Restive dividers and Precision reference and PPM? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: October 14, 2015, 06:06:25 pm
content: How accurate is a restive divider? I was always under the assumption that they were as accurate as the resistors used, and both resistors would drift the the same way. So if you had some foil .1% resistor and with a +/-10ppm coefficient that they would drift the same way and keep .1% accuracy. I read some stuff on stack exchange contradicting that thinking. They are saying that the tempco of one resistor could be in the negative and the other in the positive causing opposite drift. And also that the smaller resistor in the divider will get hotter causing drift.If you want to take the output of a calibrated standard and lets say 5v and get as close to you can to 400mv, will a divider work? How much accuracy will you lose, and how much drift will happen with lets say a 100k and 1k divider? How does one step down a standard without accuracy loss? Would paralleling resistors on each side of the divider help average things out? What about using networks connected in different arrangements?

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2015, 06:20:24 pm
content: Quote from: rwgast_lowlevellogicdesin on October 14, 2015, 06:06:25 pmNope. The larger resistor is the one dissipating the most Watts (bigger voltage drop)....and if one of them is dissipating more watts then it will get hotter than the other. Result: They'll drift by different amounts.Quote from: rwgast_lowlevellogicdesin on October 14, 2015, 06:06:25 pmYep. It's all down to the accuracy of the resistors.(nb. A 10 Ohm multi-turn trimmer pot would be cheaper than very accurate resistors)Quote from: rwgast_lowlevellogicdesin on October 14, 2015, 06:06:25 pm100k will dissipate less heat and therefore drift less.(nb. It will also be more prone to noise so put a small ceramic capacitor from the center of the divider to ground).

author: German_EE
author_rank: Super Contributor
timestamp: October 14, 2015, 08:43:48 pm
content: I was always told that PROVIDING there is no resistive heating in a two resistor potential divider they will both drift in the same direction if the temperature varies. My 10 MHz reference has a 10K trimpot on the VCO and this is outside the thermal chamber but I still have very little drift.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2015, 09:49:25 pm
content: Resistors are typically rated for a range of possible values, across manufacture, temperature and age. Whether any of those errors are correlated across different individuals from the same series, or across manufactured lots or years, is not stated. You'd have to take the measurements yourself; or preferably, just spend a tiny fraction of that research cost on tight tolerance parts ($5/part is probably better than $10k of research labor, unless you're making a lot of them*).*And if you are, you're probably better off consulting the manufacturer, having them design and build a custom part (like those thick film resistor dividers in the Keithley and others!), and having them meet the specs you set out for them (which will tend to make the part more expensive, because they have to account for yield and liability; but again, it's not a cost that's going to go away, because either you have to test samples, or they do; and they're probably set up to design and run those tests already).Tim

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: October 14, 2015, 11:06:59 pm
content: Ok so 10ppm high accuracy vishay thin films arent good for this? How about if you do something like two 1ks to divide in half and divide in halfs till acceptable. I will check out a thick film set up too. Is there a way besides a divider to transfer 5.0000 volts down to 399mv or less with no accuracey loss?I also have a few broken meters with sheilded divider networks like the ones in uni t 61. Would repuropsing these do the trick? My plan is to let the standard settlt then connect dmm to divider and then divider to vstandard so if running the divider for less than 30 seconds will fix drift issues that will work too.

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2015, 11:40:01 pm
content: Quote from: rwgast_lowlevellogicdesin on October 14, 2015, 11:06:59 pmYou could try it and see.Quote from: rwgast_lowlevellogicdesin on October 14, 2015, 11:06:59 pmIf all the resistors are the same value the drift will be minimized. It's going to be expensive though...

author: Rerouter
author_rank: Super Contributor
timestamp: October 15, 2015, 01:31:12 am
content: easier way is to tightly couple the 2 together, mount them touching side to side and bond then with thermal epoxy, now even with dissimilar heating, there is less temperature difference between the 2, This still relies a little on them both drifting at the same rate,now how do you find that out? Hair drier, if the result of the division starts changing more than acceptable then they either have dissimilar drifts or opposite drifts, but same family and technology tend to at-least be same direction,Kind of like how on time keeping, I lucked out on 2 10ppm crystals from different suppliers having only a 0.3ppm relative temperature drift from one another (and an 8ppm offset but that was easily worked around)

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: October 15, 2015, 02:36:07 am
content: Hmm ok thanks for the tips, last question at what point is johnson noise a factor, and how visible is it on a scope? Is it LF or HF, my DSO is only 25mhz

author: SteveLy
author_rank: Regular Contributor
timestamp: October 15, 2015, 04:09:07 am
content: No idea about the intricacies of drift issues, but the basic accuracy of a divider made up of 0.1% resistors will be 0.2%. Relative errors add when you multiply or divide, so in R1/(R1+R2), 0.1% + 0.1% = 0.2%.Keep in mind that 0.1% is 1000 ppm, so if you don't cook them drift should not be an issue IMHO as far as the basic accuracy is concerned. Just run them at well below their power ratings and she'll be right.

author: Fungus
author_rank: Super Contributor
timestamp: October 15, 2015, 06:10:05 am
content: Quote from: Rerouter on October 15, 2015, 01:31:12 amNot needed. The tiny amounts of heat we're talking about here will easily conduct down one leg of the resistor and into the other one. Put a big copper pad on the PCB to help things along.I say just go for it and see what happens. A 10ppm coefficient would mean you need a rise of 100 degrees to see a difference with three digit accuracy. If you start with 0.01% resistors then you'll be fine.

author: splin
author_rank: Frequent Contributor
timestamp: October 15, 2015, 10:19:24 pm
content: Quote from: SteveLy on October 15, 2015, 04:09:07 amNo. The worst case error depends on the divider ratio and the tolerances so at 1:2, .1% the maximum error would be .999/(.999 + 1.001) = .05%.At higher ratios the error increases; e.g. at 1:1001.001/(1.001+99*(1- 0.001)) = .2% which agrees with your statement.But increase the tolerance to 20% and the maximum 1:100 ratio error increases further to 49.5%, rather more than the sum of the tolerances you suggested.