What is the next best ratio measurement device after the 3458A? - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 18, 2016, 09:41:29 am
content: Hi,I would like to measure ratios. The voltage ratios between two decade resistors connected in series for example (like Fluke 5450A calibration). To avoid trouble with my wife, I have to postpone the purchase of a 3458AAccording tohttps://www.eevblog.com/forum/testgear/8-5-digit-dmm/msg270530/#msg270530and some manuals there is no DMM which specifiy a transfer accuracy better than 0.1ppm (0.05ppm+0.05ppm HP 3458A). Even the Datron 1281 is specified with 0.2+0.05ppm in the manual. (The Fluke 8508A seems to be linear to 0.035ppm FS, but a 8508A isn't an option)I own a (noisy) Solartron 7071 (7081) and a Datron 1071 (7.5 digit but no transfer specifications found) as the highest resolution meters. These meters should be also far away from 0.1ppm linearity.Inhttps://www.eevblog.com/forum/testgear/hp34401-measurement-of-linearity/msg356304/#msg356304Dr. Frank showed a very good linearity of his Fluke 5442A (below 0.1ppm of input).Therefore, my question: If I could verify the linearity of my Fluke 5440B also to below 0.1ppm (I have access to some 3458A, but I have to carry the 5440B to the 3458A) isn't it a good ratio measurement machine in combination with my beloved HP 3456A acting as nullmeter? (I also own a Fluke 845 AR Nullmeter, but the setup with the 3456A could be fully automated).Or is there any cheaper multimeter out there with very good linearity?What do you think?

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 18, 2016, 11:15:31 am
content: Hello,The referenced link was a one pass linearity test only, and one quadrant.Somewhere else, I have published a multiple pass test on both quadrants.The median linearity of my 5442 was about < 0.3 ppm, additionally both quadrants behaved differently in gain and offset.As you see, this 0.1 ppm linearity is not achievable each time, that's a general phenomenom of A/D and D/A characterizations.The 3458A linearity test vs JJA in hpj 4/98 also shows fluctuations during different passes. The 0.02ppm linearity is therefore a typical, median value only.If you characterize your 5440, you may also achieve a median linearity of 0.2 ... 0.3ppm for one quadrant, but that's no guarantee.So, the Fluke 720A with its 0.1ppm guaranteed linearity of input is the next best solution.FrankEdit: Added complete 8 pass linearity measurement

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 18, 2016, 11:50:14 am
content: I would like to have an automated setup to make an offset compensated ratio measurement with short time between the measurements. With such a setup it should also possible to make all the ratio transfers in the 5450A automatically. My Knick JS 3010 can act als voltage and current calibrator. I would use this to supply voltage/current to the series resistors.Rewiring is only needed to transfer the 10k reference to the 5450A 10k.Therefore, I don't want a KVDAnother option/upgrade could be a computer controlled switchable 1:10/1:1 hamon divider which divides the output of my 5440B. Very similar to the described technique in the 5440A handbook, but automated.@Dr. Frank: How do you calibrate the 1R resistor in the 5450A? The manual says one should use a 1R reference resistor (same as stated in the 5700A calibration process). Do you transfer the 10R to 1R?I would like to know the 1R also as precise as possible (within my capabilities) to transfer this further to my 0,1R shunts.Thanks.Philipp

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 18, 2016, 12:03:54 pm
content: I use my 3458A only, and live with the mediocre calculated uncertainty for the 10 Ohm range at 1 OhmFrank

author: acbern
author_rank: Frequent Contributor
timestamp: September 18, 2016, 03:45:31 pm
content: I am using SR1010s for transfer of 10k to other resistance values. But these are not cheap either, depending on where you get them.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 18, 2016, 05:03:53 pm
content: Fluke 8508A was designed and build based on 1281 at UK Wavetek/Fluke after Fluke acquired Wavetek. The ADC(and Vref etc) were almost the copy of the 1281, so you cannot expect the linearity anything much better than that of 1281. This also evidenced by the same slow speed and panel arrangement of both meters(binding posts on the left, dual displays, similar number of buttons)The 8508A manual does specify the linearity, however, there is this Transfer Uncertainty of 0.12ppm+0.1ppm for reference, which specified by HP/Agilent as the same figure in 3458A manual. That 0.1ppm is of range of 20V and is 2uV, compared with 0.5uV(0.05 ppm of 10V for 3458A), four times larger than 3458A. Therefore, 8508A is not superior in this regard, it may not even be the next best thing after 3458A.In order to check the linearity, probably 11 points is enough: 0%, 10%,,, 100%. If that is the case, there is an old but effective way of doing it:build you own divider string consist of 10 identical resistors(like a Hamon), and buy or build a battery operated, isolated(thus with very low leakage, very high CMRR) null meter to check with the 5440B outputs with the taps of the sting. I sometimes use my Metra Hit 30M(0.1uV last digit, battery operated) for this purpose. The divider ratios of the string can be calibrated by another 1/10th of the voltage (floated, need battery powered again) by 'moving check' of voltages of every resistor-voltage with the help of the same null meter. Have a look at this article for similar configuration: 'A Sub-ppm Automated 1-10 Volt DC Measuring System'.BTW, 8508A has much better uncertainty than 3458A when measure 1 Ohm, probably because it is the only hi-digits DMM with 100mA test current on low Ohm range.

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 18, 2016, 09:09:56 pm
content: Hi zlymex,thanks for your great additional info.I found the 0,035ppm FS here:http://www.infinet.com.tw/001/member/download/fpm/thereferencemultimeter&ratiomeasurements.pdfIn the end, they say the same as you.A divider is a nice idea to check linearity without a 3458A. I think andreas uses a divider to calibrate the linearity of his LTC2400 ADCs. I will have a look at his technique. If I remember correctly he hasn't tuned the steps to be exactly 1/10th. He is measuring from one point of the divider (x/10) to 0/10 and to 10/10 or something like that.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 19, 2016, 12:31:05 am
content: Quote from: e61_phil on September 18, 2016, 09:09:56 pmI recall I had that info 6 years ago which looks like a market campaign seminar material to me that I tends to ignore. If 8508A has that kind of remarkable linearity, why not specified in the datasheet in the first place? How can one get a linearity better than repeatability/noise?BTW, Fluke does't specify the noise either, another critical piece of data that Fluke is ashamed to mention IMO.However, there is this paragraph in the specification tells the right thing:http://assets.fluke.com/images/products/fpm/bench_instruments/8508A_Extended_Specs_Rev_B.pdf--------I quoteWhen making measurements on thesame range, these errors will affect both measurementsand effectively cancel, leaving shortterm noise and linearity as the dominant errors.The 20 minute Transfer Uncertainty specificationsare provided to describe the performanceobtained when making ratio measurements onthe same range.--------End of my quote.Therefore, Fluke does know these two dominant errors are very important, which make up the Transfer Uncertainty.

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 19, 2016, 06:19:04 am
content: Yes noise and drift is a great problem for ratio, I think.It isn't easy to meet the 0.1ppm in 10min specs with the 3458A, but possible (my experience with some LTZ1000 tests). Therefore, it must even much more difficult to measure the 0.02ppm linearity. Are there some addinational tricks I don't know?

author: zlymex
author_rank: Frequent Contributor
timestamp: September 19, 2016, 07:04:09 am
content: Quote from: e61_phil on September 19, 2016, 06:19:04 amTricks I can think of- repeat measurement + recording. Later the data may be unpicked and averaged to decrease the type A uncertainty.- automatic switch of different voltage to ease the thermal EMF variation and speed up the test duration- reverse switch (better to be auto and repeat) to eliminate large portion of thermal EMF- stable and nice environment, good power supply and filter, wind shield on binding post, low thermal EMF test leads, enough warm up time.

author: tszaboo
author_rank: Super Contributor
timestamp: September 19, 2016, 08:49:11 am
content: Probably some analog circuits + a universal counter. It would not be that hard to make an analog integrator, null detector, make it as slow as you want, and then use the counter to make a ratio. You need a few good quality film capacitor analog switches few opamps and off you go?

author: wiss
author_rank: Frequent Contributor
timestamp: September 19, 2016, 10:06:26 am
content: Maybe try the method in this document?https://www.eevblog.com/forum/testgear/dmm-linearity/msg698554/#msg698554

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 19, 2016, 12:13:03 pm
content: Quote from: NANDBlog on September 19, 2016, 08:49:11 amSuch an easy approach won't work for more than 4.5 digits. The HP Journal which describes the ADC from the 3458A has described the problems a bit.Quote from: wiss on September 19, 2016, 10:06:26 amThat is very interesting!

author: tszaboo
author_rank: Super Contributor
timestamp: September 19, 2016, 03:05:48 pm
content: Quote from: e61_phil on September 19, 2016, 12:13:03 pmWell, I did it for some 1-2ppm linearity/accuracy, in noisy environment. It was not needed to be better. It is not rocket science. Take a look at the LT1043 datasheet 0.005% V/F Converter. With simple components, like a LM358 and a LT1009, they created a 50ppm accurate converter.It is not that hard to match the specs of high end meters for 1 range. To start with, you dont need an accurate voltage reference. You need a clock source, which has small drift in a 10 second timeframe. Any timer counter can do that. The oldschool 1989 active parts are easily outclassed today's laser trimmed opamps and chopper amplifiers. There are analog switches which behave nicely, almost no leakage, almost like the Jfets they had to use 30 years ago.It is actually a good experiance to build these kind of circuits, PWM dividers and voltage to time/frequency converters are really nice. Especially, if you want to have a 3458a.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2016, 04:23:24 pm
content: One difficulty with an analog integrator is linearity of capacitors and even resistors. So if you really want, it can get pretty expensive and large with vacuum capacitors.However it is possible to test DNL this way. The difficult part is INL.

author: gamalot
author_rank: Super Contributor
timestamp: September 19, 2016, 06:33:35 pm
content: Application Note 14 ?http://cds.linear.com/docs/en/application-note/an14f.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2016, 07:30:15 pm
content: In the AN14, there is a version with 1:10^8 range, that is 1 Hz to 100 MHz. But this is not linearity. It even looks like using adjustment pot for linearity improvements. Another Version called high linearity claims 7 ppm (INL I suppose), which I would consider very good for a U/f converter.Usually U/f converters have quite some problems with linearity due to non constant frequency combined with dielectric absorbtion and charge injetion. This is more like the old style before multi cycle charge balancing converters with CPU / µC control became the standard. Another trouble is that you can't synchronize with the line frequency very well.DNL tests are relatively easy, even to high levels from histograms.INL at some point could be checked with stacking of references - its not a full test of cause but only a few points to see slow varying components like square or cubic contributions.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 20, 2016, 12:19:27 am
content: Quote from: DiligentMinds.com on September 19, 2016, 08:11:41 pmGood advice. A very common failure of 3458A is the 'big bad chip'(U180 on A3 board). Here is another photo of collection of the replaced U180, most of which result in halt of the meter. There are many other meters that also suffer from less critical problems such as out of daily drift spec due to the same chip.There is only one way to solve the chip problem, that is, to replace the whole A3 board, unless you can find the source for good U180s.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 20, 2016, 12:47:40 am
content: Quote from: wiss on September 19, 2016, 10:06:26 amThat is a very intersting article, the technique is similar to the DIY divider of NIM, where they also use multi-test and equation solving.However, I don't think it works here because- the measurements are all direct, floated, with large noise, large CMRR problems and bias current difference of Hi and Lo input of the meter.- OP requires a ratio device to test the linearity of a source, not requiring a device to test the linearity of a meter.

author: wiss
author_rank: Frequent Contributor
timestamp: September 20, 2016, 07:21:55 am
content: Quote from: zlymex on September 20, 2016, 12:47:40 amYes, the measurements are floated and this puts significant requirements on the grounding scheme. And the bias currents has to be low enough to not effect the measurement, it will only work for a high-Z meter. And the resistors has to be of low enough resistance to not generate high thermal noise.QuoteBut a "stack of references", which this essentially is, will be linear, the voltages will add as they should, no?Nowhere I require the resistors to actually be linear, only to have a stable voltage over them.

author: tszaboo
author_rank: Super Contributor
timestamp: September 20, 2016, 08:41:51 am
content: Quote from: DiligentMinds.com on September 19, 2016, 08:11:41 pmWell, if the goal is to have an accurate meter, sure, a used 3458A is a good buy, though a 34470A is probably better, if you sacrifice some accuracy, and get it "half the price", more features. On the other hand, the gained knowledge about these circuits is really valuable.I did design circuits, which are more accurate than a 6.5 digit multimeter in one or two ranges. I tested recently released opamps, passive parts, topologies. Redesigned boards, because mechanical stress related drifts, replaced capacitors because micro-phonic effects. The few ppm region is really fun.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 20, 2016, 09:32:18 am
content: One more option to get a sub-ppm linearity check for not much money is to use the AD5791 evaluation board. My sample is below 1ppm INL over 0-10V scale (measured by HP3458A) and there is a space on the board to put LTZ1000 reference on (which I did). As the stability of the DAC output ratio is better than 0.2ppm, it should be possible to calibrate at least some points using HP3458A to about that level of non-linearity (or even compensate for it with a secondary correction DAC). Also this DAC can be used in a bridge configuration to measure the ratio directly using a null-meter output to control the DAC for balancing the bridge.CheersAlex

author: zlymex
author_rank: Frequent Contributor
timestamp: September 20, 2016, 01:21:34 pm
content: Quote from: wiss on September 20, 2016, 07:21:55 amGrounding scheme may solve the problem of external noise and interference, but no help for common mode error of the meter if the meter is "floated".Low bias current reduce the error due to Hi(Red color pen) input of the meter but not for Lo(Black color pen) input, because the Lo input leakage to ground is not specified, may be much larger than bias current even for high-Z meters. Also, there may well exist noise and interference at Lo input to the case(internal noise and interference). So if the Lo input is connected to the middle of a resistor string, it is problematic for sub-ppm level measurement. In the Cutkosky Divider system such as MI's 6000B (http://www.mintl.com/media/pdfs/6000b), where the DMM must be floated, but that DMM has a guard input terminal, and there is a dedicated DAC to generate the appropriate guarding voltage to solve the floating problems.Linearity refers to the differences of a range of supposed-to-be proportional values from the specified. A stable "stack of references" on it's own has no linearity unless their individual voltage values are precisely specified, but that will void the beauty of the article, isn't it?

author: wiss
author_rank: Frequent Contributor
timestamp: September 20, 2016, 01:49:39 pm
content: Quote from: zlymex on September 20, 2016, 01:21:34 pmIf there is a high enough V-low bias should show as an error in the measurement, it would not show as less error than there is (unless the other errors are "exactly" the opposite of that introduced by the bias), I certainly agree.And I do not think I would like to try this down to 0.1 ppm...QuoteWhat I mean is that if you stack V_1 on top of V_2 the sum should measure (V_1 + V_2), no matter what the actual values of V_1 and V_2 are.Problems could be introduced by thermal EMF in switches and the like...

author: zlymex
author_rank: Frequent Contributor
timestamp: September 20, 2016, 03:07:15 pm
content: Quote from: wiss on September 20, 2016, 01:49:39 pmThat is correct.However, the stack of voltages of not precisely known(although they are stable and roughly equal in value) cannot be used to check the linearity of Fluke 5440B.

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 20, 2016, 10:04:10 pm
content: Quote from: zlymex on September 20, 2016, 03:07:15 pmI thought it is a minor problem, if i connect the HP 3456A with the LO input to the 5440B. The 5440B should be a low impedance source and will drive the LO input an the guard of the 3456A. If neccessary one could also connect the guard from the meter with the guard of the 5440b instead of the output. The HI input has very low input current, so it shouldn't be a problem on a resistor chain.Is my assumption wrong?

author: TiN
author_rank: Super Contributor
timestamp: September 20, 2016, 10:20:03 pm
content: Quote from: zlymexThat photo with U180's is real pain to see, at least for me. Still scares me at nightQuoteBig difference of 3458A is it's linearity and resulting ACAL feature, to significantly remove temperature/drift of working references.34470A nowhere close in that area, making it nearly useless for ppm-level stuff, as it's challenge to distinguish DUT drifts/changes from meter's.But I'm sure you know that already, just to be clear for other readers

author: zlymex
author_rank: Frequent Contributor
timestamp: September 21, 2016, 02:45:39 am
content: Quote from: e61_phil on September 20, 2016, 10:04:10 pmI agree with this. LO input of a DMM should be connected to a low impedance source but should not be "floated" such as connecting to the middle of a resistive divider. I tried this with my 3458A(when I connected to 10k standard resistors in series to an 20V source) and got a very noisy result with the guard terminal connected to earth and LO,

author: wiss
author_rank: Frequent Contributor
timestamp: September 21, 2016, 07:23:25 am
content: I connected the low end of the divider to the divider case that was also connected to mains-earth-ground and all MEG of multimeters to that same MEG power outlet as the case, for unsyncronous meters (my PREMA and Keithley 192 for example) you get a very characteristic beating between line frequency and conversion clock frequency when you screw this up, with a synchronous meter I just got an offset...

author: e61_phil
author_rank: Frequent Contributor
timestamp: September 22, 2016, 02:58:25 pm
content: Quote from: Alex Nikitin on September 20, 2016, 09:32:18 amJust to get it right: If I would build a bridge with a AD5791 (specified INL 0,19LSB ~ 0,2ppm) and measure the residual voltage. My uncertainty in a voltage transfer (for example the voltage measured on a 10k resistor (well known) and a 1k resistor (unknown) 1:10) would never be smaller than 2ppm? Is it that simple?

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 26, 2016, 07:04:17 am
content: Quote from: zlymex on September 20, 2016, 12:19:27 amHello zlymex,can you please unfold, which error messages are related to the failure of U180?What do you think, is the real root cause inside U180 to let the 3458A halt?Maybe excessive drift of its internal resistors, so that certain ratios of these are outside limits?Is error 202 'Slave Test: Convergence' also related to such a failed U180?Thank youFrank

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 26, 2016, 07:23:41 am
content: Quote from: e61_phil on September 22, 2016, 02:58:25 pmSorry, the AD5791 is specified as 1ppm INL plus 1ppm DNL; both have to be taken into account.It's the stability of the INL only, which is 0.2ppm.Please check hpj4/89, page 22 and following, about the 10:1 transfer calculation related to DNL and INL errors.Therefore, you can't expect better than about 11ppm 10:1 transfer from that AD5791.If you characterize some ratios with a 3458A, I really doubt, that this would be a stable solution, due to varying code, noise, temperature influence (specified???), and so on.So, if you need a 10:1 ratio only, instead of random ratio numbers between 1.0 and 0.1, better build yourself a simple, fixed 10:1 Hamon divider, with self-adjust capability. (6 stable PWW resistors required, only)This will probably give better 10:1 transfers (at low voltage levels) than even a 3458A, <= 0.2ppm ofoutputis possible.Frank

author: zlymex
author_rank: Frequent Contributor
timestamp: September 26, 2016, 10:10:27 am
content: Hello Dr. Frank,There are 11(out of 13) chips were marked "??" in Chinese which means dead/halted in the photo I posted. This is the most serious error that U180 may cause to an 3458A. The halt may be occasional or only on DC range(according to the hand writing on the chips).There is one marked as "AC measurement Ok", which probably means DC will be halted as well.There is only one saying "workable but large tempco", I assume there is no error message associated with it.I've just asked the one who provided the photo(thy888), he said they were all halted with error message 114, and the auxiliary message being either Multi-slope ADC error or Time Out error.There are two root causes, one is the severe mismatch of resistors, the other is the bad input switching JFETs of the ADC. There are two paths for the signal, one is the high resolution one(50k resistor) the other is the high speed one(10k resistor).I once try to fixed an 3458A with large daily drift but failed, there is no error message, just like mine. At the end, I suspect it was due to the U180, had it replaced, and the meter is Ok. There was a thread at 38hot that record my work:http://bbs.38hot.net/thread-436-1-1.htmlhttp://bbs.38hot.net/thread-4104-1-1.htmlFor that error 202(Slave Test: Convergence), it is caused most probably by open JEFT(cannot be closed) of the high resolution switch for 50k resistor, but the high speed JFET switch will probably work, this can be verified by setting the NPLC to a very small value(so that only high speed path is used) and see if the meter will work. And yes, those 10k/50k resistors and JFET switches are all part of the U180 chip that cannot be replace alone easily.Edit: add a pic from HPJ 1998-04

author: Kleinstein
author_rank: Super Contributor
timestamp: September 26, 2016, 04:33:13 pm
content: Likely they will not reuse the old U180. It's failure prone and likely quite expensive. Today would likely not us a single converter for high speed and super good INL, as there are rather good fast converter chips (LTC2378.. and similar) that could be used for the very fast tasks and AC.They may still end up with a new similar chip though, but if they don't need high speed this could be a simpler version or than could get away with separate resistor array and switching chip, maybe even of the shelf parts. My guess would be more like continuous integration, integrator with AZ OP and than an ADC chip for the residual charge - so more like in between the 34470 and the Datron 1281. One thing they should really add is a temperature controlled fan and better RF shielding (way more cell phones and WLAN today).