Someone should work out all the voltages possible from an LM317 and a quad r-pack.
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tIt would be more irritating if there were more applications where you could exploit this. I've known about ratio transformers for about forty years no w, but have yet to find an application where I could use one.
And they don't have to be rack-mounted to be that good. A ferrite core woun d with Litz wire could be hooked up as a ratio transformer, though making t he connections would be both fiddly and labour-intensive. I've long cherish ed the idea of winding one with round-to-flat cable. You couldn't put on al l that many turns, but you wouldn't have to sort out the wires.
The one time that I had occasion to actually try to source such things I found that many of those arrays don't guarantee matching -- they're basically some number of "plain old" precision resistors all on one substrate, with no guarantees about matching beyond what you'd get from a bag of similar-precision resistors.
Clearly we weren't looking in the right place, or perhaps the market has developed in the last 12 years. Or we weren't looking in exclusive enough boutiques.
We ended up solving that problem another way. Had there been a $30/ea solution, I think that we would have still solved the problem with that alternate method -- like Joerg, the group I was in had an aversion to boutique parts. In his case it's often expense; in ours it's because we were building products with 10-15 year lifetimes.
I think that misses at least
r+(r|(r+r))
to use my "notation".
Yes that is what I needed, otherwise you don't get to take advantage of the ratio matching of an integrated network. (Or of resistors from the same reel possibly?).
Also some of the LT5400 parts have two values in the package, so you need to actually get the formulae so you can plug in the values.
I think that's my 1.666 case.
Well, they're used all the time in capacitive gauges.
Cheers
Phil Hobbs
No no, in my sketch the bottom resistor is moved over underneath the right hand one!
Oh all right, sorry. I plead a cold coming on.
Oh well, ok. Get some rest and drink lots of liquids.
My question is why? Even brewing beer does not take that kind of precision .
Okay, here's a first cut, assuming all equal values and ignoring the adj pin current, using the circuits John D listed:
# Circuit Ratio LM317 LT1964 LT1761 TL431 TLV431
13 "(a|b) + (c+d)", 0.800 2.81 -2.75 2.75 5.61 2.79 19 "(b | (a+c)) + d FB a-c" 0.800 2.81 -2.75 2.75 5.61 2.79 4 "a+ (b+c+d)", 0.750 2.92 -2.85 2.85 5.82 2.89 2 "a + (b+c)", 0.667 3.13 -3.05 3.05 6.24 3.10 7 "(a|b) + c", 0.667 3.13 -3.05 3.05 6.24 3.10 9 "a + (b + (c|d))", 0.600 3.33 -3.25 3.25 6.65 3.31 11 "a + ((b|c)+d)", 0.600 3.33 -3.25 3.25 6.65 3.31 17 "(b|(a+c))+d", 0.600 3.33 -3.25 3.25 6.65 3.31 1 "a+b", 0.500 3.75 -3.66 3.66 7.49 3.72 5 "(a+b) + (c+d)", 0.500 3.75 -3.66 3.66 7.49 3.72 15 "((a|b) + (c|d))", 0.500 3.75 -3.66 3.66 7.49 3.72 12 "(a+(b|c)) + d", 0.400 4.38 -4.27 4.27 8.73 4.34 14 "((a|b)+c) + d", 0.400 4.38 -4.27 4.27 8.73 4.34 3 "(a+b) + c", 0.333 5.00 -4.88 4.88 9.98 4.96 8 "a + (b|c)", 0.333 5.00 -4.88 4.88 9.98 4.96 6 "(a+b+c) + (d)", 0.250 6.25 -6.10 6.10 12.48 6.20 10 "(a+b) + (c|d)", 0.200 7.50 -7.32 7.32 14.97 7.44 16 "(a) + (b|(c+d))", 0.200 7.50 -7.32 7.32 14.97 7.44 18 "a + (b | (c+d)) FB c-d" 0.200 7.50 -7.32 7.32 14.97 7.443 s t s
That´s not my experience. They are made on the same substrate and then sp rayed with vaporized metal film (if that's the correct term) and the reason that they match so well on a quad pack is that within the small area of th e resistor pack, the vapor density is uniform, so that each element "sees" the same amount of metal
Cheers
Klaus
Bridges for precision temperature measurement and capacitance bridges are two "common" applications for the physics types.
By "that good", I presume you mean "fairly decent", not 1ppb!
Not all the rack mount ones are as good as 1ppb, some are as bad as
0.1ppm/K (100x worse).Probably assembling ~1,000 similar Z-foil resistors into a series-parallel divider would get well within an order of magnitude, of the good ones, but..
Fluke use a "statistical array" made from resistor networks in their voltage standards.
See fig 4, I think it's possible there are more elements than that shown.
(This is what I was using the LT5400 and my gain calculating program for).
Anyone know what are these "Taitanium-Nitrate" arrays? I think they mean tantalum nitride but even if so have not been able to find any with anywhere near required ratio tracking.
Perhaps they use hundreds of them as you suggest.
Things may have changed in the last 15 years, or we may have been asking the wrong vendors. I dunno -- I just know we didn't find anyone that would sign up to what we were looking for.
Maybe they are custom thin-film arrays.. if the stability of the TaN resistors is good enough, a clever arrangement on a thin-film substrate might make it work. Something like the transistor arrays used in MOSFET-input op-amps.
If they're all on one substrate it might not be all that costly.
They're pretty tight-lipped about what exactly is inside the LT5400.
s
Quite a few resistor arrays are intended to be used as pull-ups or pull-do wns and don't need to be at all precise.
I first got enthusiastic about them when I - briefly - worked for Chessell Recorders in 1979. Chessell didn't end up using them - the twit who took ov er the project didn't see the point. He also rejected a positive feedback o f 1.0003 because he feared that it might cause oscillation.
The 1979 parts were closer to $3 than $30. I think they came from Beckman.
Ratio transformers? Ferrite-core ratio transformers wound with Litz wire?
A little more information could be interesting.
I imagine you are thinking of the Baratron capacitative pressure sensors - my Ph.D. thesis included a discussion of that approach back in 1969, based on work that had been published by the Philips Laboratory quite a bit earlier.
ould
One of my colleagues at Cambridge Instruments used one in a calibrator for
6-digit DVM (20-bit). Not for Cambridge Instruments - it took longer than e xpected at his previous employer, who carped enough that we got him - and w ere very glad about it.The quality is determined by the physics, not the size. More space does giv e you more room to be picky about the lead dress.
Rayner and Kibble devote quite a lot of their book to singing the praises o f ratio transformers. You don't have to work too hard to get 1ppb absolute accuracy out of a bifilar-wound ratio transformer. The more useful beasts w ith eleven intertwined windings are hard to get better than 100pbb.
Since it's all fixed by geometry, thermal drift isn't much of an issue (bel ow the Curie point of the core).
Bad design.
Worse design.
t .
-- Bill Sloman, Sydney (but in Nijmegen at the moment)
I didn't see much information on details of construction in the thin R&K book.
There's a lot more detail in B. Hague/TR Foord's excellent book on bridge methods (and even more in the copious references), enough to lead me to think I might not be able to make a 1ppb/K transformer the first time or second time. Here's my (commercial) one.
At 30lbs and >$12K new it's not very practical as a component.
Apparently. And that one is not by an unknown name. McGregor et al. at NBS found they had to ovenize the ratio transformer (they were shooting for a few ppb total error).
Certainly could be, and perhaps silly and/or obvious.
Any other ideas for getting a DC ratio stable to within, say, 10ppb/K, and WITHOUT ovenizing?
Best regards, Spehro Pefhany
I don't understand that. LM317 ref voltage is 1.25, so some obvious available voltages should be 1.25 and 2.50.
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