Cheap AD584 voltage reference off Ebay. - Page 1
Forum section: electronics / general

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 16, 2019, 01:20:00 am
content: I have a decent bench DMM, a Keithley 2100, plus a couple of Philips 2525s and half a dozen handheld DMMs (mainly cheapish ones with the best being a UT61). I was looking at getting a voltage reference just to check the calibration of the cheaper DMMs. Anyway whilst looking I found an AD584 reference so cheap I couldn't build it for that so I ordered one just to have a play and if it was rubbish I've not lost a lot.Hooked up to a new 9v PP3 the 5V reference SHOULD give 5.000 as per the advert (if it was to be believed) but what I get on the Keithley is 5.012. Now the Keithley IS out of calibration date but connected to the UT61 it gives 5.014! I replaced the AD584 board with a cheapo 7805 as a comparison and got 5.001 on the Keithley.So looks like it was a waste of time but as with other things I can measure to several decimal places but can't be sure that I am getting the right answer.I can't imagine that the Keithley and Uni-T are both out by the same amount and that the voltage regulator is out by just enough to be more accurate according to the two DMMsSo what it the best way to get at least 3-4 digits of accuracy just as a lab standard? It would be nice to get even more accuracy but to be honest most of the stuff I deal with doesn't actually require the accuracy of my main DMM (but it would be nice).ThanksEDIT: If anyone knows how to make the Ebay one accurate then I have the STL files, which I created from Fusion 360, to 3D print a nice case for it :-)

author: beanflying
author_rank: Super Contributor
timestamp: February 16, 2019, 02:30:06 am
content: Depending on the version of the AD584 (or clone) it is still within spec at +-15mV.Trimming circuit is here in the datasheethttps://www.analog.com/media/en/technical-documentation/data-sheets/ad584.pdfMy KK moon version (Laser Cut Acrylic Case version) drifted a few PPM over the course of a year and has approximately a +1PPM/C tempco but otherwise fairly tolerant of 10M or GigOhm loadings without more than 1PPM change to the output.One of the other options to consider is roll your own Vref based on the Max6350. One of the benifits is the very close initial accuracy (+-1mV in practice they are even better than this) for those without a decent reference or calibrated sourcehttps://datasheets.maximintegrated.com/en/ds/MAX6325-MAX6350.pdf

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 16, 2019, 07:40:15 am
content: Quote from: beanflying on February 16, 2019, 02:30:06 amThanks, I'll have a look at this later. If I trim it so that the Keithley reads 5v then I can use it as a transfer standard but then I might have everything slightly off....

author: SvanGool
author_rank: Regular Contributor
timestamp: February 16, 2019, 12:50:38 pm
content: Instead of wasting money on non-calibrated references from doubtful sources, I would use a 10 V reference, using the full 10 V scale of your Keithley 2100 (1 year 44 ppm after calibration) from:Europe: LM399 10V Precision Reference Calibrated (34 ppm 1 year)https://www.ebay.com/itm/LM399H-10V-Precision-voltage-Reference-Calibrated-LM399/254066484205US: VREF10-003 (30 ppm 6 months, with free re-calibration within a year)http://www.voltagestandard.com/Both suppliers are well known on the forum and are active members.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 16, 2019, 01:25:49 pm
content: Quote from: SvanGool on February 16, 2019, 12:50:38 pmThat looks ideal. Thanks

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 16, 2019, 09:51:17 pm
content: I have another thread re my Keithley ....https://www.eevblog.com/forum/testgear/keithley-2100-error-614/msg2205438/#msg2205438So I changed the cheap ref to 10v powering it from the bench supply. So at 10v we get the followingKeighley 2100 - 10.00581Philips PM2525 - 9.98UNI-T UT60E - 10.00UNI-T UTD4102C - 9.99and at 5vKeighley 2100 5.01068vUNI-T UTD4102C 5.00vPhilips PM2525 4.98vUNI-T UT60E 5.01v

author: beanflying
author_rank: Super Contributor
timestamp: February 17, 2019, 07:14:28 am
content: Quote from: SvanGool on February 16, 2019, 12:50:38 pmEveryone has to start somewhere on there way down the rabbit hole that is 'knowing' what a volt really is or isn'tI spent a lot of years not knowing the truth only to find out that it doesn't really exist as there is always more zeros and less uncertainty to chase

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 17, 2019, 12:49:44 pm
content: Quote from: SvanGool on February 16, 2019, 12:50:38 pmThanks for this, do you know who the member is re the Europe one? Just a few questions re size, power supply requirements etc. I'm going to probably go with that.

author: SvanGool
author_rank: Regular Contributor
timestamp: February 17, 2019, 02:10:54 pm
content: Quote from: cowasaki on February 17, 2019, 12:49:44 pmIt is "RoadRunner" and you may find him here:https://www.eevblog.com/forum/buysellwanted/deeu-simple-lm399-reference-board-improved-second-version/msg1970513/#msg1970513

author: alex-sh
author_rank: Regular Contributor
timestamp: February 18, 2019, 08:51:40 am
content: @cowasakiI can highly recommend both sellers!

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 18, 2019, 10:46:28 am
content: Quote from: alex-sh on February 18, 2019, 08:51:40 amI’m buying the European one on Wednesday.