Statistical measurement of resistors - Page 1
Forum section: electronics / general

author: BradC
author_rank: Super Contributor
timestamp: December 11, 2018, 03:26:04 am
content: G'day all,My Ultrohm Plus resistors landed last night.Among the order I have a set of 15 10K resistors. These were measured pre-shipping and I received a sheet with the listed deviations from nominal with an uncertainty of <0.5PPM (thanks Edwin!) at time of measurement.20 odd days later they land in my mailbox and I decided to measure them before they've had a chance to settle down get over their jetlag.I used hp3457a (which is now 3 months out of cal) on the 30K range and 100NPLC, took 4 wire absolute measurements and recorded those in a spreadsheet along side the supplied pre-shipping deviations.Because my meter is out of cal, and even when it's in cal the 30K ohm range 1yr rating is +/- ~55ppm, I decided to look at this from an angle that does not have any reliance on the absolute accuracy of the meter and just see how the relative numbers fall out.I took the mean absolute value of the measurements and used that to calculate deviations from the mean across the family. I used the supplied deviations to calculate absolutes, and did the same thing. I didn't need to do that for the calculations, but having those values meant I could calculate a family mean for the pre-shipping values (which was 10.00000147K).These resistors (as-measured pre-shipping) deviated -10.8ppm to +23.5ppm from nominal(10.0K) and -10.9 to 23.4 from family average. My local measurements deviated -12.2 to 23.8 from average (which I was pretty damn happy about given the kit I'm using). Measured family absolute mean was 9.999612K.The calculated differences between my deviation values and the supplied values range from -2.9ppm to 3.6ppm with a std-dev of 2. From that I can see a pretty strong correlation between my measured values and the supplied values. Using the two means, I can now speculate that my meter is reading ~39ppm low (which is well within the 1yr spec and is right on the edge of the 90 day spec - and it's now ~90 days out of cal).Now, I'm relying on 2 things. The short term stability of my meter, and the 20 day stability of a set of freshly minted (and therefore drifting) PWW resistors.The 24 hour accuracy rating on the meter is 10PPM + 5 counts, so one can intimate that could be the worst case short term stability. The resistors are unknown, but given my calculated deviations are within a handful of PPM of the supplied measurements, that might indicate that my local measurements are not too wide of the mark.This doesn't take into account any family-wide drift of the resistors, but it does indicate there are no real outliers and everything calculates out quite nicely given the measured deviations are under a quarter of the potential local error sources.These are 100PPM resistors, and the max deviation from 10.0K was 23.5ppm with a std-dev of 10 at time of shipping.As this was only an exercise in validating assumptions and I don't plan to do anything with these figures I haven't calculated an error budget, didn't do anything to minimise the error sources and haven't bothered to calculate any uncertainties. What I did do was validate some assumptions I had about my meter and measurement techniques.I plan on subjecting these resistors to some accelerated lifetime ageing and keep tabs on how they are moving relative to each other prior to putting them into the devices I bought them for, so there might be more to follow in the next couple of months.