors from networks in other decades. With a matching requirement of +/-0,037%, that is not easy to do in any case. If a PCB layout is ever constructed, it will be much easier to bite the bullet and spend another 10-15 Euro's on trimpots and also make the second decade with trimmers. The third decade needs a matching of +/-0.37% and that can be met by using 0.1% individual resistors or a network with that precision, or also use trimmers.How that will turn out is largely depending on the available resistor values and precision for the networks. The AORN, NOMCA and the OSOP series look good, but the first two are limited in the available resistor values, and pack only 4, 7 or 8 resistors. The OSOP supports 1K, 2K and 10K values with 10 resistors in a package.For the first decade, I have found with my current KVD prototype, that the resistors should match the 10K value per resistor pretty well. Unless I'm wrong with that, this means that the first decade needs to have a small resistor of 100 Ohm in series with the 10K, in order to adjust it around the 10K value with a shunt of a 1M resistor and a 25K trimpot, following JD's and Kleinstein's suggestions. With the trimpot at 50%, the result is 10K0, and there is a +/- 1.2 Ohm trim possible. That span will most likely be too much for a single turn pot in order to get a match within +/-0,0037% because for 10K the span is only +/-0,37 Ohm. Values can still change to a tighter span, but that depends on the specifications of the available networks.Thank you all in advance for taking the time to contribute!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2016, 03:44:41 pm
content: You only need the series resistor if you need to get the adjusted value to 10 K exactly, so that to resistance of the whole KDV is at 100 K. Just for matching to the second decade one can add an adjustable resistor in parallel to the second decade. In this case the total resistance of the KVD might end up at 99 K or so, but this is usually not such a big problem.With the resistor networks it's not that practical anymore to do individual matching. One could still use adjustment for the whole decade resistance and rely on matching inside the decade.An important factor for the mechanical form are the switches. Also don't forget about the extra contacts needed to do the adjustment in circuit. With the 10 K resistors one also has to have an eye on isolation. So a printed board might not be that good here - without a board these SMT resistor array may not be that practical anymore. For the lower resistance and less precision lower decades a board might be OK.

author: Dbldutch
author_rank: Regular Contributor
timestamp: October 25, 2016, 02:43:18 pm
content: Good points! Thank you.After hemming and hawing, I decided to forget about the use of resister arrays. At first they looked really good because of the resistor-to-resistor low ppm values, but because you can't select them on value, the whole price advantage goes out the window.So, I guess that if you really would like to have a reduced TC effect on the KVD, you need to go for low PPM/C resistors and because they are too expensive to buy in bulk and then do the precision matching, you have to use individual shunts.Looking at the TC effects on the first 3 decades, the PPM budget of the decade must be pretty low to stay within the +/- 0,0037% (or 37 PPM/C) matching requirement for the first decade. (BTW, where is this 0,0037% number coming from?) The second is +/- 0,037%, or 370 PPM, and the third is still +/- 0,37%. If you select 5 PPM/C resistors, my hope is that you can reach some reasonable stability in your hobby environment. Provided you can stay within a few degrees temperature variation between calibration and then taking the measurements.In any case, I found 10K2 5 PPM/C 0,1% resistors (PTF5610K200BZEK) so they are easy to shunt towards 10K0 for less than 2 Euro's and shunting them with a 510K resistor and a 19 turn 5K trimmer (T93YB502KT20) may actually work. For the second decade, I found a 2K1 resistor (PTF562K1000BYEK) 10 PPM/C 0,1% and if shunted with a 42K2 and a 19 turn 1K trimmer you can trim just above the 2K0 value, so a decade shunt can be used to trim the decade to decade trimming. The third decade can be created with a 1K resistor (PTF561K0000BXR6) with 15 PPM/C and 0,1%. With this precision you may not need an individual shunt, otherwise you can use one with 100K and a 19 turn 10K trimmer. A decade shunt will do the rest, same as with the other decades, but they will only need 1% or less precision to stay within the matching requirement.For my second KVD prototype, I need to purchase some of these parts and do some experimenting myself, while learning to deal with this TC circus, which I am pretty new at.

author: JS
author_rank: Frequent Contributor
timestamp: October 26, 2016, 05:21:29 am
content: Kleinstein, why wouldn't you trim the resistors within a decade using a network?The resistors are in series and adding the trim circuit is just the same as in the schematic before. If the matching from factory isn't good enough the trim could be added, and the better the matching of the original network the lower the effect of TC of the trim circuit.The value of 37ppm is there for a very good reason, the 250µV instead of 270,27µV for the desired 40ppm in the design. IIRC, the 40ppm are there to try to get 20ppm of absolute accuracy. As a comment 10ppm where archived at the end of that project, but not assured by design. That design to that level of confidence is to be able to build it with common metal film resistors, if you are going for a more expensive option with pretrimmed arrays with very good tracking or 0.1 resistors with 5ppm/C you should probably aim higher, thus better matching of the first decades are needed. Also I'd try to get higher power rating for R3 to prevent self heating. If you are aiming for 20ppm you are probably good enough with the original design.Using TH resistors I don't know if insulation is still a problem, probably not as long as you keep the PCB clean. Even then, to avoid insulation errors and still have an easy way of building the trim you could use small PCBs mounted over stands and each PCB would receive one end of the 2 resistors (R1 and R3), have the trim pot and connect to the other end of R1 and R3 of the next stage. The alternative is using guard traces from a dummy divider in parallel with the main one guarding each node of the primary from the respective node of the dummy divider. As I said, with big TH resistors, you could even add cut outs, probably not needed of extra efforts. With SIP TH networks the guarding could be done.JS