Matching two 0.2ppm resistors - Page 1
Forum section: electronics / general

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 17, 2020, 09:16:05 pm
content: From time-to-time I am faced with the following dilemma:I am building a circuit which requires a pair of high stability matched resistors. Ok, so I go through my bag of such and pull out two 10K resistors rated at 0.2ppm and measure them....several times, at different temps....and determine:R1 = 10000.05473 ohmsR2 = 10000.13278 ohmsDelta = 0.07805Ok, seems cool. But having gone to all the trouble of measuring, characterizing, etc. I would like to eliminate that 0.08 mOhm difference. Seems like it should be doable, right?How would you go about it?This is not a question about the claimed 0.2ppm stability of said resistors--lets take that as a given. Its about matching w/out losing that stability.Thanks,Randall

author: TimFox
author_rank: Super Contributor
timestamp: August 17, 2020, 09:40:11 pm
content: You could put a 1.25 Gigohm resistor in parallel with R1, the larger nominal 10 k; that would bring it down to 10,000.05278, about 2 m(0.2 ppm) from R2. The tolerance and tepco of the 1.25 Gwill not matter.

author: razvan784
author_rank: Regular Contributor
timestamp: August 17, 2020, 09:40:29 pm
content: Cal' it out in software.Or add to one (or both) resistors a trimmer with properly chosen series and parallel resistors, like they do it in the Fluke 720A for example.To get a series of equal, nominally 10k resistors, the 720A uses a series of 9.896K resistors, each in series with (a 5k trimmer + 8.45k fixed resistor) || 100 ohms.The drift of those additional resistors is strongly attenuated because they only contribute a small fraction of the total resistance. The fixed resistor in series with the trimmer helps with trim range linearity.You can play around with the values in a spreadsheet.

author: srb1954
author_rank: Super Contributor
timestamp: August 17, 2020, 10:15:19 pm
content: Shunt resistor should be paralleled with the higher value resistor, R2

author: TimFox
author_rank: Super Contributor
timestamp: August 17, 2020, 10:24:57 pm
content: Oops--I got the resistor number backwards. Yes, the shunt goes in parallel with the larger value (as I stated, but got the number wrong).

author: ArthurDent
author_rank: Super Contributor
timestamp: August 17, 2020, 10:55:24 pm
content: I would add a piece of copper wire in series with the lower value and trim it to match the higher value 10K resistor. It won't take much to add the 0.08 ohms and probably the copper will have an opposite tempco so it might be more temperature stable. You could wind about 10" of #30 magnet wire on a high value resistor for a test to check it out.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 17, 2020, 11:13:39 pm
content: Quote from: razvan784 on August 17, 2020, 09:40:29 pmThanks for the suggestion. Indeed, I have a 720A sitting in the to-be-repaired pile with a broken, wait-for-it...5k trimmer (at least that is the suspect from failing the auto calibration).I would like to avoid trimmers if at all possible.I can't cal this out which is why I'm asking. These resistors will contribute to mismatched CMRR and non-linearity, I just want to get it right and forget about it.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 17, 2020, 11:19:02 pm
content: Quote from: TimFox on August 17, 2020, 09:40:11 pmThanks--I was not aware that reasonably priced gigawatt, I mean gigaohm resistors were obtainable. That is the simplest solution in terms of components. Finding a 1.25Gohm resistor seems to be non-trivial? Is there somewhere that actually sells those values? Quick scan of Mouser shows reasonably priced 1 and 1.5 Gohm only but maybe my search fu is failing me.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 17, 2020, 11:20:38 pm
content: Quote from: ArthurDent on August 17, 2020, 10:55:24 pmThanks, forgot about that trick...I will give it a try. I have plenty of WW#30.

author: garrettm
author_rank: Frequent Contributor
timestamp: August 18, 2020, 12:54:09 am
content: Quote from: ArthurDent on August 17, 2020, 10:55:24 pmProblem with this approach is that you are adding extraneous inductance to one of the resistors. For DC it might not matter, but go up in frequency and that DC match is worthless.If you go this route, I'd wind the wire on a sturdy wafer of mica like Fluke does to lower the inductance. Probably more important than matching sub ppm resistance is thermal EMFs generated by the added junctions of the series resistor. The circuit should be symmetric in terms of number of junctions so as to cancel out the effects of these voltages. So you will want a dummy wire in series with the other resistor to balance the number of junctions on each side.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 18, 2020, 01:20:24 am
content: "Problem with this approach is that you are adding extraneous inductance to one of the resistors. For DC it might not matter, but go up in frequency and that DC match is worthless."You don't know if the two 10K resistors have any inductance. Generally if you're looking for this degree of precision, you are talking D.C. only. If it is a problem just wind the added resistance noninductively.

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2020, 03:36:13 am
content: 0.08 ohms is available in this series, $2.00 in singles from Mouser.https://www.mouser.com/datasheet/2/427/lvr-1762341.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: August 18, 2020, 04:00:48 am
content: You could use a Peltier device to heat one and cool the other until they match!

author: Gyro
author_rank: Super Contributor
timestamp: August 18, 2020, 12:37:33 pm
content: Check them again after they've been soldered.

author: KT88
author_rank: Frequent Contributor
timestamp: August 18, 2020, 01:08:43 pm
content: Quote...that would make it DAC with digital control!

author: free_electron
author_rank: Super Contributor
timestamp: August 18, 2020, 01:11:10 pm
content: what you are after is tracking. absolute values don't matter.simply buy resistor made for that purpose !some companies sell resistors built on the same substrate so they are thermally coupled. i believe you can get bifilar traced thin film resistors. ( the meanders lie parallel to each other ) Susumu may have thatthat will give you two resistors that track each other thermally. so that problem is gone. as for the delta between them : that can be solved using a small value cermet trimmer. Use the non-potted version to avoid surface leakages.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 18, 2020, 03:00:45 pm
content: Quote from: Gyro on August 18, 2020, 12:37:33 pmNo, *already soldered*. Usually. I put them on to an adapter card and measure that and solder to it. Especially those smd types.But, yes, this has bitten me in the past.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 18, 2020, 03:21:45 pm
content: Quote from: free_electron on August 18, 2020, 01:11:10 pmUnfortunately, this is not a panacea. You are correct that tracking is supposedly good. The ones I considered were the LT5400-1A. Each package has four resistors. If I use two packages and interleave them then tracking should be (from datasheet) max of +-1ppm, which is good but not stellar. Then there is the absolute drift spec of +-8ppm typical (25ppm max!). In my application that does matter because I need a gain resistor and it can not be from the same package since it needs to be around 100 ohms.Susumu does make good product...it is also usually expensive.What I am actually using are two Alpha Electronics HC Z.http://www.farnell.com/datasheets/2078701.pdf?_ga=2.130320907.524586686.1597764025-1610028266.1560978470&_gac=1.58644184.1594401426.Cj0KCQjwo6D4BRDgARIsAA6uN1-crzMRZlIHSTFksT6zGl62PaovPGQJf5qUidNMwjJwDM1HYRMnmDMaAuu6EALw_wcB

author: guenthert
author_rank: Frequent Contributor
timestamp: August 18, 2020, 05:56:22 pm
content: Quote from: RandallMcRee on August 18, 2020, 03:21:45 pmI don't see the 0.2ppm there though. That would be an extremely tight specification for the resistance value. 0.02% (200ppm) would be quite good already. And that's what the 'Q' tolerance class stands for. I can't claim that I fully comprehend their data sheet (partly because I'm not familiar with MIL-PRF-55182/9 Specification), but stability over time seems to be given as 0.005% change (50ppm) over 10,000h (shelf life).

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 18, 2020, 10:43:16 pm
content: Quote from: guenthert on August 18, 2020, 05:56:22 pmThe 0.2ppm in the title of my question refers to the TCR of the resistor pair. Guess that was not clear. Sorry!As far as I know you cannot buy off-the-shelf resistors that guarantee 0.2ppm for the max spec (an LT5400 gives it as 1ppm max and 0.2ppm as typical). This is why I mentioned "take it as given that the resistors are 0.2ppm". The HCZ is +-1ppm TCR. It's not clear whether that is max or typical, but I did measure 0.2ppm.But you can certainly measure TCR and select pairs that meet these requirements. I built a DAC using 64 selected resistors combining different TCRs to get to my goal. That achieved about a 0.4ppm TCR for the first 8 resistor leg.

author: bgugi
author_rank: Contributor
timestamp: August 21, 2020, 05:51:48 am
content: depending on your layout, you could probably add trimmable traces on your pcb.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 21, 2020, 12:02:44 pm
content: Julie trimmed the resistors in their KVDs using short lengths of manganin wire. With the right gage you should be able to make the correction with an inch or so of wire in series and not mess up the stability. Manganin is nice because of the low emf against copper and the fact that you can solder it. There are somewhat better resistance wires, but they usually have to be spot welded.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 21, 2020, 03:46:11 pm
content: Quote from: Conrad Hoffman on August 21, 2020, 12:02:44 pmThanks--do you know of a source for small quantities of manganin wire??

author: KT88
author_rank: Frequent Contributor
timestamp: August 21, 2020, 04:50:27 pm
content: I was looking for Manganin wire on e--y recently and found even a roll of NOS wire for a few Euros. There are also sellers that offer wire in multiples of meters...

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 21, 2020, 04:53:36 pm
content: https://en.wikipedia.org/wiki/ManganinMaybe you could get Kanthal to sample some?https://www.kanthal.com/en/products/material-datasheets/wire/resistance-heating-wire-and-resistance-wire/manganin-43/Orhttps://pelicanwire.com/alloy/alloy-290/eBay seems to be loaded with the stuff.Goodfellow probably has it, but they tend to be expensive.I've often had good luck getting samples of things from the application enginering people. Calculate what gage range might make sense for you, as samples tend to be only what's on the shelf at the time. Or, maybe somebody on the forum will have some if you only need a small amount. Post what gage might work for you.

author: KK6IL
author_rank: Regular Contributor
timestamp: August 23, 2020, 01:27:15 am
content: I can give you a few feet of Manganin wire, 30 ga, 3.89 ohms/ft if I read this 50 year old faded ink correctly. Just PM me an address and I'll throw it in the mail. Anyone else also welcome.JohnKK6IL@AMSAT.ORG

author: guenthert
author_rank: Frequent Contributor
timestamp: August 23, 2020, 05:24:01 pm
content: Quote from: KT88 on August 21, 2020, 04:50:27 pmHow 'bout ISOTAN (CuNi44, quite similar to Constantan), claimed to be easily solder-able as well.https://www.conrad.de/de/p/thomsen-widerstandsdraht-28-m-5-m-429031.html

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 23, 2020, 07:45:41 pm
content: IMO, Constantan solders, but not very well. It also has a high thermal emf against copper. I've used it for resistors, but I'm not sure it's a great choice for precision work.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 24, 2020, 07:32:06 am
content: Quote from: RandallMcRee on August 18, 2020, 10:43:16 pmHello Randall,0.2ppm matching of resistors is an extreme undertaking, please think of similar arrangements like in the Fluke 720A, or any Wheatstone bridges.I assume from your descriptions, that you want to use both resistors in a similar bridge arrangement, or in ratio mode.The drift over temperature, after soldering and over time of the individual resistors, and any residual resistances like solder joints, PCB traces, and any T.C. mismatch will render a fixed trimming to 0.2ppm uncertain / invalid very soon after initial set up.Therefore you should definitely provide a 10-turn trimmer adjustment for about +/- 20ppm range.In ratio / bridge mode, that would be a 0.39 Ohm Thin Film resistor and the trimmer in parallel, between both 10k resistors.Then you need a provision to balance both resistors at the trimmer output to 0.1ppm. i.e. either a bridge arrangement with Null detection, or another detector in your complete setup which can really resolve this extremely small difference.I assume that you have a long scale DMM of some sort, which can be used in an absolute voltage comparison mode.The other method would be to determine the INL of your DAC of the different binary steps in a frequent manner and correct that mathematically. This also would require a 8.5 digit DMM with < 0.1ppm INL, like the 3458A.Frank

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 24, 2020, 11:05:24 pm
content: Just look sharply and intently at the resistor with the highest value until that exorbitant thing goes down.But don't blink or it will be up again.Are these resistors THT?Just moving your probe half a mm on the most offending leg will do.

author: mzzj
author_rank: Super Contributor
timestamp: September 05, 2020, 07:01:22 pm
content: Quote from: ArthurDent on August 18, 2020, 01:20:24 amThe resistors itself probably differ lot more than 0.2ppm in inductance. Likely laser-trimmed to slightly different geometry.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: September 06, 2020, 10:47:54 pm
content: Quote from: mzzj on September 05, 2020, 07:01:22 pmWell, my application is DC-10Hz or thereabouts. I'm not too worried about inductance or stray capacitance.