Digital Calibration help?  - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 08, 2013, 11:56:54 pm
content: my boss asked me if i could some how find a way to factory calibrate a voltage referencethe chosen voltage refrence is 2.5v (can do 3V also) and we need an out of 2.048vmy first choice was a 10 turn pot and a stepper motor but that was apparently too much for him so this was my idea and have someone do it manually got me laughed at ... soo this is what i thought uptake a fairly cheap AD5174 1024 step SPI epot and plop it in the middle of a voltage divider and have pogo pins come down and hit some pads and with the SPI pins plus ground and have volt and sense right before a voltage follower and have one of the equipment boys calibrate an high precision volt meter than just throw something together with one of the arm dev boards i have to calibratethe vref is only 2ppm/c so that with the 0.1% resistors in the divider it does not have to adjust it tooooo much so i think 1024 steps is MORE than enough ... might even bump it downthis is only for brownie points and will just be one possible solution but i thought that i would run this by you guysor if you guys know of a simpler solution to this let me know!side note: with the cost of this project +/- a few bucks per part is not really going to make a dent so im not too worried about the added part

author: amspire
author_rank: Super Contributor
timestamp: April 09, 2013, 04:33:06 am
content: Quote from: BiOzZ on April 08, 2013, 11:56:54 pmYou can make a custom reference, but if you have a 4 1/2 digit dmm or better, they are very linear.If you like, you can make up a meter checking reference using one of the reference ICs accurate to 0.05% or better just to check the meter. The MAX6350 is 0.02% 5V. Then just rely on the multimeter reading.It is easy to make a very precise 2:1 divider, but it is really hard to beat the linearity of a good multimeter. It will probably be better then many D/A converters.Using PWM using a chip like an Arduino is superb at precisely dividing a reference voltage. You can look at hi s discussion:https://www.eevblog.com/forum/projects/general-purpose-power-supply-design-7488/msg99807/#msg99807Here is the performance it can be achieved with the PWM output attached to a 74HC cmos type gate attached to the 5V reference supply. To buffer the PWM output after the R/C filter string, you want a good precision low offset opamp as a unity gain voltage buffer.The thing with any custom reference you make is you will still need to regularly double check it is still accurate.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 09, 2013, 10:35:20 am
content: I think it should work. Things to note:1) you want to have the full 1024 steps, because this gives you much finer control. The resistor itself is nominally 10k ohm, and has a 15% tolerance, so it can be way out. having 1024 steps gives you the needed control and you don't care that it's 15% tolerance anymore, because 1 step is 10k/1024 which is less than 0.1% of nominal.2) I just did a quick math, a voltage divider with 100k on the bottom and a 18k+10k epot at the top, each step of the epot will give you about 150uV to 175uV change in the resister divider's output, ranging from 1.9531V to 2.1186V. If you play with the numbers more you can probably get tighter tuning range and use more of the epot, thus smaller steps. My example only uses a small bit of the 10k epot.3) the epot has a tempco of 35 ppm, that's ok, and probably better than the tempco of the other 0.1% resistors in your divider.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 09, 2013, 01:27:37 pm
content: I don't get it, are you trying to calibrate a reference that is part of a facrory line or are you trying to calibrate onboard references in manufactured products?

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 09, 2013, 03:43:04 pm
content: Quote from: codeboy2k on April 09, 2013, 10:35:20 amGood idea i complacently forgot to check the accuracy of the epot but i am quite happy herethanks for your help!Quote from: hlavac on April 09, 2013, 01:27:37 pmits calibrating a voltage reference that will be inside of a product

author: ve7xen
author_rank: Super Contributor
timestamp: April 09, 2013, 08:01:13 pm
content: It's not enough to measure the actual reference voltage and store it in EEPROM/wherever for your application to use the exact value? Or do you actually need to provide an exact voltage output for some reason?

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: April 09, 2013, 10:01:10 pm
content: In the product that is going to be calibrated, how is the calibration adjustment done, i.e analogue or digital, a pot or a calibration word. Is the connection to the product by a header or just multimeter leads.

author: Neilm
author_rank: Super Contributor
timestamp: April 10, 2013, 06:37:04 pm
content: Is there any reason you couldn't get a 2.048V reference? It will save you having to adjust anything saving production time. Also it will reduce the eliminate any issue of the pot value changing due to any EMI.