EEVblog #972 - Operating Chips Outside Their Spec - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: February 15, 2017, 11:39:00 pm
content: Dave tests the AD8436 True RMS converter chips inside the 121GW multimeter to see if it works well below it's voltage rail specification.Bonus thermal testing in the chamber.http://www.analog.com/media/en/technical-documentation/data-sheets/AD8436.pdf

author: jfasoc
author_rank: Contributor
timestamp: February 16, 2017, 04:29:50 am
content: The whole video I was waiting for you to test how low it will work.

author: nidlaX
author_rank: Frequent Contributor
timestamp: February 16, 2017, 04:38:11 am
content: Can't wait to get one of these. I passed on the $200 U1252Bs so I can save my multimeter budget for a 121GW!

author: KerryW
author_rank: Regular Contributor
timestamp: February 16, 2017, 04:38:56 am
content: Have you thought about using Batterizers with this meter?

author: EEVblog
author_rank: Unknown
timestamp: February 16, 2017, 04:39:53 am
content: Quote from: jfasoc on February 16, 2017, 04:29:50 amSorry, didn't occur to me to do that!

author: JS
author_rank: Frequent Contributor
timestamp: February 16, 2017, 05:33:15 am
content: Dave, we all want a batteroo inside our meters instead of the low voltage operation!!JS

author: integritetus
author_rank: Newbie
timestamp: February 16, 2017, 05:42:16 am
content: I am curious as to whether Analog Devices will guarantee that the AD8436 will operate to its specifications at a supply voltage of 3.6 V, or whether they are saying that it works but the design user is on his own to characterize operation at voltages below the minimum 4.8 V.Dave mentioned this his DMM design partner is doing testing. I would expect that they could characterize some number of parts across voltage, temperature, and frequency, but I wonder how they would characterize the part across process variation (that is, the variation in process parameters for the IC process in which the AD8436 is fabricated). I would expect that only the part designer/manufacturer could characterize the part across process (unless the design user were supplied with a significant number of parts binned by process parameter).For a sufficiently small production of units, a design user could actually in theory characterize each part received. But I suspect that is not practical for this project.

author: kandrey89
author_rank: Contributor
timestamp: February 16, 2017, 06:27:29 am
content: Is Dave making an EEVBLOG branded multimeter?Is it branded only, or did Dave help design it?Is there a pre-sale, spec page or something, can't find anything anywhere about it?ETA and other details?

author: tszaboo
author_rank: Super Contributor
timestamp: February 16, 2017, 09:13:03 am
content: Some time ago: We were testing a FET driver. Specification said maximum voltage 18V. Sure we can go higher than that! They give large amount of headroom on these specs, and it will work fine at 20, if we want to.Of so we started cranking up the voltage on the lab supply. At 18.3V the smoke flew out of the part

author: Supercharged
author_rank: Regular Contributor
timestamp: February 16, 2017, 05:19:53 pm
content: AM i the only one who saw a Wifi or Bluetooth module at 4:18?Wireless Confirmed!

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 16, 2017, 05:37:34 pm
content: Quote from: kandrey89 on February 16, 2017, 06:27:29 amFirstly, I'm pretty sure those questions are answered somewhere on the forum.Secondly, you just need to think about it for one second... just try and quantify the engineering required to have a decent product. It's clearly going to be either a rebadged existing model or as has been suggested a rebadged exisiting model with maybe a tweak or two. Personally I don't have any issue with that as its better to have a tool that works than a flakey v0.00001 of something.But... I've been madly googling images and for the life of me I cant work out the manufacturer. I'm was going on the basis that its got to be a reasonable manufacturer so that eliminates Hung Lo brands. My line of search is working on the DMM glass. It's likely that this model will have the same glass as an existing or planned model since an EEVBlog run will only form part of a larger commercial run..Am I warm? Any suggestions?(If the price isn't outrageous I'll be getting one)

author: testian
author_rank: Regular Contributor
timestamp: February 16, 2017, 05:59:28 pm
content: Quote from: Supercharged on February 16, 2017, 05:19:53 pmDefinitely looks like one

author: Len
author_rank: Frequent Contributor
timestamp: February 16, 2017, 10:20:14 pm
content: Quote from: kandrey89 on February 16, 2017, 06:27:29 amhttps://www.eevblog.com/forum/chat/new-eevblog-branded-multimeter-coming/

author: DavidDLC
author_rank: Frequent Contributor
timestamp: February 16, 2017, 10:22:29 pm
content: Quote from: Supercharged on February 16, 2017, 05:19:53 pmNo you are not

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 06:26:52 am
content: Quote from: NivagSwerdna on February 16, 2017, 05:37:34 pmI think you'll find Dave's involvement is a bit more than that.Dave has already done a (sort of) rebadge with the BM235 ... but the 121GW is something special.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 17, 2017, 06:59:21 am
content: That part is rated to well above double the battery voltage and uses less than 1mA. Perhaps you could use a spare PWM on the microcontroller to drive a charge pump.Quote from: NANDBlog on February 16, 2017, 09:13:03 amSwitcher driver chips for mains applications often have an internal zener clamp to allow a simple resistor to supply startup power.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: February 17, 2017, 09:42:40 am
content: Hee Dave, is this what you were working on in#900 STM32 Development Board?

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2017, 11:09:18 am
content: Operating chips outside their spec is a bad practice, I don't agree with this.Ask to the manufacturer to change their specifications....The statement " the manufacturer (Analog Device) says it can work at lower voltage than specified" is not a profissional statement....It means that good working in this condition is not garanteed by the manufacturer of the chip.You can't specify that your multimeter will work with 3.6V if the chip is only specified to work with 4.8V minimum voltage, that's not acceptable.I worked in quality control and I would reject your multimeter....Sorry Dave....

author: EEVblog
author_rank: Unknown
timestamp: February 17, 2017, 11:13:15 am
content: Quote from: NivagSwerdna on February 16, 2017, 05:37:34 pmNope, and nope.QuoteNope again.

author: EEVblog
author_rank: Unknown
timestamp: February 17, 2017, 11:15:11 am
content: Quote from: oldway on February 17, 2017, 11:09:18 amYou don't get a say in itApart from not buying it of course.

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2017, 11:17:30 am
content: And what about using 1.2V rechargeable batteries ?No way !The only solution is to use 5 batteries instead of 4 or a 9V battery.NB: 1.5V batteries often leaks, I have a Fluke 87 IV badly damaged by leaking 1.5V batteries....I never had such problem with 9V battery.So I prefer using a 9V battery in a multimeter...So I prefer a multimeter with a 9V battery....

author: f4eru
author_rank: Super Contributor
timestamp: February 17, 2017, 11:20:20 am
content: Question: why use a rms calculator chip? I would rather use two adc channels and sample both I and U at the same time, and calculate the result. This way you can also measure power.Also note that you don't need absolute add precision for your 6000 counts, you can use 12 bits, for example, the massive oversampling gives you the needed precision.Concerning chips out site specs, it can be OK, if the risks have been properly rated, and the outside spec validated as thoroughly as possible. A eol test is a good bonus. I seen it done at a big Corp for million automotive products. It was done properly.

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2017, 11:33:03 am
content: Quote from: f4eru on February 17, 2017, 11:20:20 amI don't think Dave intend to make a multimeter as the Metrawatt Gossen Energy....https://www.gossenmetrawatt.com/resources/tt/metrahit_energy/db_gb.pdfIt is far too expansive !Even this multimeter seems to use two rms converter and not your solution, as max bandwith is only 100Khz.

author: EEVblog
author_rank: Unknown
timestamp: February 17, 2017, 11:46:26 am
content: Quote from: f4eru on February 17, 2017, 11:20:20 amBecause most multimeters use front end chipsets, and these integrate well with external True RMS chips.QuoteIt's not that cheap or easy to get fast decent 12bit converters and integrate them into the design.It's much easier to use a proper True RMS converter chip that integrates into the measurement system nicely, can handle high crest factor waveforms, and can use the existing chipset converter and firmware.And the meter is 50000 counts, not 6000 counts.

author: f4eru
author_rank: Super Contributor
timestamp: February 17, 2017, 11:48:39 am
content: Expensive? Noooo, it's not! There are plenty of us today with integrated adc 12 bits or higher. Take one that is fast enough. The cost of the Micro is already included

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2017, 11:49:52 am
content: Quote from: f4eru on February 17, 2017, 11:20:20 amIt may be acceptable in the United States and China but not in Europe .... In Europe, it is necessary to respect the specifications of the manufacturer, it is not negotiable.It is probably a difference of mentalities.

author: f4eru
author_rank: Super Contributor
timestamp: February 17, 2017, 11:58:53 am
content: As an example: an ads 7850 as stand alone fast adc is alreadycheaper than the rms to DC converter, adc nonwisstanding......

author: joeqsmith
author_rank: Super Contributor
timestamp: February 17, 2017, 12:50:16 pm
content: Quote from: NivagSwerdna on February 16, 2017, 05:37:34 pmApple lists the software as well now.https://itunes.apple.com/us/app/eevblog-121gw/id1173156224?mt=8The meter looks good from the back.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 17, 2017, 12:59:41 pm
content: Quote from: oldway on February 17, 2017, 11:09:18 amThere is no such thing as "bad practice". Only more or less appropriate for a given situation.In this case, for example, the minimum supply voltage will be guaranteed over the entire temperature range, which is not required in a handheld DMM.Obviously there is some risk, e.g. they could chnage their process in the future, but that's one of things that need to be taken into account.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 17, 2017, 01:03:08 pm
content: Quote from: oldway on February 17, 2017, 11:49:52 amThat is just a ridiculous generalisation. You do not speak for all of Europe.The multimeter either meets its published specs or it doesn't. How it achieves that internally is irrelevant.

author: Zbig
author_rank: Frequent Contributor
timestamp: February 17, 2017, 01:21:39 pm
content: Quote from: joeqsmith on February 17, 2017, 12:50:16 pmDave, I hate to be that guy, but I honestly think the mobile app as it looks today, is a disservice to the image of your soon to be released product. I know that you've stated more than once that it is just a kind of a proof of concept, to verify meter's basic functionality. I say it'd be better to start with no app at all than this. It just screams amateur personal late 90s Geocities home pages, the UI looks truly awful. It's not always that anything is better than nothing. The app will be associated with your product, the look and feel of this abomination will affect how the actual meter is perceived, IMO. How would you like a color scheme, fonts, drop shadows, etc. like this on the actual meter's faceplate?EDIT:Has anyone clicked the "EEVBlog 121GW Support" link on the iTunes listing? It leads tohttp://www.ueitest.com/so now is seems we know who the OEM is :-)

author: madires
author_rank: Super Contributor
timestamp: February 17, 2017, 01:47:46 pm
content: Quote from: oldway on February 17, 2017, 11:09:18 amNot quite. The answers could be:- No!- Yes, but at your own risk.- Yes, it will be stated in the next revision of the datasheet.- Yes, we'll send you a confirmation written on paper if you wish.- Yes, but not officially.- any other shade of greyUnless you get an official confirmation or an updated or custom datasheet, you have to perform a risk analysis besides running tests. The most prevalent risk in this case is that the measured value could have a larger tolerance. But no risk of electric shock, exploding DMM or other showstoppers.

author: tszaboo
author_rank: Super Contributor
timestamp: February 17, 2017, 01:49:07 pm
content: Quote from: f4eru on February 17, 2017, 11:48:39 amIf we calculate with a conservative (fluke 87) 20 Khz bandwidth for the TrueRMS core, you need to take samples at around 200KHz, otherwise it is not really true rms. An ADC with those specs there is the ADS8339 which is 3.5 USD from TI. You need to buffer and amplify the signal. I will assume the AD8436 is getting a 1V signal, so the amplification is only 5. Assume 0.5% max error, you need an opamp doing less than 1mV offset, 2MHz GBW. Something like a OPA197 can do it, 60 cents. You will need to dedicate an SPI port of your micro to it. Also coding needs to be done, verifying that the RMS calculations are correct. Luckily, there is only one "root calculation" in the code. I think with the usual big company impotence, a few months should be enough to develop this.Compare this with the price of the AD8436, which is ~3USD. You gain nothing, but more chance of error for more money, and more code. While you could just solve it with an IC. #analogisbetterAt this particular chip, I would suspect the following:At low power supply voltage, over the temperature range, it might have linearity errors. Maybe they had problems with the output buffer going only to supply voltage -1V. Sometimes they just dont test the part with a certain scenario, because they haven't thought about that.

author: coppice
author_rank: Super Contributor
timestamp: February 17, 2017, 02:05:18 pm
content: Is that a commercial chamber in the video, or something else repurposed? If it's a real chamber, who makes it?

author: EEVblog
author_rank: Unknown
timestamp: February 17, 2017, 02:12:22 pm
content: Quote from: Zbig on February 17, 2017, 01:21:39 pmMeh.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 02:25:29 pm
content: The App available on release doesn't worry me in the slightest.I think Dave has said the protocol will be open - so it's only a matter of time before we have someone with an EE bent and some App writing skills sit down and put together something far more useful.

author: Zbig
author_rank: Frequent Contributor
timestamp: February 17, 2017, 03:08:34 pm
content: Quote from: coppice on February 17, 2017, 02:05:18 pmI believe it's an incubator for pet reptiles.

author: coppice
author_rank: Super Contributor
timestamp: February 17, 2017, 03:58:30 pm
content: Quote from: Zbig on February 17, 2017, 03:08:34 pmAh, right. It seems to be this onehttps://www.amazon.com/Exo-Terra-PT2499-Incubator-Unit/dp/B004AJLREA. It seems to get a lot of negative feedback from customers over reliability.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 17, 2017, 04:26:56 pm
content: Quote from: EEVblog on February 17, 2017, 11:13:15 amFair enough! Looks like I wasn't warm then.Looking forward to seeing the final version!

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 17, 2017, 04:29:07 pm
content: Quote from: coppice on February 17, 2017, 03:58:30 pmAs an owner of some tortoises, whom have just come out of the fridge, I would agree with that. I think this might be a peltier device... it would struggle to keep a stable 5C required for wildlife hibernation.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 17, 2017, 05:57:26 pm
content: Use of a device outside of spec has dangers that vary with the product. The most significant ones come for very high volume or very long production life products. In my career the products I associated with were usually produced for a decade or two. Over that interval common parts can see not just process variation but complete process changes. The manufacturer will validate their new process to the spec sheet, no more, and anything not defined on the spec sheet is at risk. Large volume producers may purchase from multiple suppliers of the same device. Some of which will work while others don't. A famous example of this came in the WWII VT fuse which was based on a single vacuum tube (valve for those of you speaking Old English). As I recall there were eight makers of this type tube, only one worked in the application.For a product like the EEVBlog branded meter, which will likely be produced for a short time, maybe in a single production run, the risks are much smaller.

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2017, 06:10:46 pm
content: Quote from: madires on February 17, 2017, 01:47:46 pmNo risk of electric shock ? You adjust the low battery sensor at 3.6V but the Analog Device rms converter chip is only garanteed to work down to 4.8V....and if this chip stops to work at 3.8V (no low battery alarm) and you measure an 600V ac voltage and it indicates no voltage at all....You think it safe, but it isn't ...How can you garantee something if you use parts out of specifications ?

author: Len
author_rank: Frequent Contributor
timestamp: February 17, 2017, 06:37:32 pm
content: Quote from: coppice on February 17, 2017, 02:05:18 pm

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 17, 2017, 06:45:51 pm
content: Quote from: oldway on February 17, 2017, 06:10:46 pmProduction test.

author: madires
author_rank: Super Contributor
timestamp: February 17, 2017, 06:46:00 pm
content: Quote from: oldway on February 17, 2017, 06:10:46 pmIf the RMS converter only works down to 3.8V you would adjust the low battery warning accordingly. And if rechargeable batteries would be a problem you would put a "non-rechargable batteries only" sticker on the DMM. And we shouldn't forget that any DMM could fail for whatever reason and show the wrong voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2017, 07:03:07 pm
content: The datasheet for the RMS converter chip already shows a few things that happen at low supply voltage: there is an increased error expected for large signal amplitudes. Depending one the ADC following this might not be a problem, or only at a high crest factor. AFAIR the high crest factor tests in the video where done at low amplitude (fixed peak-peak value). There is no real risk of showing grossly wrong voltage due to the low supply, more like maybe a few percent off for extreme cases. This can be an acceptable compromise between power consumption and accuracy for an handheld DMM. For a very high crest factor signal, the RMS value might not be that good to judge if a voltage is dangerous anyway.It might be a good idea to include tests of the RMS function in the QC, just in case.When using the digital method, there is no absolute need to have such a fast ADC. Digital RMS even works with sub-sampling. So even the 15 kSPS inside an AVR µC could be good for software RMS up to about 40 KHz (BW limit of the ADC when sub-sampling). The SW method would be mainly an option with an µC internal ADC. The downside is that is does not work so well with small amplitudes, so one might need some extra auto-range steps (e.g. another 1 or 2 gain stages). However it also has an advantage: settling time can be lower and is not slowing down on low amplitudes.

author: SeanB
author_rank: Super Contributor
timestamp: February 17, 2017, 08:16:11 pm
content: Quote from: CatalinaWOW on February 17, 2017, 05:57:26 pmDon't worry, most of those specs for a chip are "typical" and can vary in some cases widely, with only very broad minimum and maximum values. There are a lot of parameters that are "typical" and which are only tested on a sample basis, and those often are not guaranteed, even if they are actually an important reason for choosing one chip over another in selection. Things like noise levels, actual open loop gain, true bandwidth and such are only sampled, not tested on each lot, they might only be done on a few chips from each run of wafers.A lot of the lower figures like supply voltages are often "we never tested below xx", and similar for things like gain, bias current ( and some can be nasty, with bias current that depends on the input voltage applied to BOTH inputs, and the supply voltage and temperature, and some can swap from source to sink at some point) and saturated output voltages.

author: oldway
author_rank: Super Contributor
timestamp: February 17, 2017, 08:48:34 pm
content: 4.8V is min supply voltage of AD8436, not typical supply voltage.It has to be respected.There some reason why Analog Device has specified such a voltage : below this voltage, working of the chip is no more garanteed.http://www.analog.com/media/en/technical-documentation/data-sheets/AD8436.pdf

author: f4eru
author_rank: Super Contributor
timestamp: February 17, 2017, 09:55:12 pm
content: Quote from: NANDBlog on February 17, 2017, 01:49:07 pmWrong price comparison. You already need a front end and an ADC anyway.You got many advantages: a good power meter. Flexibility. Low cost. More precision on measurements (thanks to oversampling). Simpler hardware...Nope. Analog is not always better, especially when dealing with lowish frequencies.Personally, I did this exact implementation of an rms and power calculation, at only 10 ksps, with acuracy better than 0.05% on an EFM8BB1. A 32 cent micro. With integrated 12 bit ADC!!!!!! Used in an industrial product.I think the actual state of digital uCs and ADC allows us to make much better cheap multimeters.

author: EEVblog
author_rank: Unknown
timestamp: February 17, 2017, 11:25:26 pm
content: Quote from: mikeselectricstuff on February 17, 2017, 06:45:51 pmYep. Chip performance doesn't magically change at any time, it's fully characterisable.Every unit (like any quality meter) is fully tested and calibrated at the factory. Ithasto be fully calibrated on every range, that's how modern meters work, the cal values are stored in the EEPROM.And in the this case it's tested at different frequencies and signal levels on all AC ranges.There is no risk here for the consumer, the risk of production parts changing falls upon the manufacturer.

author: thm_w
author_rank: Super Contributor
timestamp: February 17, 2017, 11:50:35 pm
content: Quote from: oldway on February 17, 2017, 08:48:34 pmIt could have been an original spec goal but has little relevance to actual chip performance, it may have been related to first batch of chips and later batches were improved, who knows.It should be respected, but it doesn't have to be.You say you are in QC, did you go over the original design to ensure all parts are within manuf. specifications? Did you probe the board while in various conditions to ensure all parts stay within these ranges?Often designs will have parts running out of spec but its not intentional, it was either overlooked or unexpected. Not saying its a good thing of course, but it happens.

author: oldway
author_rank: Super Contributor
timestamp: February 18, 2017, 12:05:18 am
content: I do not agree.As long I know, there is only ONE calibration value by range.There is no calibration value for every single battery voltage below 4.8V.It may be tested at different frequencies and signal levels on all AC range, it would be tested also at different battery voltages below 4.8V.Ask Analog Device a special batch of AD8436 specified with 3.6V min power supply voltage or increase the battery voltage.I hope that you will make a serious industrial multimeter, not another Chinees crap one !Don't operate parts outside their specifications, that's not serious, that's a principle used in crap low cost instruments.QuoteYou seems to know beter the AD8436 than Analog Device themself !Why did Analog Device specify that min power supply is 4.8V if, as you say, lower power supply voltage has little relevance to actual chip performance ?How low can we go ? 4V, 3V, 2V, 1V ?As you said...who knowsIt does no matter if one prototype is working correctly or not !You can't make a serious project on the base of "who knows, perhaps it works"...Specifications have to be respected in a serious product....

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 18, 2017, 12:24:52 am
content: Quote from: SeanB on February 17, 2017, 08:16:11 pmAnyone who designs based on "typical" values without some type of backup is riding for a fall. "Typical" values are of most use to the marketing department.I have no problem with someone doing a "typical" based design if:a) They fully understand how the chip works and thus how it is likely to respond in untested situations.b) They do sufficient post purchase testing to verify that the components they will physically be using are "close enough" to typical. That requires enough analysis of their design to know what "close enough" is and what testing is required to verify that the parts installed are there.

author: EEVblog
author_rank: Unknown
timestamp: February 18, 2017, 01:21:34 am
content: Quote from: oldway on February 18, 2017, 12:05:18 amYes, but it'stestedat different frequencies and voltages on each range.There is no calibration value for every single battery voltage below 4.8V.It may be tested at different frequencies and signal levels on all AC range, it would be tested also at different battery voltages below 4.8V.There is a 3.6V voltage regulator, it doesn't matter what the battery voltage is.QuoteIf it bothers you then you don't have to buy it.QuoteThe entire point seems to go completely over your head.In a case like this it doesn't matter, as long as the meter is 100% tested.QuoteYou seems to know beter the AD8436 than Analog Device themself !Strange that AD themselves tell us it's not a problem.Not that it matters anyway when you 100% test and characterise something.QuoteAgain, you completely miss the pointI'm done.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 18, 2017, 01:36:42 am
content: The Mooshimeter does RMS calculations in software with just a 32MHz 8051. In fact, it can do that on both voltage and current at the same time and also calculate real power.

author: Brumby
author_rank: Unknown
timestamp: February 18, 2017, 05:55:34 am
content: It seems all the characterization, testing and calibration under the sun isn't enough for some people, but when I see this:Quote from: EEVblog on February 18, 2017, 01:21:34 amany apprehensions about the use of the chipshouldevaporate, IMHO.As a result, the only question that I can see in any of this is - Why did AD put that minimum voltage specification in the datasheet?I half expect a bureaucracy based answer more than an engineering one.

author: riscy00
author_rank: Contributor
timestamp: February 18, 2017, 08:30:21 am
content: What the model of the red oven.What the maximum temperature it goes to?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2017, 08:46:08 am
content: The DS from AD already shows two curves (fig 11 and 13) that show what happens when the supply is reduced. They show the errors going up / the maximum well working amplitude is going down when the curve go towards 4.8 V or a little below.Operating at a low voltage (like 3.6 V instead of 4.8 V) is more like is would need modified specs for the rest. The more obvious point is that the optimum amplitude level is going down. It might mean the accuracy is a little lower, but as it looks, the difference is not that large.So the main part that is missing at low voltage is the full test and specs for this. I can somewhat understand AD that they don't want to do the low voltage tests, if most users will use it with something like a 6 V or higher supply. They may end up with a different version that is the same silicon, but tested for 3-5 V instead of 4.8 -12 V.

author: EEVblog
author_rank: Unknown
timestamp: February 18, 2017, 09:19:59 am
content: Quote from: riscy00 on February 18, 2017, 08:30:21 am

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 18, 2017, 09:38:53 am
content: Quote from: Brumby on February 18, 2017, 05:55:34 amProbably to guarantee optimum performance, and operation over the whole temperature range. Not everyone will need either.

author: EEVblog
author_rank: Unknown
timestamp: February 18, 2017, 09:42:23 am
content: Quote from: mikeselectricstuff on February 18, 2017, 09:38:53 amSomething like that.The nominal 1% error graph for input level vs supply voltage goes down to 4V, so why 4.8V? Who really knows.

author: f4eru
author_rank: Super Contributor
timestamp: February 18, 2017, 12:49:38 pm
content: Quote from: NiHaoMike on February 18, 2017, 01:36:42 amat which samplerate?seems a quite capable device. I wouldn't give up range switching and DMM Form factor. Nor a few 100hz bandwidth

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 18, 2017, 05:50:59 pm
content: Quote from: f4eru on February 18, 2017, 12:49:38 pm8kHz on each channel. Plenty for mains frequency use. I think one of the reasons why it's not 48kHz or above is Bluetooth bandwidth limitations for the real time graphing mode. (Even 2 8kHz 24 bit channels uncompressed is pushing the limits of BLE.)What I would really like to see is software support for an external shunt or Hall probe. The built in shunt has a 10A full scale with uA resolution which is very impressive but not quite good enough for measuring very low power microcontrollers. The option to get sub microamp resolution or being able to measure current/power greater than 10A would be very useful.

author: tszaboo
author_rank: Super Contributor
timestamp: February 18, 2017, 06:55:29 pm
content: Quote from: f4eru on February 17, 2017, 09:55:12 pmA decent multimeter multiple KHz bandwidth on the AC range. Fluke 87 as a reference has 20KHz. Your implementation has less than 1 KHz, and who knows how it will handle crest factor.The EFM8BB1 has 0.1% maximum slope error on its ADC. I highly doubt that you magically were able to create a 0.05% accuracy front end with it, I rather think you are just ignoring all the practices of proper data acquisition. You dont end up magically with better accuracy with oversampling, you get more resolution, aka more bits of the same bad measurement.

author: David Hess
author_rank: Super Contributor
timestamp: February 19, 2017, 06:58:58 am
content: Quote from: f4eru on February 17, 2017, 11:20:20 amThis works for increased resolution but does nothing for accuracy. The linearity and drift will remain.Quote from: mikeselectricstuff on February 17, 2017, 06:45:51 pmAnd selection, grading, and qualification. Manufacturers make compromises in specifications for yields and test time but if it is worth it, then you can do your own testing.Select parts through testing for a specific characteristic like transit time in bipolar transistors, low quiescent current when operating 741s above the absolute maximum supply voltage, drift over temperature, avalanche breakdown, etc.Grade parts to match characteristics like Vbe, Vf, Vgs, hfe, ratio of capacitance, etc.Qualify manufacturers and lots for things like insulation resistance in film capacitors, reverse recovery in standard diodes, and leakage in base-emitter junctions.I had to write this reply twice; EEVBlog ate it the first time.

author: f4eru
author_rank: Super Contributor
timestamp: February 19, 2017, 10:08:31 am
content: Quotemy implementation can handle harmonic content up to a few kHz.Enough for my application, 50/60Hz with only low harmonics present in any situationQuoteYep. We got 0.05% typical, which is OK for a 30 cent part, and an order of magnitude better than we needed anyway.Also, don't forget that an RMS measurement is an average of many thousands of samples, so measurement errors often tend to even out when the measurement error affects your end result in both directions.This is not always the case. For example we found out that a small difference in sampling time between I and U channels (approx 40us) brought a small phase error that made the error in the end calculation of the active power skyrocket due to the very high reactive power always present in the application.This error stood out because it pulled the end result in a single direction a little bit for every sample, so the integrated end result summed all these errors instead of averaging them out.

author: tszaboo
author_rank: Super Contributor
timestamp: February 20, 2017, 09:25:54 am
content: Quote from: f4eru on February 19, 2017, 10:08:31 amYes. For 50 Hz is enought. For a multimeter, it is not, because people want to use it to debug audio circuits for example, so 20KHz bandwidth is necessary.I've also underlined a word in your response. Sure, chips will work better than the datasheet absolute maximum. And mulitmeters are expected to do the same. So you cannot claim 0.05 if the datasheet said 0.1. And INL errors will not integrate out, because ADC will make the same bad measurement every time the signal is there. For example, you start with 3 LSB offset error. That is 0.07% of your measurement. It also states 61dB SNR for a 10KHz signal, which is about 0.1% error again. Good luck averaging that out.

author: coppice
author_rank: Super Contributor
timestamp: February 20, 2017, 11:17:46 am
content: Quote from: NANDBlog on February 20, 2017, 09:25:54 amThe offset error shows up as DC. That DC component generally drifts a bit with temperature. Any reasonable implementation of AC RMS calculation will run a continuous noise shaped DC estimator, and subtract this DC from the incoming samples before they are squared. That's your offset errors killed. The AC offset due to AWGN is not easy to completely eliminate, because it can't be continuously estimated well, and it varies with temperature, and from sample to sample of chip. It is, however easy to mitigate to some extent. Find the noise accumulated by the RMS calculation on a number of devices, at room temperature, and find their average. Now put the square of this average into the code on every device, such that it is subtracted from the accumulated squares, just before calculating the square root. You have now removed a compromise average chunk of the noise offset. Now your results should look pretty good, at least at room temperature. However, the temperature coefficient of the reference, and other parts of the MCU, are unlikely to hold the accuracy to 0.05% over a wide temperature range.

author: f4eru
author_rank: Super Contributor
timestamp: February 20, 2017, 07:56:20 pm
content: QuoteOf course you can do better than the datasheet claims ! it's even the whole subject of this topic ! "Operating Chips Outside Their Spec"And there are many ways to do it !Doing it properly involves a risk evaluation.One method how I would do it in this case, if the manufacturer gives no indication:1) get a few chips with different date codes from different suppliers, samples, etc...2) characterize them to see the lot variation on the parameter of the Datasheet. You only need to measure the parameter you want to be outside spec, in our case the INL3) characterize the temperature variation (or voltage if variable, or any other variable external parameter for your application)4) Calculate or evaluate which is the limit of the chip INL which will make you fail your end calculation. Let's say you could demonstrate that 0.08% INL on the chip gives 0.05% accuracy in your application, due to the sample spread when measuring AC RMS. So our limit is set to 0.08%5) Evaluate the spread, evaluate statistically how much percent of chips will probably fail in your application (over 0.08% at the ADC).6) Set a strategy. In our case, we decide:- Alternative uC with better specced adc costs 10 times the price- we produce 1000 parts a Year.- we measured the deviation of INL from the chips in the same batch to be under 0.01%- typical INL is around 0.04%- we buy a reel of EFM8BB10F2G-A-QFN20R (1500 parts) every 1,5 Years, for about 500 Euros, we could resell a roll of non useable 1499 parts for 200 Euros, but administrative effort reduces this to 100 Euros.- we have a 2% chance to have an entire reel outside our spec- we have an 3% chance of a marginal reel, where some parts would be outside our spec- Product lifecycle is estimated at 8 Years -> 5 rolls needed max (count ramp up and down)- Acuracy testing is part of our normal EOL QA testing, so characterising one chip from a new roll costs us only about 300 Euro effort, and two weeks waiting.So we decide that a meaningful strategy is:- we plan a reel ahead- Every time we buy a roll, we get one chip of it, solder it manually onto a board, and pass the final test- if ever we get a reel outside or marginal to our criteria ( 0.07% ADC, which corresponds to 0,04375% final calc), we reject the reel and resell it, re-order one, retest.7) Talk to your boss about the added risk, and the strategy. The strategy and following decision of management have to be written down in the project minutes (very important)go on and implement the strategy.As I said already, we didn't do all that, because we didn't need better than 0.5% anyway.

author: b_force
author_rank: Super Contributor
timestamp: February 20, 2017, 08:04:13 pm
content: I thought we were always so picky about testing things on scientific ways??How can one sample be representative for a production of millions???The video I saw was pretty much common sense, but also lacked a lot of information.A lot of companies are extremely conservative about certain specs, that all has to do with claims.In some cases they even use a 3 sigma error distribution to be as safe as possible.So first of all you use all thinkable worst case scenarios; min&max temp+max current+all I/O loaded with max current+other things that make it all bad.In many cases this is simply being calculated from their theoretical models and simulations.(I've worked in a company that produces IC's).Than you compare that with a handful of practical tests, and calculate the maximum error with a 3 sigma distribution.An other scenario is that they simple didn't test all thinkable conditions with a certain variable (the power supply voltage in this case).Therefore they can't tell with certainty if this chip will perform according specs with lower voltages.It's not uncommon at all that IC's or parts will perform fine outside their specs.

author: f4eru
author_rank: Super Contributor
timestamp: February 20, 2017, 08:19:30 pm
content: Who said one sample should be representative of millions ?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 20, 2017, 11:39:21 pm
content: Quote from: f4eru on February 20, 2017, 07:56:20 pmExcellent summary and a good plan. The only thing I don't see explicitly addressed is the risk that there will be shift in production parameters which changes that 2% chance of a bad reel, or the chance of getting unlucky and getting two or more bad reels in a row. The latter risk is easily addressed by deliberately buying the next reel very early. At least now the cost of money is so low that things like that can be considered, but still may be a problem for your business either due to cash flow issues or opportunity cost.