Known voltage source for checking calibrations... - Page 1
Forum section: electronics / general

author: george graves
author_rank: Super Contributor
timestamp: January 05, 2011, 12:27:30 pm
content: I think I'm really unlucky with buying test equipement. Either my year old Rigol 1052E isn't measuring voltage correctly, or my 6 month old Fluke 87-5 is messed up. I say that because when I hook them up to to the same source, the Rigol will read about 10% different.Anyways, I'd love to figure out what one is incorrect, so that I don't have to send BOTH in - and not only that but I'd love to check them against a known standard occasionally so that I don't feel like I feel right now (slowly loosing my damn mind over it!)I found this:http://lategahn.2log.de/index.php?calibration-standards-introductionQuoteThe voltage "standard" looks very promising. Any thoughts on such a set up?Thanks!

author: saturation
author_rank: Super Contributor
timestamp: January 05, 2011, 12:46:20 pm
content: Its OK but you'll still need to calibrate the output of your design to determine what exactly the voltage is. Over time it will drift and it needs to be characterized and stabilized.We have a very detailed discussion on voltage standards here.https://www.eevblog.com/forum/index.php?topic=1652.0https://www.eevblog.com/forum/index.php?topic=650.0https://www.eevblog.com/forum/index.php?topic=601.0I'm maintaining one to 10uV accuracy but it takes a lot of work, even if you just buy a premade reference as per the links. However, if you buy a precision hobbyist reference good to the uV, but only require it for testing to mV, it should be good for years after you purchase it from Geller or voltagestandard.com and the unit is properly made.The reason is after calibration, and excluding periodic variation with weather and climate through the year, it could drift at worst + 100uV/year [see the spec sheet of the chip you choose, 100uV is a really worse case 'cheapo' reference]. At that rate, it will need 10 years before it registers 1mV out of calibration on the mV scale.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: January 05, 2011, 01:08:15 pm
content: Quote from: george graves on January 05, 2011, 12:27:30 pmI have build it , works great !!Check pictures and details here..https://www.eevblog.com/forum/index.php?topic=1148.0

author: FreeThinker
author_rank: Frequent Contributor
timestamp: January 05, 2011, 02:39:46 pm
content: Quote from: george graves on January 05, 2011, 12:27:30 pmSods Law predicts that it will be the (Just out of warranty ) Rigol thats at faultPS Have you tried to recalibrate your Rigol, may be a quick fix

author: Time
author_rank: Frequent Contributor
timestamp: January 05, 2011, 08:41:17 pm
content: I'd put my money on the Rigol being wrong. An oscilliscope is not supposed to be terribly accurate with voltage measurements in the first place. It would make a poor DC voltmeter if thats how you are testing it.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2011, 11:36:42 pm
content: I wouldnt trust any oscilloscope to give an accurate voltage reading, (well.. maybe one worth $20k +)Depending on the settings on the oscilloscope (volts/div and probe setting) 10% error might be perfectly normal.Remember scopes were designed to graph changes in a signal over time, displaying the voltage digitally on the screen was something that was tacked on later, it wasnt what they were designed for. With digital scopes the scope cpu only has a limited number of vertical bits to work out the voltage from.The more of your signal you have on screen (number of divisions between the negative and positive of your voltage) the more bits are used and the more accurate it can measure the voltage. The rigol has 8 bits vertically, like most scopes do, so you don't have a huge number of bits for accurate vertical measuerment like a DMM.If your scope is setup so it could display 200V from top to bottom division and you're trying to measure 10V then your spreading the 8bits over 200V, not many left in the 10V area to measure your signal. Having the scope probe set to x10 also makes things less accurate but you do get the ability to measure higher voltages and it adds some protection to the scope so its a tradeoff.So the voltage display readings on the lcd are going to be most accurate when the probe is on 1x and the volts per div results in your signal taking up the whole lcd.In another thread where someone was having a similar issue i did some tests on my scope.This was with a 5.00V signal1x probe50v/div = not possible20v/div = not possible10v/div = 5.4V = 8.3% error5v/div = 4.99V = 0.2% error2v/div = 5.08V = 1.6% error1vdiv = 5.05V = 1% error10x probe50v/div = 6.63V = 32% error20v/div = 6.00V = 20% error10v/div = 5.73V = 14.6% error5v/div = 5.02V = 0.4% error2v/div = 5.01V = 0.2% error1vdiv = 5.03V = 0.6% errorIf you want more info here is the threadhttps://www.eevblog.com/forum/index.php?topic=1989.0