Tell me about precise resistor networks (6.5 digit multi meters etc). (laser cut - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 17, 2013, 10:38:49 pm
content: So if you ever took apart a precise meter you would see that they use an odd resistor which looks like a ceramic tile with metal laid and laser cut on top of it.Could this kind of performance be reached by using discrete components, like maybe foil SMD resistors mounted on a heat sink with the super thermally conductive sheet I saw mentioned in another thread (having thermal conductive properties similar to diamond)? (pyrolytic graphite sheet)Are those laser cut resistor networks used for cost reasons or strictly for performance reasons? I imagine that it might be cheaper to create your own laser cut resistor networks if you invest in the machinery and decide to build 10,000 meters.Is it possible to get resistors like that made? I want to gut a parts meter now.......

author: eurofox
author_rank: Unknown
timestamp: October 17, 2013, 10:46:25 pm
content: Quote from: SArepairman on October 17, 2013, 10:38:49 pmI have a resistor decade full of hand made precision resistors, of ourse with the right resistor wire you can make precision resistors.eurofox

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 17, 2013, 10:52:41 pm
content: Quote from: eurofox on October 17, 2013, 10:46:25 pmBut how does that compare to those laser cut "hybird" things?I wish to find more information about them. Resistor wire seems archaic.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 17, 2013, 11:34:45 pm
content: Hi,These resistor networks were made by Caddock, which is now part of Vishay.You can buy the resistor already trimmed.http://www.caddock.com/Online_catalog/networks/networks.html?gclid=COjHlMODn7oCFZMDOgodVE8AFwI believe there are some standard ratio available.I don't think that are as good as the hermetically sealed wire-wound resistors found in the top meters.Jay_Diddy_B

author: Harvs
author_rank: Super Contributor
timestamp: October 17, 2013, 11:50:03 pm
content: You can even buy them off the shelf:http://au.mouser.com/ProductDetail/Caddock/HVD5-B20M-050-05/?qs=%2fha2pyFadug1%2f5m8%252bxnOObp8xgdyc8ZABYhpvhvX3oXxdoJdd6bxSw%3d%3dOr element14 have what I believe is the custom type:http://au.element14.com/caddock/t914-1m-100-10/type-t912-t914-precision-resistor/dp/1696964i.e. they show out of stock, but I'm pretty sure that's because you contact them and tell them all the details you want. Vishay do the same thing with their custom resistors.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 18, 2013, 02:55:41 am
content: That seems not so good. The PPM of vishay foil resistors is better then those that you linked.Is there any advantage to using those over getting metal foil resistors? Especially if you thermally couple them...0.2 PPM seems alot better then 5PPM..Do those meters only use ~5PPM networks?are lower temp co networks available?They seem inferior so long you are capable of digitally adjusting for the ratio (they might not be matched as well).Then again, 100:1 ratio is hard to achieve with the ultra low temp co metal films (which only go up to 10K) with a decent impedance.http://www.digikey.com/product-search/en?pv1=37&FV=fff40001%2Cfff80482%2C4400c9&k=resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

author: Harvs
author_rank: Super Contributor
timestamp: October 18, 2013, 04:37:07 am
content: You do note the minimum order price of the parts... $97,816.25And that only gets you 250 of them. So for the price of three of them, you could buy a 6.5 digit Agilent meter...And their voltage coefficient:? Voltage coefficient: < 0.1 ppm/VIs 5x worse than the Caddock dividers.Another thing to consider is working voltage. Those dividers are available up to 5kV.As for TC, they're available at 2ppm/C. That may well be enough for in a temperature stable lab environment.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 18, 2013, 10:12:50 am
content: Quote from: SArepairman on October 17, 2013, 10:38:49 pmWell, in DMMs, there are different applications for those resistor arrays or assemblies.The first type contains a set of stable reference resistors, for the different OHM ranges ( 100 Ohm, 1k, 10k,...), dividers for DCV amplification (x10, x 100, etc.), and perhaps also shunt resistors for current ranges.Low T.C. and low T.C. matching in the case of dividers is required.As most modern instruments have SW calibration, the absolute value of resistors, or their ratio, has not to be that accurate.The advantage is, that the complete set of stable resistors is produced in one run, and their stability figures are identical. Therefore, no external T.C. matching of discrete resistors is required.Additionally, the component count on the PCB is smaller.The 2nd application type are the DCV input divider for 1kV and 100V.Here, a fixed divider ratio of 100:1 is needed.For more simple DMM designs, mutiple ratios of 1000:1, 100:1, 10:1 are required.This divider design is very delicate, if you think of 1kV input voltage.The reason is the high power dissipation of typically 100mW (1kV^2/10MOhm) , which creates non linear behaviour of the divider over increasing voltage, which leads to an additional uncertainty of many ppm in the region between 100V to 1kV.This non linear effect originates from the different power dissipations over the 100kOhm and the 9.9MOhm part of the divider, especially if you design that divider with discrete resistors.From 100V to 1kV, the 100k is loaded with 0.01 to 1mW, but the 9.9M with 1 to 99mW.Therefore, the 9.9M resistor will heat up over that input range, typically 10°C, but the 100k nearly stays at the same temperature.Well, the metal foil resistors have low T.C. of <2ppm/K,therefore 10°C difference for the 9.9M creates an additional mismatch of the ratio at 1kV of 20ppm (10 K * 2ppm/K), compared to the ratio at 100V.The Caddock dividers have low T.C. of 10ppm/K and T.C. matching of 2ppm/K, but by design, they are additionally thermally matched:The 9.9M is physically bigger than the 100k, so the heating is lesser.And both resistors are thermally coupled, i.e. always nearly on the same temperature, because they are either on the same substrate or their substrates are bonded together, so that the 100k resistors is heated by the power dissipation of the 9.9M resistor.By their T.C. matching (2ppm/K), and their nearly equal temperature, their divider ratio will also stay nearly constant. This is specified by the voltage coefficient of 0.02ppm/V, giving 20ppm at 1kV.That's similiar to what you can achieve with metal foil resistors.If you inspect the specification of the famous HP 3458A , you will find an additional 12ppm uncertainty for 1kV, which arouses exactly from the mentioned thermal effect. HP also used such a Caddock like 100:1 divider. (That's RP7, the blue component in the picture below)This is very mediocre for such an instrument (0.01ppm resolution!).Other manufacturers can do better, although all of them also use those dividers.Their trick is, to measure this power dissipation effect once (at 100V, 500V, 1kV), and then make a linearization calculation by the quadratic power dissipation formula.So they attain uncertainties of around 1ppm, even at 1kV.Well, I hope my - a bit longer - explanation has shown, that this special case of resistor dividers arrays has got some non trivial 'magic' inside.Frank

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 18, 2013, 04:09:49 pm
content: I wish we had a "like" button. That was an excellent explanation!

author: saturation
author_rank: Super Contributor
timestamp: October 18, 2013, 06:37:08 pm
content: Same here, Dr. Frank has many highly informativeyet succinct posts.Quote from: Conrad Hoffman on October 18, 2013, 04:09:49 pm

author: Andreas
author_rank: Super Contributor
timestamp: October 18, 2013, 07:08:11 pm
content: Quote from: SArepairman on October 18, 2013, 02:55:41 amThe Vishay CNS471 are specced with <2.5 ppm/K for the ratio tracking.http://docs-europe.electrocomponents.com/webdocs/0032/0900766b80032fe4.pdfhttp://de.rs-online.com/web/p/widerstandnetzwerke/3468538/With best regardsAndreas

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 20, 2013, 12:25:48 am
content: that was an excellent explanation. I have this thread saved in my favorites now.

author: quarks
author_rank: Frequent Contributor
timestamp: October 20, 2013, 04:10:02 am
content: Very informativI have some Caddock USF and they claim to have Zero nominal TC.