Adjustable voltage reference using DAC - Page 1
Forum section: electronics / projects

author: DanKalc
author_rank: Newbie
timestamp: February 04, 2014, 07:15:22 pm
content: Hi,I would like do make a simple quite precise adjustable voltage reference and i thought about using a precise reference with a DAC. I found a quite nice DAC from Analog AD5791, which as a student i can get with a sample request. This DAC has a output noise level about 1.1uVp-p. Will this work or should i use standard Kelvin-Varley divider circuit?datasheets:http://www.analog.com/static/imported-files/data_sheets/AD5791.pdf

author: leppie
author_rank: Frequent Contributor
timestamp: February 04, 2014, 07:47:47 pm
content: You can get samples without being a studentFunny thing is they DHL it to you for free. Can't imagine the costs for them.

author: dannyf
author_rank: Super Contributor
timestamp: February 04, 2014, 07:50:39 pm
content: I am in the process of making one, following an application note by Linear (AN86?).

author: branadic
author_rank: Super Contributor
timestamp: February 04, 2014, 08:20:58 pm
content: The cheapest way is, as stated in many threads you should have read before, the EDN design:"DC-accurate, 32-bit DAC achieves 32-bit resolution".http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolutionThere are some necessary modifications to make, as posted by Andreas.So no need to sample expensive parts. Personally I don't like if students use the sample request for private use, there're many companies that have changed the sample request strategie because of that fact."You can't afford it, you can't buy it!"This pretty simple lesson you need to learn first in life before you start an expensive hobby.

author: ve7xen
author_rank: Super Contributor
timestamp: February 04, 2014, 08:32:25 pm
content: Aggressively filtered PWM is extremely linear and can work well. You will need to implement it carefully but it's not that difficult. Use a fast CMOS logic variant.Downside is it's slow to settle but for a voltage reference this probably isn't a huge problem.

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 04, 2014, 10:53:59 pm
content: Quote from: branadic on February 04, 2014, 08:20:58 pmyou are just a ball of fun aren't youits not like we are going to the CEO of analog devices and putting a gun to his dogs head trying to shake him down for dacsits great when students sample things, they are the engineers of tomorrow. Fuck them, they can run a fab that costs thousands of dollars per hour but they are gonna bitch about a few samples to kids who are gonna be working for them one day? what do you think that we are selling dacs to the pawn shop to buy crack cocaine? if some college kid wants to volt/time nut it up whats the problem?a person can learn much from using expensive parts. god i hate this fucking chinaguyver shit going on in this forum sometimes. I did not know the 25 people on the planet interested in 8.5 digits are putting a huge dent in the bottom line of a multinational multi billion dollar companyconsider it fucking charity that they owe to society and something to further electronics knowledge.

author: dannyf
author_rank: Super Contributor
timestamp: February 04, 2014, 10:57:17 pm
content: QuoteThat's an interesting idea. However, in a world where dacs are fairly common, I question its practical value.QuoteBut it is difficult to make it work - ie it is difficult to stabilize it in a feedback loop.There are pwm-generated Vref meter but they tend to be very slow - to avoid the stability issue.

author: ve7xen
author_rank: Super Contributor
timestamp: February 04, 2014, 11:01:45 pm
content: Quote from: dannyf on February 04, 2014, 10:57:17 pmDoes a voltage reference imply an external feedback loop? It sounded like OP was just asking for a relatively precise voltage generator, which shouldn't need any feedback. Speed is an issue, but may not be, depending on application.

author: dannyf
author_rank: Super Contributor
timestamp: February 04, 2014, 11:14:26 pm
content: QuoteIt doesn't have to. However, many factors will impact the output accuracy - opamp's offset or drift for example, or load variability, etc.For a reference level device, you have to maintain its repeatability and that in my view requires a feedback loop to make sure that when you tell the device to generate 1.000v, it is actually generating 1.000v, or 1.001v or 0.999v.An open loop Vref generator is of no practical use unless you are looking for low (0.1%?) accuracy.Linear has an application note on that.

author: Mickle T.
author_rank: Frequent Contributor
timestamp: February 05, 2014, 06:01:07 am
content: This articlehttp://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolutionis like a students homework and nothing else. Read the comments, especially from Elio.Mazzocca_#3. Even the strongest Datron/Fluke calibrators have a INL as low as specified in this opus, but only after pre-calibration procedures (including nonlinearity too). Without a such linearization calibrators have a much worse INL.

author: branadic
author_rank: Super Contributor
timestamp: February 05, 2014, 07:32:50 pm
content: QuoteI will give you jester an answer....QuotePeople that can afford smart phones and fruit labeled products should also be able to buy a simple single chip via distributors, don't you agree?QuoteNo they can't, the opposite is the case! They unlearn simple basics. The best solution is the most simple one that can fullfil the requirements!Seems like you are one off those 21century guys that use the next stronger plattform instead of optimizing code to solve a problem with a given hardware.QuoteAs I already wrote, Andreas has given lots of hints in the www to that design and the necessary modifications, for example refer to the volt nuts archiev

author: dannyf
author_rank: Super Contributor
timestamp: February 05, 2014, 09:57:58 pm
content: QuoteHe must be a pretty good student, then.QuoteI saw lots of "wild claims" in Mazzocca's comments, as he complained.Sound reasoning would be more helpful.

author: Andreas
author_rank: Super Contributor
timestamp: February 05, 2014, 10:15:28 pm
content: Quote from: dannyf on February 05, 2014, 09:57:58 pmI guess he has never built the circuit.My results can be read on volt-nuts archives in october 2010.And if I compare some of the specs with a Fluke 5520A (e.g. noise output) the circuit (with some tweaking) is great value for money without having to sell your brand new car.with best regardsAndreas

author: AlfBaz
author_rank: Super Contributor
timestamp: February 06, 2014, 01:17:35 pm
content: Quote from: dannyf on February 04, 2014, 11:14:26 pmA lecroy oscilloscope I recently fixed was full of them (open loop Vrefs).They call it a scanning DAC. It's output is multiplexed to several outputs where at the other end a simple "sample and hold" type circuit feeding a buffer is used as part of the calibration cycle during voltage or time scale changes on the front panel. The banner spec claims a DC accuracy of only 1%

author: SteveyG
author_rank: Unknown
timestamp: February 08, 2014, 12:02:02 pm
content: Quote from: branadic on February 04, 2014, 08:20:58 pmI have to disagree with you somewhat here. There are people who obviously take the pee, ordering samples, selling them on or selling something they've made with samples etc which is absolutely out of order, but I would not have a problem supplying students with components for educational purposes. IMO educational purposes includes their own personal projects.I used to order samples from TI, Analog Devices and Microchip as a student which meant I could build projects that I otherwise may not have. Based on that experience and knowledge of their devices I would typically spec at least one TI, AD or Microchip part in any device I develop at work. The awareness and hands on experience provided by the sample scheme mean that I have provided sales for them literally into the millions of pounds.

author: dannyf
author_rank: Super Contributor
timestamp: February 08, 2014, 12:51:16 pm
content: QuoteNot sure what volt-nuts are but if you care to share your insights or a link to the particular results, it would be greatly appreciated.

author: branadic
author_rank: Super Contributor
timestamp: February 08, 2014, 01:09:01 pm
content: Never heard of yahoo, google ...? Just type in volt nuts and you will be linked to:https://www.febo.com/cgi-bin/mailman/listinfo/volt-nutsA simple click on "To see the collection of prior postings to the list, visit the volt-nuts Archives." brings up a new page, then look for october 2010 and bingo, you will find:http://www.febo.com/pipermail/volt-nuts/2010-October/000537.html

author: SteveyG
author_rank: Unknown
timestamp: February 08, 2014, 03:31:10 pm
content: Quote from: branadic on February 08, 2014, 01:09:01 pmMaybe you should have provided a link in the first place. 99% of people on here will not know what volt-nuts is.

author: branadic
author_rank: Super Contributor
timestamp: February 08, 2014, 04:55:39 pm
content: Most people out there are able to use a search engineYou haven't heard about something? Use a search engine and study yourself. Believe me, it's as easy as it sounds.

author: SteveyG
author_rank: Unknown
timestamp: February 08, 2014, 06:23:41 pm
content: Quote from: branadic on February 08, 2014, 04:55:39 pmIt could easily have been a journal or magazine though. Easiest thing to do it just provide a link to external references if you expect people to read something as per standard forum etiquette rather than resorting to sarcasm.