> Using 25-pole sub-D-connectors as programming/shorting bars allows easy and fast switching from 0.1*R to 10*R.
I like that idea. ![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif)
I agree. I also like the machined aluminium case, which will help ensure an isothermal environment for the resistors. The only improvement I would make is to upgrade the quality of the resistors [which is easy to do up to 1M\$\Omega\$].
*** EDIT ***
The DB-25 might become an issue at the lower resistance values [say, <= 10K\$\Omega\$], and so for those, a lower resistance connection system will need to be designed.
This looks like fun, and I might try this.
It seems he is doubling the connections, there are DB25 rated for <30m? connections, for the series it would add <30m? (one on each side, 2 in parallel). For the parallel would add <5.5m?, 5 connections in one side, 6 in the other, twice in parallel. For the 1ppm target is already too much for using the 10k resistors, as it would add 5ppm in the parallel mode.
Using the second connection would allow for 4 terminal resistor approach in both, series an parallel modes. You could also use 9 resistors 3S and 3P and get the resistor value and use the 4 terminal approach here as well. One would be off the network but if you select just that one to be the closer to the average of the 10 you reduce that error. Using the 4 terminal connection it could probably go to quite low values and have reasonable results, probably not 1ppm but could still be useful.
I've just got ten 10k? resistors (away from my lab and 1% are oddly expensive here) and they had an standard deviation of ~0.2% so if those numbers are ok 1ppm seems not a hard thing to get. I measured all 10, got the average 9k996 (only 1LSB changed of one single resistor before or after soldering given enough time to settle). I used the worst DB25 I ever seen and only using single connection. The one off the 3P3S mode read 9k97 BTW. Luckily for me today is 20ºC in here, nice day to measure resistors.
AVG= 9k967
Series= 99k6
3P3S= 9k97
Parallel= 998?
I'm using the oficial DMM to get my numbers, I've got it yesterday. I could get it to the university nest week and have a better look. I guess the extra ? from the parallel expected reading could be helped by the bad connections, which would make the DMM track for the LSB within the 3 ranges.
As there still are 3 extra terminals in the DB25 a temp sensor could be added inside to have as a reference.
JS