Simple Vref - Page 1
Forum section: electronics / beginners

author: Ranger14
author_rank: Regular Contributor
timestamp: September 04, 2016, 08:14:52 pm
content: I am still kind of new to electronics and I am looking to build a simply Vref.. Right now I have a couple cheap meters and a Fluke 87 Series I. But I have a couple questions and would like some input please..Also I am tight on cash and trying to keep a balance between cheap and good accuracy verification of calibration..Also I don't have a meter that is calibrated or high accuracy meter to be able to trim the chip for better tolerance.. So will have to use the chip in the stock tolerance as the reference.. What kind of tolerance should I be looking at since my meter is .1%vdcI was looking at a couple chips and wanted to see if you guys had an suggestionsChips was looking at- Trying to stay in the 2.048v or 2.5v rangemax6126max6325ref5025

author: retrolefty
author_rank: Super Contributor
timestamp: September 04, 2016, 08:28:52 pm
content: Quote from: Ranger14 on September 04, 2016, 08:14:52 pmI think the 'rule of thumb' for reference accuracy is that one needs X10 better tolerance for the reference then the unit under evaluation. So a .1% accurate meter should be compared to a .01% reference. Not easy or cheap to attain.

author: pelule
author_rank: Frequent Contributor
timestamp: September 04, 2016, 08:31:16 pm
content: By measurment theory the reference should be at least one class better than the tool to vrify against.Your best class is 0.1%, thus you need a reference with less or equal 0.05%.

author: Simon
author_rank: Unknown
timestamp: September 04, 2016, 08:49:10 pm
content: I once used a nice reference I bought from Digikey it cost about £5 and was a SOT-23-5 package but my it was spot on.

author: singapol
author_rank: Frequent Contributor
timestamp: September 05, 2016, 08:03:55 am
content: Get a cheap AD584 voltage reference or similar like this one, it has 4 voltages and is 15ppm. Good enough for all your meters. The new 87V5 or later would need a better reference like MAX6235 1ppm.http://www.ebay.com/itm/AD584-Precision-Voltage-Reference-2-5-7-5-5-10v-Calibrated-By-Agilent-34401-/261972330425If you are serious about calibration then have a good read below: If you are a volt nut (person with OCD or obsessive ,compulsive disorder with regards to voltage measurement). A voltage reference need some burn in time approximately 200hrs. or more (about 3 weeks 8 hours/day or 1 week to 2 weeks 24/7) but if you are not particular then it's initial tolerance will do.Calibration-Philosophy in practice- Fluke:http://www.introni.it/pdf/Fluke%20-%20Calibration%20Philosophy%20in%20practice.pdfeffect of longterm drift on vref-http://www.analog.com/media/en/technical-documentation/application-notes/AN-713.pdfvref_tutorial_ad:http://www.analog.com/media/en/training-seminars/tutorials/MT-087.pdfvref design note intersilhttps://www.intersil.com/content/dam/Intersil/documents/an17/an177.pdf

author: MosherIV
author_rank: Super Contributor
timestamp: September 05, 2016, 08:17:06 am
content: QuoteI would have to ask, what are expecting from off the shelf voltage references?Are they good enough to calibrate meters? No, not on their own. They may be good enough to verify meters if one of them is known to be in calibration and therefore the reading can be trusted.Will they be absolute and stable?No and No. Look at the specifications for them and you will see that they all quote 'Initial accuracy' - ie how much off the specified voltage. Even the best ones are no better than mV.The specifications also specify a drift and thermal drift, they are oftern the same number eg 30ppm. The drift means that the reference will change over time.So back to my original question :what are expecting from off the shelf voltage references?

author: ebclr
author_rank: Super Contributor
timestamp: September 05, 2016, 08:26:36 am
content: This can be a very cheap and useful acquisitionhttp://pt.aliexpress.com/item/AD584-High-Precision-Voltage-Reference-Module-4-Channel-2-5V-7-5V-5V-10V/32631545933.html?spm=2114.02010208.3.10.dwqGdH&ws_ab_test=searchweb201556_0,searchweb201602_3_10057_10056_10065_10037_10055_10054_10069_301_10059_10033_10058_10032_10017_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_2&btsid=558de984-f16a-4650-a92a-c3f34be5bc14

author: NottheDan
author_rank: Frequent Contributor
timestamp: September 05, 2016, 10:26:23 am
content: Just, throw the calibration certificate away. It isn't worth the paper out is printed on. Mine arrived on Friday and the cert is the same as the one shown here:http://www.markhennessy.co.uk/ad584_references/

author: setq
author_rank: Frequent Contributor
timestamp: September 05, 2016, 10:56:07 am
content: I'll get shot for this as it's a crap solution, but it works well enough for amateur use. I'm using an ST branded LM723 as a voltage ref. There I admitted it. It's a tempco zener. It gets a "real voltage reading" from someone I know who works at a cal lab every couple of years and hasn't drifted much at all. It's good enough to check that all my 3+1/2 digit meters agree at least which is all I use it for.

author: tszaboo
author_rank: Super Contributor
timestamp: September 05, 2016, 11:28:56 am
content: If you want to calibrate/check a multimeter, it is batter if you measure a reference voltage, which is close to the full scale of your meter. Because the accuracy is usually defined as a factor of full scale + factor of measured value . Eg. 0.5% of reading + 0.5% of range. Or 0.5% of reading + 5 digit.Now if your measured value is only 2.5V on a 6V range, your total error for your full scale is 2.4x bigger. So I recommend getting a 5V reference. That will be better for the 6000 count fluke 87.

author: oldway
author_rank: Super Contributor
timestamp: September 05, 2016, 12:42:53 pm
content: You said you are not a professional, but only new to electronics.Try not to be a precision maniac.Your Fluke 87 is a reference, you don't need to check his calibration.I wonder how you could need something better...Use your Fluke 87 to check your cheap multimeters.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 05, 2016, 06:58:28 pm
content: Quote from: oldway on September 05, 2016, 12:42:53 pm+1 for thisIf you are not just trying to confirm the accuracy of your existing DMM(s) and you want to build your own reference as an educational project then sure, build it from parts and it should be a good learning experience. But if you are primarily just trying to gain confidence that your measurements are reasonably accurate then I think your Fluke 87 is likely to be a very good reference.In general, I think having one really good DMM like the 87V plus one or two other less expensive DMMs is the way to fly for most EE enthusiasts. I have a Fluke 179 which isn't quite in the same league as the 87V but it has proven to be a very good reference.Early on I wasn't so sure about the 179 and I went down the path of trying various ways to measure circuits with precision to 100ths of Amps and thousandths of Volts, and sometimes better.I tried various DMMs and also purchased items such as these:http://shop.voltagestandard.com/product.sc;jsessionid=340DCC10A9E023DD0FB530C6731D6FDE.p3plqscsfapp001?productId=1&categoryId=1http://shop.voltagestandard.com/product.sc;jsessionid=340DCC10A9E023DD0FB530C6731D6FDE.p3plqscsfapp001?productId=4&categoryId=1At the end of the day the 179 is still giving me lots of confidence. Even more interestingly, I have a few Radio Shack DMMs that cost in the $45-$20 and they show remarkable consistency with the 179.As mentioned in this thread, it's pretty easy to become an OCD VoltNut and there will be plenty of people here to be members of the support group for this condition but unless you have a real need to achieve more accuracy and more precision (those are two different things), I think you should consider yourself fortunate to have a 87V as a reference; build your test equipment and do measurements around the 87V as the reference at least until you come up with some good reason to doubt the 87V.YMMV, good luck!PS, if you build our own ref make a post here to show the design and results, I think there will be plenty of people here that will find both the approach and the results to be interesting.

author: retrolefty
author_rank: Super Contributor
timestamp: September 05, 2016, 07:59:02 pm
content: DMMs are like watches, the more you have the less likely you are sure of the time or voltage.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 06, 2016, 04:34:14 am
content: Quote from: retrolefty on September 05, 2016, 07:59:02 pm+1, for sure

author: oldway
author_rank: Super Contributor
timestamp: September 06, 2016, 08:41:05 am
content: If you make your own Vref, you can also doubt about the chip you buy.There are a lot of fake chinese Vref chips on the market.How will you test and verify your Vref if you even doubt about your Fluke 87 ?You should have calibration instruments and you don't have such instruments.Send your Fluke 87 for calibration if you think that's necessary...It will cost you halve of the price of the new multimeter but you will recover your sleep.

author: tggzzz
author_rank: Super Contributor
timestamp: September 06, 2016, 08:44:52 am
content: Quote from: Ranger14 on September 04, 2016, 08:14:52 pmGiven the cash/balance part of that statement (which every engineer should include in their statement!), you should modify the latter part of the statement. You should be looking forsufficientaccuracy. That, of course, means you should define what accuracyyourapplicationrequires.That will start you along the path of being able to understand and specify the different components that combine to make "accuracy".Remember that when you are up to your neck in alligators, it can be difficult to remember that your job is to drain a swamp. Beware the seductiveness of the voltnut addiction!

author: Ranger14
author_rank: Regular Contributor
timestamp: September 11, 2016, 03:27:55 am
content: Sorry for the late response but work has been busy..My Fluke 87 is a Series II not V but the manual recommends calibrating at 3.5v at .01% reference .. My plan is just to build a fairly accurate voltage ref to make sure my meters are still within calibration.. Also I was going to age the reference at least 250hrs and though about using the Fluke 87 Series II to trim the reference to 2.5v or something else.. I have tested the Fluke 87 against some what calibrated pieces and it's still withing spec from what I understand ..Here is what I am looking to buildhttps://www.tindie.com/products/JSaturnus/voltage-standard-reference-multimeter-tester/