Repeatability when measuring resistor - Page 1
Forum section: electronics / general

author: Anders Petersson
author_rank: Regular Contributor
timestamp: May 28, 2020, 04:46:12 pm
content: Hi. First post on this excellent forum!What's the expected precision/jitter when measuring reference resistors?I measure +- 2 ppm sample-to-sample. This obscures any small effect the day cycle may have. I use 4-wire mode with a new 6.5 digit DMM6500 and multiplexer card 2000-SCAN to measure a 100 ohm Vishay Y1453100R000V9Lhttp://www.vishaypg.com/docs/63187/zseries.pdf. Standard deviation 104 uOhm = 1.04 ppm.This performance comes at 10 NPLC and filter of 50 samples, giving one reading every 2 minutes. Line Sync on, Auto Zero on. It seems I will need to take measurements over an hour or so to trust the full resolution. Is that normal?Lowering to 5 NPLC and 10 filter (a tenfold decrease in sampling time) doesn't make much difference, increasing standard deviation only from 104 to 107 uOhm (however, this compares 24 h with 1 h sampling).I made an aluminium box for the resistor as shown in the attached photo. The aluminium box is connected to the shield of the thin 1 meter 4-wire cable, connected to DMM ground. However, I didn't ground the DMM low-side (in spite of dire warnings in the DMM6500 manual?!)For background, my end purpose is to monitor the DMM6500 drift, in order to maintain calibration measuring PT100 temperatures. Therefore stability is more important than accuracy. 1 mK resolution is equal to 0.385 mOhm or 3.8 ppm. DMM6500 has one-year uncertainty of in total 105 ppm when measuring 100 ohm. If unchecked, the DMM could therefore add 26 mK uncertainty, more than the PT100 calibration accuracy of 15 mK.If I understand correctly, Vishay Y1453100R000V9L stability is 50 ppm according to datasheet "load life at 2000 h, 0.1 W/+70 C". (Or is that only after stabilization "PMO"?) That's somewhat better than the 105 ppm of the DMM -- at least it's a second opinion. And I suppose this means measuring 1 ppm resolution isn't really necessary...Any tips for me? Maybe I expect too much by scrutinizing the very last counts of this 6.5 digit DMM?EDIT: Changed title "jitter" to "repeatability"

author: Kleinstein
author_rank: Super Contributor
timestamp: May 28, 2020, 05:16:16 pm
content: Jitter is normally used for phase noise, not normal variations.The 100 Ohms, 4 wire resistor measurement can have slightly higher noise than more simple voltage readings. This comes from a few reasons: one can be frequent switching between the high and low side of the resistors. The test current should be around 1 mA and the voltage reading thus in the 100 mV range, were the input amplifier can contribute quite a bit to the noise. The 1 mA current source inside the meter may also very well have some noise / variations. Usually the different current sources share the same active part (OPs) and these are thus a compromise for both large current like 1 mA and small current like 1 µA. Quite a few, even higher rated DMMs use a non AZ OP for the current source - so there can be quite some extra 1/f noise for the resistance ranges. I don't know the details on the relatively new DMM6500 - so maybe it's better than average in this aspectThe relatively high uncertainty for the 100 Ohms range does not give much hope for exceptional good performance.Even with higher grade meters the protection of the Ohms low input (and often also Ohms sense high) is just a resistor chain, that can add to the noise.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 28, 2020, 05:17:31 pm
content: From the pic, it looks like there is a cold solder joint between the white/red wires and the resistor's terminal, like if the terminal was not heated enough. Don't know about the other terminal, might be the same.A solder joint is well made when the solder "flow" and "wet" the surface of the terminal and the wires by itself. The soldering iron should be removed only after the solder starts to flow and wet the surfaces.If you are worried the terminal might get too hot during the soldering process, you may attach an inverted tweezers or a surgical clamp on the terminal, right near the resistor's body, in order to dissipate the heat while allowing the time for the solder to nicely flow and wet the terminal all around.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: May 28, 2020, 10:23:18 pm
content: Quote from: Kleinstein on May 28, 2020, 05:16:16 pmThanks, I changed the title to "repeatability", hopefully that is more correct.Quote from: Kleinstein on May 28, 2020, 05:16:16 pmThis is probably at the heart of the matter. Anyway, the DMM is clearly delivering more than promised, graphing 1-2 digits more resolution than 6.5 digits. When using the scan card, I've seen quite a lot of bugs that I plan to report. Apart from that, I'm very impressed by all features.

author: The Soulman
author_rank: Super Contributor
timestamp: May 28, 2020, 10:49:06 pm
content: Did you test with and/or without scan card?That could also be a source for (small) errors.

author: ramon
author_rank: Regular Contributor
timestamp: May 29, 2020, 12:45:08 pm
content: Quote from: Anders Petersson on May 28, 2020, 04:46:12 pmI would like to see that question answered too.From my own experience (and from the accuracy information provided by several DMM specs) I have come to the conclusion that any attempt on doing long term ppm measurements without environmental control (mainly temperature and humidity) for both DMM and resistor is completely futile.The following graph is a 10K wirewound resistor from a precision manufacturer in Italy that was 1ppm flat for almost 4 days (the two spikes were probably me on the room doing something).As that measurement was made without any temperature control I think that those 4 days were the conjunction of Jupiter and Uranus, and the correct lunar phase cycle too.I have no access to a proper resistor reference. As a beginner, I wonder if we can all be enlightened by a reply from any ppm expert throwing a 30 days graph or raw log that shows 1 ppm flat for at least 1 month.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: May 29, 2020, 04:12:19 pm
content: Quote from: RoGeorge on May 28, 2020, 05:17:31 pmThanks for noticing. I opened up and saw the wire was correctly soldered on the backside, not visible in the picture.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: May 29, 2020, 04:28:47 pm
content: Quote from: ramon on May 29, 2020, 12:45:08 pmTrying my skills at analyzing your graph: I see a recurring spike every midnight, between 0.5 and 1 ppm. This hints there is indeed a temperature dependence. There are also deviations on the scale of a few hours which I suppose come from changes in the environment. The general precision (reproducibility?) is +- 0.5 ppm. I don't think that comes from the environment, and the effect will be there even for short-term measurements. Where you have +- 0.5 ppm, I have 2 ppm... today when I try NLPC=5, filter=5, I get +- 4 ppm.

author: Bill158
author_rank: Regular Contributor
timestamp: May 29, 2020, 05:43:49 pm
content: I am not worrying about the specs of the meter or resistor. The biggest mistake I see in your picture is that you "twisted" your Force and Sense wires together and then soldered them to the resistor leads. That is a BIG no-no for high precision measurements on resistors.Separate the wires and then decide which colors will be the Force leads and which colors will be the Sense leads. Then solder the Sense leads closer to the resistor and the Force leads farther away from the resistor on the resistor leads. This will then help eliminate any soldering resistance affecting the measurement. This is because the Sense leads are relatively immune to any solder joint resistance that may occur. Sense leads are generally high impedance and draw very little or no current while the Force leads are conducting what ever current the meter is forcing to make the measurement, therefore there will be some voltage drop from the wires to the resistor leads due to the solder joint.Also wrap the wires around the resistor leads before soldering so that the wires are providing the contact and NOT the solder.Read this thread on EEVBLOG about Standard Resistors and their construction. The Fluke 742A pictures are very good examples.https://www.eevblog.com/forum/metrology/teardown-standard-resistors/Bill

author: ramon
author_rank: Regular Contributor
timestamp: May 30, 2020, 07:16:56 am
content: Maybe we can just call it noise instead.Meter specs usually report best accuracy/TCR for measurements near 10K range. Lower ranges and higher ranges will have their extra issues : cabling, soldering, high test currents or voltages, self-heating, availability (or lack) of low noise precision high resistance values ...My noise measurements on 100 ohms range for a 127.07 wirewound resistor (same Italian manufacturer as the 10K resistor) are not as good as my previous 10K log (and not that much better than your results). I don't know yet if this is DMM, resistor issue (or both). So far, I have tested only with wirewound resistors (I don't have any Vishay hermetic resistor, or reference resistor).Maybe it would be better to reformulate the question in this way:What was the lowest noise you have got on a resistor measurements for <x> range in <h> hours?For my R6581T :at 10K ohms range with 10K wirewound : 0.5 ppm (less than 4 hours)at 100 ohms range with 127.07 wirewound : 1.5 ppm (less than 4 hours)The old and experienced person that sold me the R6581T told me that controlled temperature was needed for this meter. And I just can say that he was right.When you keep doing measurements for several days without temperature/humidity control then bad things happens (at least here in Taiwan were we can have 24 degrees at 90% humidity and next day 30C degrees at 60%).I invite others to share their experience, graphs and logs here too.

author: MiDi
author_rank: Frequent Contributor
timestamp: May 30, 2020, 12:52:39 pm
content: Quote from: Bill158 on May 29, 2020, 05:43:49 pmHi Bill158,could you please explain why there is a significant difference if sense + force are at same joint and if sense is closer to resistor case?In both cases the joints at the sense wire form a T-connection.

author: JohnPi
author_rank: Regular Contributor
timestamp: May 30, 2020, 05:44:41 pm
content: Imagine that the Force wire drives 1 mA through the wire, solder connection and DUT. If the wire had (say) 20 mohm and the solder connection 3 mohm, it would actually require 123 mV to drive this. The DMM doesn't have any problem doing this. However, if you measure the voltage at the Force terminal of the meter, you would see 123 mV and conclude (incorrectly) that the DUT resistance was 123 mohm. Instead, using the Sense line, you measure the voltage drop across the DUT alone -- getting 100 mV. Since the Sense line draws negligible current, a) all the 1 mA actually does flow through the DUT, and b) the resistance of the Sense line and its solder connection has no effect on the measurement.Separating the locations of the Force and Sense connections lets you control precisely what is being measured -- it's the DUT (plus some small resistance of the wires leading into the DUT itself), but is not affected by any solder or measurement wire characteristics.This 4-wire arrangement (do the same on the GND end) is also called a Kelvin connection.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2020, 06:14:33 pm
content: It is kind of intuitive to have the sense wires close to the resistor element. However as a 4 wire resistor one would measure exactly the same resistance if one swaps the sense and drive pins. So it does not really matter which contact is more to the outside, unless the test current is so high that a weak contact can not carry the current.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 30, 2020, 07:08:29 pm
content: Drive (in DMM6500 called input) and sense wires are NOT interchangeable, and the sense wires must stay near the resistor, and not on the other side of the solder joints (relative to drive wire soldering point), or else resistor's terminals resistance will be included in the measured Rx, which is not what we want.Also, it is not a good idea to solder the drive and sense wire to the same point, because it might happen for the sense wire to collect the voltage even before the soldering joint of the drive wires, so this time we might measure (Rx + Rterminals + Rsolder joints) together instead of measuring only the Rx.That is why for very precise measurements the connections should be made with 4 solder points, and sense wires should be near the resistor, just like it is shown in fig.35 in the user manual:https://www.tek.com/-/media/files/dmm6500-900-01b_user_aug_2019.pdf

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2020, 08:10:17 pm
content: This order of wire placement is especially important on low-value resistors used to measure current. Good “shunt” resistors come with four separate terminals to facilitate the four-wire connection, including some SMT units.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: May 30, 2020, 08:42:19 pm
content: Quote from: RoGeorge on May 30, 2020, 07:08:29 pmHere's the figure from the user manual.I can certainly solder like that, but like MiDi's question, I don't exactly understand why the order matters. If sense is closer to the resistor, the DMM will only measure between these points, yes, but if the connections were reversed, the extra length of leads between the sense and drive solder points should not affect the measurement as they're no longer carrying current -- the resistor leads become part of the sense wires. But this is only true if the solder points are as close to the resistor regardless if they're sense or drive.It seems to me the IET SRX 1Ok in this post is wired with sense further away:https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg891673/#msg891673Anyway... not a hot topic for meI will resolder.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 30, 2020, 09:04:28 pm
content: I was heaving in mind the drive wires are already soldered somewhere, then moving the sense wires soldering place from the interior to the exterior will not be good.If instead, all 4 wires remain soldered and only the banana plugs are switched at the instrument panel, then the circuit doesn't change, indeed.However, for that switch at the front panel to be allowed, we need to tacitly assume the wires for the drive and sense are the same, which might be identical for this instrument, I don't know if they really are, but for the general case of 4 wires probing the wires might not be identical. Force wires can be for hundreds of Amps (e.g. wiring a 200A/50mV shunt in some other equipment), or maybe the sense wires can be shielded while drive wires might be not, and so on.

author: Bill158
author_rank: Regular Contributor
timestamp: May 30, 2020, 09:36:33 pm
content: Ok so to go a little deeper into this subject, without writing a "doctoral theses". JohnPi has correctly explained the effects of the solder connection and the resistance added by it.In looking at your picture I can see that the Red/White pair is twisted together and then "tacked" to the resistor lead. The solder hasn't even flowed around the resistor lead. I can't tell how good the Black/Yellow wires are soldered to the resistor lead but it doesn't look good either. Let us assume that the White and Yellow wires will be the Sense leads back to the DVM. The best possible connection for the Sense wires would be to wrap them around the respective resistor leads 2 - 3 times tightly and then solder them. This way the Sense wires are making as tight a contact to the resistor leads without any solder between them. This provides the best chance of preventing "Thermo Electric" effects of the Sense wires through solder and then the resistor leads and any possible noise that might be generated here also. Solder between the Sense wires and the resistor leads can generate a Thermal EMF, quite small but could be noticeable. As JohnPi says the Force leads have enough compliance to overcome the slight drop across the solder joint IF they are just "tacked" onto the resistor leads. It would still be better to also wrap the Force leads 2 - 3 times around the resistor leads, farther away from the resistor body, and then solder those also. This is just to make sure of a very good connection to the leads and NOT generate any noise.Then any "noise" in your measurements will be because of either the resistor, your measurement leads or your DVM's voltage measurement circuits. I see that the DMM6500 is a Tektronix meter. In looking at the spec sheet to measure 100 ohms it Forces 1 mA which results in 100 mV across the resistor, which puts the DVM on it's lowest range which is usually the most noisy for digit jitter. I am not sure just how much noise this DVM is adding to the measurement from the spec sheet in a quick look at the specs. As usual they are trying to make the spec as confusing as possible and look as good as possible at the same time. But Tek also states that making the measurement through a "card" may "introduce additional noise". It is not clear from your post if you are making the measurement through a card or directly from the front terminals. All DVMs have some digitizing "jitter" with the last digit which is why there is usually a "+/- number of least digits" in the spec. I don't see this in the spec so I am not sure as I have NO experience with this DVM.Anyway all I am trying to do is to explain how to make your "standard resistor" box so that you can eliminate that as much as possible from the measurement.Then there is the subject of the measurement leads from the DVM to the resistor and Guarding them to prevent any noise pickup in the leads. I don't see any Guard terminal provided on this meter. Guarding is another whole subject that I don't want to get into right now. But somehow you need to take care of shielding of the leads and the resistor box. I would have to download the DMM6500 manual and then see what and how a Guard or Ground is handled for this meter's terminals. OK so I did just download the complete manual. A quick scan of the manual shows me that the Force Low (INPUT LO) lead is at ground. So all shielding of measurement leads should be connected to the INPUT LO lead. You may have to work this out by trial and error. The diagram that you just posted really doesn't show any Guarding or Grounding outside of the meter, i.e. leads and resistor box. It is not clear from your picture if there is shielding around your measurement leads. I do see some sort of "pigtail" that might be coming from the measurement leads.Bill

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2020, 11:03:04 am
content: The soldering of the wires is not that critical, as long as one does not produce a cold solder junction. Thermal EMF at the solder joint is often not an issue, as the temperature difference over the small solder drop is usually very small.With a relatively low resistance the guard is not important - this would be a thing with a resistor in the MOhm range.A point contribution is more from thermal EMF at the plugs at the meter. For this reason the meter may offer a mode where the voltage is measured with the test current on and off instead of a classic zero reading. Not sure the DMM6500 offers this and if it is active.The resistor from the initial post looks better than most resistors insider the meter - so I would not worry so much about the DUT. It is more about variations inside the DMM. It is reading a small voltage and also the current source to generate the 1 mA test current.

author: dietert1
author_rank: Super Contributor
timestamp: May 31, 2020, 02:58:25 pm
content: I like the proposal to use a precision resistor for checking/calibrating RTD measurements.When i saw your box in the first image i thought: 1 ppm of 100 Ohm is equivalent to 100 MOhm leakage. Could you slide some insulator tube over the blank wires of the resistor in order to avoid them touching the desiccant bags?And as you wrote, it is recommended to ground all guard/shield. I normally use a 50 or 100 Ohm resistor between shield and PE, so i can see when there is a ground loop.I think that a standard deviation of 1 ppm is a perfect result for a non-ovenized resistor. The "0 ppm/K" is marketing - when you look up the data sheet the say like 0 ppm/K typical and +/- 1 or 2 ppm/K max. You could try and determine the TC of your reference resistor. Getting below 1 ppm means research.Regards, Dieter

author: Anders Petersson
author_rank: Regular Contributor
timestamp: June 05, 2020, 11:36:39 am
content: Thanks for all suggestions! That's a great welcome for me.Clarifications: The box was connected to the shielding of the cable and connected to DMM ground, but the DMM sense itself was not grounded. The measurements were done through the 2000-SCAN card.I have resoldered as per the attached picture. This moved the averaged reading closer to the nominal 100 ohm value (from +36 ppm to +26 ppm, later increasing to +30 ppm again). Now I have also connected Force Low to DMM ground. However, the changes don't help with the noise. I get standard deviation 100 uOhm or on some runs even 200 uOhm (= 1 or 2 ppm). The attachment shows a 24h log, where at the end I increased the averaging from 5 to 50 readings without much improvement.Connecting through the front panel first gave 88 uOhm standard deviation, but that increased to 106 uOhm over one hour. This is without grounding however and much longer cables.Measuring via the rear card right afterwards gave 120 uOhm stddev with the same average (100.0030 ohm) so maybe the front panel makes a slight difference.But I've discovered the measurements are sensitive to nearby radio transmissions, even with the shielded cable and box. 1 W transmission adds magnitudes more noise. This means that wifi, wireless keyboards etc could all have a measurable effect.My feeling is that even without deliberate radio interference I'm simply at the limit of what my setup will measure. Since the noise frequencies seem to be in the order of seconds and minutes, I can average half an hour of logging to about 1 ppm reproducibility. If the resistor is stable, I'll get a good sense of any DMM drift well within the 100 ppm span offered by the DMM datasheet.I will test the temperature dependence later, using peltier elements.

author: Dr. Frank
author_rank: Super Contributor
timestamp: June 05, 2020, 12:45:25 pm
content: Quote from: Anders Petersson on May 28, 2020, 04:46:12 pmHi,the instabilities you observe depend on the DMM, proper statistics, the cables used, the resistor (DUT), and on environmental conditions like temperature and EMC (and some other, minor influence parameters). That's whole a lot of things to take care off, let's briefly summarize:1. The DMM drifts over time, temperature and shows noise effects, either internal or induced from external EMC.Temp (T.C.) and Time drifts are specified, but you should always use a room with constant temperature, like your basement.The external noise can be mitigated by proper shielding (cables, DUT in metal box), and by using the correct statistical function.That long averaging over 1 h makes no sense. Instead, use the longest NPLC, I think KEI is always NPLC 5, and let the DMM do statistics over about 1min, which is a good metrological practice.Your measurement then always consists of the average value (~ 100 or 10k) and the Standard Deviation which should be between 0.2ppm for higher grade DMMs, and maybe a few ppm for your DMM.Use the logging function, to detect long termed changes.Use the Kelvin, or 4 wire method, and additionally the Offset Compensation method, see page 4-22 in the manual.The 4W method eliminates all cable and solder joint resistances. So your 2nd solder configuration with 4 distinct solder joints is ok. The quality of the joint does not matter.The Offset Compensation eliminates any thermo electric voltages, so the solder joints and all connections between your DMM and your DUT do not matter also.2. The cables between the DMM and the DUT should have a high isolation, like PTFE, best then to have all 4 cables inside a shield, latter connected to your DUT case, on the other side to your DMMs Guard, which it unfortunately does not have.The isolation resistance of other cables, like PVC, may create leakage errors between plus and minus, on the order of several ppm @ 10k, if you drill plus and minus cables.So the 2nd best thing to drill the hi sense and hi force cables, and separate them from the drilled lo sense and force cables.Using 4W configuration, you can randomly interchange sense and force lines. It's not correct, that drilling sense and force is a no-go.Get rid of any EMC sources in your lab, like switch mode PSUs, LED lamps, and alike.3. The DUT itself also has timely and temperature drift effects.In this molded case it may drift about 20ppm / year w/o load (mostly not specified, just my experience), and also may show humidity effects, which you fight nicely.The T.C. is specified, but Vishay makes fuzzy specs. Usually, these have up to 2ppm/°C. (typical T.C. plus max. spread!)You could add a thermometer, like a precision NTC or a Pt100, bring it in close contact to the resistor, inside a separate aluminium block, and then you can always determine R(T).Initially you could change the temperature of this assembly and keeping your DMM constant, so to determine the real T.C. of your resistor.Afterwards, you could always mitigate the T.C. by inversely calculating the resistance from the temperature change.Question is, if your resistor is more stable over time than your DMM, the type you've chosen is not that appropriate for longterm studies.Another remark: 100 Ohm as a reference resistor has further disadvantages.At first, self-heating by the test current will give additional errors to be mitigated.Low Ohm resistors in general are more difficult to measure by the DMM, should be reflected in its spec.Therefore, the best Standard Resistors have usually 10kOhm, like the SR104, or FLUKE 742-10k, or the 'center' resistor of the 5450A.Maybe you have a look here, where we already discussed in length the characteristics and measurement on precision resistors.Here's how I started a few years ago:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg464413/#msg464413and this developed further, with better methods and cables, see pictures. DMM and DUT were not warmed up. Otherwise they are both in agreement, StD and repeatability is 0.2ppm with the 3458A, drift of these VHP202Z is about -1ppm/yr.Frank

author: Bill158
author_rank: Regular Contributor
timestamp: June 05, 2020, 08:18:54 pm
content: Quote from: Anders Petersson on June 05, 2020, 11:36:39 amI am still a little unclear about the above statement. Please look at the diagram on pg. 5/60 of the manual fig 106. This shows that the "INPUT LO" is connected to DMM ground so the "SENSE LO" will also be connected to DMM ground, at the resistor.Quote from: Anders Petersson on June 05, 2020, 11:36:39 amNominal 100 ohm value could have been affected by soldering on the leads of the resistor.Quote from: Anders Petersson on June 05, 2020, 11:36:39 amSee above about fig. 106. I am slightly concerned that you have created a "ground loop" as the "FORCE LO" is already connected internally to the DMM ground. This would be dependent upon exactly how and where you connected the cable "LO" wires, along with where the shield is connected to ground (or grounds).Ideally you probably should connect the cable shield to the resistor lead where the "LO" connections are made and connect the shield to the box also. Then leave the cable shield open at the end of the cable where it plugs into the DMM, but make the shield cover as much of the cable wires up to where they plug into the DMM. I am trying to eliminate a ground loop going through the cable shield, created by having the cable shield connected to ground at two different points separated by the length of the cable.This all may not cure your "noise". I agree with Dr. Frank, you may just be at the limits of what this DMM can measure.Bill

author: Tryer
author_rank: Contributor
timestamp: June 07, 2020, 04:31:17 pm
content: Hi everybody,initiated from this thread, i wanted to see, what the repeatability of my old 3457A is (new for me).Found a resistor near 300 Ohm and connected it to the 2w input of that DMM.I measured similar values as Anders Peterson. 120..150e-6 Ohm, in the 300 OHM range (stdev ~ 0.5ppm).The 3457A has a "nice feature" with front and rear inputs. Playing around and displaying the valueswith 1 s/measurement, i noticed that if i switched the inputs to rear (nothing connected there) andthen within 200-300ms back again, the value made a +20ppm jump, and then it came very-very slowly (minute)to the previous value with an e-function shape. This was repeatable.It did not happen by interrupting the resistor connection.I mean the DMM showed the same value again after reconnection.Bridging the K101 solved that problem but the nice feature went lost.I desoldered the K101 and K103. Comparing i could see that the K101 had ~10mOhm higher contact resistance on the Hi side andwas knock sensitive.I put them back changing their positions. The effect remained but got much smaller (~ +6ppm jump).Question: is that a known effect? what happens there? is it related to aging of the relay?Unfortunatly, i forgot to check if that happens also to in DCV.The relay type used is S4EB-L2-5V-H35. Info about?Any inputs/remarks are welcome.

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2020, 05:38:28 pm
content: I would never expect any precision measuring a 300 Ohms resistor in 2W mode. There are so many sources of error. You have the cables, the connectors, the front-back switch, the relays don't know what else. Your ppm results are a miracle.Certainly they are high by the cable resistances (at least 0.1 Ohm = 333 ppm of 300 Ohms). In case you have two more cables, you can check this by repeating the measurement in 4W mode.Regards, Dieter

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 07, 2020, 06:51:09 pm
content: Regarding interference, you might keep some large ferrite toroids around and try a few wraps through them to kill pickup. I know this works for my precision capacitance measurements, but haven't tried it with resistors.

author: Tryer
author_rank: Contributor
timestamp: June 07, 2020, 07:34:35 pm
content: Quote from: dietert1 on June 07, 2020, 05:38:28 pmHi Dieter and thanks for your remarks,it is only about repeatability and not accuracy. I am looking at the noise of that instrument and the effect of the relay switching. My PPM results are also refered to repeatability. They only say that after e.g. 100 samples my standard deviation is abt 150e-6 Ohm and this is 0.5ppm of the (end)value. Where is the miracle?My quetions were about the effect: i have some resistance in the relay contact which is not stochastic, and decays as if it were a discharging curve of an RC combination, with no irregularities and repeatable. Yes, in a 4 wire constellation it will be suppressed more or less. I will check that and report. The problem is that i have to remove the bridges below the relays.Greetings, Tryer

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2020, 07:55:54 pm
content: There are mercury wetted relays like coto 3400 and 3500 series. For most other relays a contact resistance instability of 10 mOhm is absolutely normal and the front-back switch may develop contact resistance of 1 Ohm unless you operate it every now and then. The problem arises mainly for contacts with small currents ("dry switching").Mechanical contacts may be good during the first years but later they become unstable. This applies to potentiometers and fuse holders as well. Even many connectors have this, e.g. IC sockets. Instead of buying new equipment you just have to move the switches and re-seat connectors.Regards, DieterPS: Your observation about slowly relaxing error may be caused by out of range measurement.

author: Tryer
author_rank: Contributor
timestamp: June 08, 2020, 07:47:23 am
content: Quote from: dietert1 on June 07, 2020, 07:55:54 pmT have checked that by interrupting the connection from the outside (pulling out the banana connector). It comes immediatly to the end value.What confuses me, is the fact that it decays smoothly. And it happens only if the relay contacts participate.regards, tryer

author: dietert1
author_rank: Super Contributor
timestamp: June 08, 2020, 08:46:42 am
content: Ok. that means your banana connector is better than the front/back switch. If you operate that switch 20 or 30 times to clean the contacts, the effect may vanish. Really, you cannot expect ppm measurements in 2W mode, except with resistors above 100 KOhm or so.Recently i happened to measure the old mains switch of a HP 3456A DVM. Switching dry (or almost dry) it had between 2.5 and 1.6 Ohms. I assume that resistance also vanishes after power-on.Regards, Dieter

author: Tryer
author_rank: Contributor
timestamp: June 08, 2020, 12:24:19 pm
content: Look how it looks like. Every time the same. You must imagine the 3457 measures in a loop once per second.Then, at the point of the jump, it had switched to rear for 200ms, switches back to front and continues measuring in the same loop.That means i have a contact with ON-resistance x mOhm, jumps after interruption to 6+x mOhm and then slowly back to x mOhm in such a smooth way. At least i would expect wild small jumps that happen so fast for a very small time-not to capture with an fs of 1 Hz. Why does it take so long ?But OK, i have so many things that i cannot explain, one more or less doesn't matter.Thanks again and regardsPS : And of course i have tried to clean the contacts switching the relay with a current of up to 1 A, many tens of times.As i told the other relay has a similar behaviour but reduced by a factor of 3.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: June 08, 2020, 01:56:57 pm
content: Perhaps these are some kind of thermoelectric processes. Time is needed to equalize the temperature.It is also possible this is a consequence of thermal heating of the measured resistor. After all, a current is supplied to it and it begins to come into equilibrium with the environment.

author: Tryer
author_rank: Contributor
timestamp: June 08, 2020, 03:01:16 pm
content: Quote from: MegaVolt on June 08, 2020, 01:56:57 pmIf the current is interrupted by pulling the connector out and reconnecting again this effect does not happen. It comes directly to the end value. So, it cannot be the thermal heating of the resistor.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: June 07, 2021, 08:21:13 am
content: One year later... Now I have bought a Vishay VHP101 100 ohm resistor (Y4078100R000T9L), which should be the best in its price class with the advertised "typically 0 TCR". (I'm aware these specs can be debated.) I mounted it like the previous Vishay Z series (Y1453100R000V9L) in a shielded metal box. I have also upgraded from Keithley DMM6500 (6.5 digit) to Keithley 2010 (7.5 digit).K2010 settings: Four-wire mode, offset compensation, 5 NPLC plus 20 samples averaging (12 seconds in total). I use scan card 2000-SCAN to alternate between the two 100 ohm resistors.A three day run was telling. At first, there's a big dip for 1.5 hour while the K2010 warms up. The specs say the warm-up time is 2 hours so this is ok (although much slower than DMM6500).After that, there's a clear correlation between the readings of the old Z series (yellow) and new VHP101 (green) resistors. The VHP101 starts out at roughly -6 ppm and ends at -1 ppm. The Z series starts at +2 ppm and ends at +7 ppm. I blame the K2010 for this overall +5 ppm drift. K2010 specifies the 24-hour accuracy as 15 ppm of reading + 9 ppm of the range. While this specification isn't the exact same thing as drift, I guess it's an indication of what to expect. (DMM6500 specifies 20+20ppm so almost the double).I haven't calculated the short-term noise yet, as was the original topic of this thread. Logging the temperature could be helpful too, but so far the drift continues unabated so I don't think it's due to temperature variations over the day.EDIT: The K2010 short-term noise is about +- 1 ppm peak-to-peak for both channels, which is roughly half the noise I originally measured with DMM6500.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2021, 11:30:26 am
content: With many DMMs the accurcy of the ohms current source is limited and may show quite some 1/f noise. This is also reflected by the often relatively poor accuracy in the ohms ranges compared to the voltage. Often a singel current source circuit is used and this is more like a compromise over the large range. So the lowest and highest ohms ranges are kind of compromises.For really accurate resistor measurments there are special resistor bridges.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: June 07, 2021, 01:30:27 pm
content: Thanks Kleinstein, the numbers in the K2010 datasheet agree with you. Here's the accuracy in ppm for the voltage and resistance ranges:Voltage:100mV 37+91V 25+210V 24+4100V 35+51000V 41+6Resistance:10ohm 60+9100ohm 52+91kohm 50+210kohm 50+2100kohm 70+41Mohm 70+410Mohm 400+4100Mohm 1500+4Resistance measurements are about double the inaccuracy and go off the chart in the high ranges.The unexpected conclusion of this experiment is that both resistors are useful as reference for my other measurements. I'm not likely to splurge on a dedicated resistance bridge. (But with test equipment addiction looming, who knows...)

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2021, 02:35:47 pm
content: Most of these DMMs use a constant current source and measure the resultant voltage to determine resistance. And they don't have different test currents for each resistance range. You can use another meter to check what is the test voltage when measuring a 100 ohm resistor with the 2010. I think you will be surprised and understand why accuracy is 5 digits and not 7 digits.Regards, Dieter

author: Anders Petersson
author_rank: Regular Contributor
timestamp: June 07, 2021, 03:16:15 pm
content: I don't need to test -- the current is specified in the datasheet. Both DMM6500 and K2010 have the same: 1 mA for both range 100 ohm and 1 Kohm.1 mA over 100 ohm is 100 mV. That suggests that the accuracy of the 100 mV range applies. The K2010 100 ohm range (52+9ppm) has an additional 15 ppm 1 year inaccuracy compared to 100 mV (37+9ppm). Perhaps this is reasonable for the additional current source to measure ohms.DMM6500 however specifies 30+35ppm for 100 mV and 85+20ppm for 100 ohm, so it has a bigger discrepancy between the two ranges than K2010.Just interesting to ponder.

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2021, 06:39:58 pm
content: The interesting fact is that these meters measure 100 mV to 5 digits or worse, although they are called 7.5 digits. The explanation is that avoiding thermal EMF and drift at a level of some uV or below is expensive (speak: missing). Even specialized instruments like the micro-ohm meter 34420A don't do it much better although it would use 10 mA for the 100 ohm resistor.The drift you logged was a factor 10 below measurement uncertainty and not significant.Your arrangement with two high quality resistors was a first step towards making a bridge. I got a SR1010 from ebay with 12x 1 KOhm precision resistors and wired it as a bridge to characterize a 500 Ohm hermetic precision resistor and demonstrate its faults at a 0.01 ppm level. It's in the thread with resistor temperature coefficient measurements.Regards, Dieter

author: Anders Petersson
author_rank: Regular Contributor
timestamp: June 07, 2021, 09:04:26 pm
content: Quote from: dietert1 on June 07, 2021, 06:39:58 pmLet's see.. K2010 1-year specs for 100 mV was 37 ppm of reading + 9 ppm of range. I suppose this could be called 5.5 digits. But even 3458A offers 9 ppm of reading + 3 ppm of range (100 mV), so then 3458A would count as 6 digits. As long as we realize that DMM resolution doesn't equal accuracy I'm ok with the drift. I think short-term noise is a more serious problem. My +- 1 ppm noise limits the K2010 to 6 usable digits.I'm also annoyed that calibration labs consider any reading within the 1-year tolerance as 'pass' without warranting adjustment. A freshly calibrated unit then has an uncertainty of the 1-year tolerance, and then it's supposed to drift for a year until calibration again.Quote from: dietert1 on June 07, 2021, 06:39:58 pmWell it was significant for my understanding.I now understand that drift is a fact of life even for an aged instrument, making a stable reference all the more important.Quote from: dietert1 on June 07, 2021, 06:39:58 pmIt's a 45 page thread... the closest post I found was this and it doesn't go into detail:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg2700574/?topicseen#msg2700574By "bridge", are you referring to a wheatstone bridge? How would that work?

author: dietert1
author_rank: Super Contributor
timestamp: June 07, 2021, 10:43:42 pm
content: In my understanding an uncertainty of 50 ppm means 1 part in 20 000, so this would be 4.5 digits of resolution.I think your way to of using two precision resistors to attribute an observed drift to your instrument instead of a precision resistor is the same idea as using a bridge. In a bridge circuit you can assert balance and precision by exchange of elements, thereby removing unknowns.Example setup:https://www.eevblog.com/forum/metrology/hp-3456a-terminal-block-and-wires/msg2759646/#msg2759646.Results:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg2751598/#msg2751598andhttps://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg2767686/#msg2767686.Regards, Dieter

author: MegaVolt
author_rank: Frequent Contributor
timestamp: June 08, 2021, 08:00:37 am
content: https://www.allaboutcircuits.com/technical-articles/hardware-techniques-of-linearizing-resistive-sensor-bridges/

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2021, 08:28:28 am
content: The calibraion uncertainty is essentially allways quite a bit high than the resolution - it has to. Often digits in addition to the cal uncertainty also can be used, e.g. to see short time changes of the DUT.Still the erros and noise in the resistance mode of the usual DMMs is quite a bit higher than with votlage. It starts with often measuring a small voltage like 100 mV, which is not the best range with most meters. The 7 digit meter may well be only 6 useful digits for the 100 mV range.There is additionally the uncertainy in the current source: In most cases this includes 1 ref. resistor and 1 resistor pair to transfer the reference voltage to a higher reference point. The Keithley2002 is an exception here with a charge pump for this. There are than usually 2 OPs that can contribute an offset. Of these one is often non AZ a high impedance version to also cover the µA level current. So this OP can contribute quite some low frequency noise.Some of the old Keithly 19x meters measure resistance with a kind of simplified bridge circuit and not a constant current source. So they may actually be relatively good with resistance, although the performance can vary (different from the current source case) over the range.The bridge circuits for precision resistor measurements are usually more like a Kelvin bridge or similar. So one can compensate lead resistance and use 4 wire mode. An alternative configuration (especially low resistance) uses a current mirror. For the high resistors (e.g. >> 1 M) the better way is to apply a constant voltage to the resistor and than measure the current - so more like a conductance measurement.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: June 09, 2021, 07:52:02 am
content: Quote from: dietert1 on June 07, 2021, 10:43:42 pmThat can be a useful measure but it's better to call it accuracy or drift-free resolution. Like Kleinstein said, the extra resolution is still good for something.Quote from: dietert1 on June 07, 2021, 10:43:42 pmThanks, yes I agree it's the same principle. This removes a big error term. Some remaining terms are DMM nonlinearity and noise.