JS,
I think you refer to the Vishay NOMCA series, with 7 or 8 resistors in an SMD package. I could not find others that looked suitable. The NOMCA series have similar specs as the AORN series, and cost about the same (about 5 Euro's) for twice the resistors. Go figure.
What really surprises me is the fact that these (pretty) high precision resistors with a (pretty) good TCR are so inexpensive, compared to through-hole resistors. The only downside that I see is that the power rating per resistor is low with only 100mW. I will not use my KVD with inputs higher than 30V. Correct me if I'm wrong, but I think that self heating by the current is no real issue then, certainly not on my more typical 5 to 10V inputs, let alone if the KVD is used as a bridge component.
So it still looks to me a viable alternative for a hobby application for the KVD.
Going down that path a bit further, I found that both Mouser and Digikey do not have the 2K resistor version in stock for both the AORN or the NOMCA series. You would have to use 2 x 1K in series.
As you alluded to, the 10K resistors of the first decade need to have a method for trimming each individual one, to get them within the matching specification of +/-0.0037%.
If I'm going for the NOMCA series, the absolute TCR is +/-25 PPM/C, and a tracking (I think they mean from resistor to resistor within the package) of 5 PPM/C. Putting a 100K trimpot in parallel, with a typical 100 PPM/C, as you propose, would degrade the total PPM I think.
In my opinion, it is better to use a small resistor in series (can be 50PPM/C) and a 100 Ohm 1 turn trimmer (100 PPM/C) in parallel to the small resistor. The maximum PPM influence with that combination should be less than 2 PPM/C on the 10K, if my calculation is correct. With a 1 Ohm/100 Ohm combo, you can adjust the 10K with an additional 0-0.9 Ohm, and stay close to the absolute 10K value. The value of the 1 Ohm depends on the absolute variations of the 12 resistors (for a 1.1 KVD input) that can be selected from the total of 16 that are available with 2 NOMCA16031002AT5 packages. If they vary too much, you can increase the value of the 1 Ohm.
By using the NOMCA16031001AT5 1K packages for all the other resistors, you could potentially select enough to match for 11 2K resistors and avoid the trimming. What I mean is that by using 1K resistors for decade 2, you need 22, and you need 11 for decades 3, 4, and 5 each, and 10 for decade 6, for a total of 65 resistors. This is one resistor short for 8 x 8 resistor packs, so you can add a normal 1K resistor for the last decade, or add another package. In that case you have 9 packages x 8 = 72 resistors to select from, to get a matching set (+/- 0.037%) for the second decade. Should be possible, I was able to select better matching than spec with a set of 100 1% 2K05 resistors. In this case you can also combine two 1K resistors in series to help matching.
The expenditure budget would be 11 x NOMCA1603 for 5 Euro's each, 11 trimmers for say 1 Euro each and 12 1 Ohm resistors for say 0,10 Euro's, or a total of about 66 Euro's. Not bad. I spent more on my first prototype with batches that totaled about 600 resistors of 1% metal film through-holes to match them to specifications. Actually, I was able to match them to much better than spec.
One of the advantages I think are that it will be easier to control the TC of the individual decades and the total board, compared to using individual resistors, especially when mounted Manhattan style, which is what I did.
Are there any objections to following this route or do any of you see pitfalls?
Tks!