What's in a precision multimeter that gives it 0.0024% accuracy? - Page 1
Forum section: electronics / beginners

author: SebG
author_rank: Regular Contributor
timestamp: August 19, 2014, 04:29:38 am
content: Hi. I'm was working on a project that required accurate voltage and current measurements. I calculated that theworst caseerror in my measurements would be 0.4% using 0.1% resistors and voltage references and including op-amps voltage offsets. So I wondered how accuracies as good as 0.0024% can be achieved in a multimeter such as Fluke 8846A:http://us.flukecal.com/products/data-acquisition-and-test-equipment/bench-multimeters/8845a8846a-65-digit-precision-multime?quicktabs_product_details=2Does it use, say, a high resolution (e.g. 24bit) ADC with a ultra precise Vref and resistors? Or does it just use like 0.1% resistors and voltage references that are stable with temperature (i.e. low ppm/oC) and then be calibrated and somehow compensated using another already precise multimeter?Since most multimeters have range selection and different types of measurements it probably needs some collection of op-amps, analog switches and voltage dividers; so how is such good accuracy still achieved? I suspect dual slope ADC would improve accuracy and remove most noise from the reading at the cost of samples/s but still probably not good enough.Anyone know anything about the topic that can explain? Are there any good schematics of a precise multimeter out there?

author: coppice
author_rank: Super Contributor
timestamp: August 19, 2014, 06:01:53 am
content: Quote from: SebG on August 19, 2014, 04:29:38 amAchieving phenomenal accuracy at one temperature and at one point in time is not hard. You just calibrate. Instrument makers are used to a lot of elaborate calibration processes on their production lines. Its very different from consumer goods, where calibration processes are loathed and detested.If you pay enough its surprising how low a temperature coefficient you can get for resistors and voltage references. Getting things with superb ageing performance is more of an issue, but a precision instrument is rarely specified for superb ageing. The are designed for a frequent recalibration schedule.Getting really impressive accuracy takes more than just using some high grade components. You have to pay attention to every detail, like board and component surface leakage, moisture control, and so on. All sorts of things which you dismiss as a minor detail in most designs become things that swamp your error budget in high precision ones.

author: Andreas
author_rank: Super Contributor
timestamp: August 19, 2014, 06:09:33 am
content: Hello,on 6,5 Digit multimeters usually a LM399 reference (1ppm/K) and special resistors (low T.C. low ageing) are used.The reference is pre-aged and selected for low ageing.The ADC is a multi slope (around 24-28 bit) integrating type.A schematic of the HP34401A can be found in service guide (34401-90013) on the web.With best regardsAndreas

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 19, 2014, 07:47:31 am
content: Generally, a verystablevoltage reference is always needed.The LM399 is very common in 6,5 digits DMMs.The reference itself is not "precise", as its initial value is +/-2% or worse, but it's stable regarding temperature and time.Then, a very linear (+/- one ... several digits) and high resolution A/D converter (> 24bit) is used.For the different ranges and modes, you need resistors and resistive dividers. They also are stable regarding temperature and time, but again not "precise", again meaning that their initial tolerance might be as high as 1% only. They are the most volatile components in these instruments, as they will drift and cause errors over temperature and time, in the order of several ppm/°C and maybe 10 ppm/ year. You may increase their stability by cost, in using very sophisticated, hermetically sealed, oil filled metal foil resistors, which have very low drift figures.Most modern instruments are then calibrated by software only, i.e. the voltage reference is determined precisely against external, traceable references, and all divider ratios for the different ratios and ranges are determined and stored in non volatile memory. During measurement, the correct result is calculated from all those calibration constants.Consequently, the next step is an instrument, which needs one stable 7V (LTZ1000) and one stable 10kOhm (hermetical sealed metal foil resistor) reference only, and then determines all the range calibration constants on its own, by using its internal ultra high resolving and ultra linear A/D.By that techniques, it may eliminate all temperature and time drifts of its internal resistors, leaving the drift of both core references only.Such "autocal" instruments are the HP3458A DMM, or the Fluke 5720, and 5440B calibrators.Frank

author: tszaboo
author_rank: Super Contributor
timestamp: August 19, 2014, 09:00:02 am
content: Besides artefact (sic) calibration and software techniques, if you want your initial accuracy lower, that is also possible. Well, if that is your goal, because an EEPROM and a MCU is usually cheaper.So Linear has 0.0025% voltage references, and there are Vishay foil resistors and networks with 0.005% initial accuracy. TI and analog also makes programmable amplifiers with 0.15% accuracy.So you can get your initial accuracy down to,say 0.01% withcarefully selected components.

author: EEVblog
author_rank: Unknown
timestamp: August 19, 2014, 10:04:17 am
content: It's really about stability, not absolute accuracy.These super high accuracy meters only use a reference (and other parts) with tolerance of 2% or so, e.g.:http://www.linear.com/product/LM399But they are super stable with temperature and time.So that horrible 2% reference is calibrated at the factory, and the correction factor stored in the unit.After that it's all about the short term stability and long term drift of the reference and other parts.

author: macboy
author_rank: Super Contributor
timestamp: August 19, 2014, 01:32:45 pm
content: Quote from: NANDBlog on August 19, 2014, 09:00:02 amFrom what I see, Linear's best voltage references are 0.025% not 0.0025% initial accuracy. That figure really is about as good as it gets for initial accuracy. Anything that needs better needs to be calibrated.

author: tszaboo
author_rank: Super Contributor
timestamp: August 19, 2014, 06:30:12 pm
content: Quote from: macboy on August 19, 2014, 01:32:45 pmYou are absolutely correct, I've missed a zero.I'm not against calibration, just making a point. There are some benefits for using high initial accuracy and stable components. You can neglect secondary order effects for example. Or use tighter tolerances. Also if you are doing trimming instead of calibration, then the trimming range is much smaller, giving you higher resolution and higher accuracy in the end.But I must say, nothing really beats the heated zeners.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: August 20, 2014, 01:45:00 am
content: Even with the 0.025% you have to consider the context of its specification. For example the Fluke 45 calls out "0.025% + 6 counts" but only against the max range for the given setting.I.e.300mV max range, 4.5 digits:0.3 * 0.00025 = ±75uVIf the meter were then in 300mV mode with slow acquisition, and you put in +100.0000000000mV, your meter might read out +100.00mV + 7mV + 6 counts = 100.11mV. It might also read +100.00mV -7mV +6 counts = 99.99mV. This results in an error of -0.01% to +0.1099% relative to your input signal. Ironically, to get the best accuracy, you want your input signal to be the closest to the upper end of the range as you can get. The lower you go, the more you're fighting the precision gremlins.

author: SebG
author_rank: Regular Contributor
timestamp: August 20, 2014, 07:44:27 am
content: This is all interesting information. Thanks everyone.It seems a good multimeter is just a stable device that is calibrated but then I wonder how are the calibrating devices so accurate. lolQuote from: Dr. Frank on August 19, 2014, 07:47:31 amHow would autocal be implemented. Could someone please elaborate on this.

author: tszaboo
author_rank: Super Contributor
timestamp: August 20, 2014, 07:52:53 am
content: Quote from: SebG on August 20, 2014, 07:44:27 amYou can read the whole shebang int the Agilent Technologies 3458A Multimeter Calibration Manual, appendix B (03458-90017)

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 20, 2014, 09:08:00 am
content: Quote from: SebG on August 20, 2014, 07:44:27 amBrief description:For all instruments, you need to know (calibrate) the RATIO value of the internal amplifiers and dividers.For example, if you calibrate the 1000V range, if the A/D has a 10V basic range, then you need to measure this internal 1:100 divider ratio.For such "ratio transfers", you normally use external transfer standards, like the Fluke 720A Kelvin Varley Divider (for ratios from 1:1 down to 1:10), or the Fluke 752A Hamon divider (1:10, 1:100), which give below 1ppm ratio accuracy.An A/D converter itself is also a ratio device, as it outputs the ratio between the reference and the unknown voltage.The accuracy of this A/D ratio measurement depends in first order on the linearity and the resolution only.Therefore, the 3458A and the 5720 are capable of calibrating all of their internal ratios by their own internal ultra-linear A/D or D/A converters.Best in class is the 3458As A/D converter, with 0.02pm (typ.) linearity of input, the 5720s D/A converter is linear to about 0.1ppm.You may read about the 3458A techniques in the hp journal April 1989.www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1989-04.pdfThe 5440 features two different methods, the first is its highly linear D/A converter for low value ratios (e.g. the 13V reference => 10V transfer).The second method consists of an ovenized resistor divider chain of ten 200k resistors, where these resistors are switched in series, or in parallel, to determine the bigger ratio values, i.e. 1:2, 1:25, and 1:100.That switching scheme is the same as in a Hamon divider.In the 5440 service manual, this very interesting technique is described in the appendix "Recent Innovations in Direct Volts Calibrator Design", and then especially in the chapter: "Improving Perfomance With Internal Calibration"Also here, page 253:http://msc-conf.com/wp-content/uploads/proceedings/MSC1983.pdfFrank

author: con-f-use
author_rank: Unknown
timestamp: August 20, 2014, 09:21:49 am
content: Quote from: SebG on August 20, 2014, 07:44:27 amThey are calibrated against immutable physical principles (except if you measure mass or mol).

author: EEVblog
author_rank: Unknown
timestamp: August 20, 2014, 10:14:20 am
content: Quote from: SebG on August 20, 2014, 07:44:27 am

author: macboy
author_rank: Super Contributor
timestamp: August 20, 2014, 05:02:30 pm
content: Quote from: XFDDesign on August 20, 2014, 01:45:00 amThe "+6 counts" works both ways, not just positive! So the reading at maximum specified error would be in the range of 99.87 (100.00 - 0.075 - 000.06) to 100.13 (100 + 0.075 + 000.06).It is not ironic that you want the signal to be as close as possible to the upper end of the range. This is why there are many ranges. Pick the lowest one that will work with the signal to maximize accuracy and resolution. Sadly, due to the power-of-ten ranges, some readings will be nearly 10x as good as others.