Once you have a few resistors on a schematic, it's tempting to do what you want using existing values. That reduces the BOM and the pick-and-place setup. There are far more possibilities if you are willing to compromise things some, or put resistors in series and parallel. This is a 4-page schematic with 9 different resistors. Doing this is an enormous PITA.
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Saved a DAC reference chip! Actually, having the DAC voltage ratiometric on +15 is a benefit here.
That opamp should be stable with the tantalum load and a 1u ceramic at the DAC, but I'll verify that.
Yuk! That would involve an entire new chain of value juggling.
The resistor ratio needs to be 1.4:1. I can get 3.125 volts with three resistors.
The +15 charges a timing ramp through resistors, and the DAC feeds some timing comparators, so it's nice if they are ratiometric. 317's are not very good references.
Data sheets suggest that an LM317 can't make 3 volts from 5 volts, so I'd have to power it off +24.
On a sunny day (Sun, 14 Nov 2021 09:39:03 -0800) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:
Really?
What I do not get is: you sell expensive things in low quantity and try to save on a trimpot? or waste board space with funny resistor combinations? I mean I use 3 resistors to get a LM317 to output some specific voltage all the time.. mostly different ones. Sure you can use 1 Ohm resistors to do most things, but then you need many,,,,
10 Ohm would work too here.
Power it from the 15V you made with the first one.
Trimpots are expensive and need manual turning. DACs are automated and can have cal factors stored in cal tables. I'm using 16-bit DACs and there are no 16-bit trimpots.
Some of my customers absolutely forbid trimpots, which I think is extreme. Once in a while they make sense. They say that if there's a pot, their techs will turn it.
I have space on this one for a few more 0603s. It's inefficient to load a reel onto the pick-and-place just for one resistor.
The LM317 is huge.
OK, but the little opamp works better. Same number of parts, much smaller, more stable, uses available values. That's the game here.
On a sunny day (Sun, 14 Nov 2021 10:27:20 -0800) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:
All true As far as trimmers go, I remember the story of that guy who brought in a defective radio and said: "All those screws in those metal cans were loose, I fastened all of those."
I have trimpots in use in many things, some have been on for decades.
One good use for small trimpots is to set the gain in wideband - hundreds of MHz - things like photodiode amps. Especially when the product is not digital.
Analog multipliers are expensive and relatively slow. PIN diodes are great but don't work at low frequencies or DC. A tiny DIP switch and resistors are OK if you don't mind coarse resolution. There are some super wideband many-bits digital RF attenuators, for around $100 each.
Selected resistors are a nuisance.
I think the wonderful Murata 2mm pots are extinct. They worked at 1 GHz or so. And were horrible to adjust.
It is a pity that the drop-box image doesn't show much of the schematic.
<snip>
Actually there are, or at least there used to be - Vishay did a rather expensive trim-pot with multiple brushes which was a lot easier to set precisely. I earned a few brownie points with final test by modifying a board which had been a swine to set up by replacing the standard Bournes 18-mm 25 turn trimmer with the Vishay drop in part. What we lost on the expensive part we more than saved on the setting up time.
So do graduate students. Covering the slotted adjusting screw with a blob of epoxy after the pot has been set up can be a wise precaution. <snip>
It isn't. The author is showing an understanding of things that John Larkin doesn't have to worry about.
The Review of Scientific Instruments published essentially one-off designs to be used by physicists who more or less know what they are doing. The circuits aren't designed for volume production. Giving graduate students a trim-pot they can twiddle - under supervision - while watching what happens can help the education process no end, and it is lot less risky that swapping soldered-in select-on-test resistors, particularly when you have an over-confident graduate student doing the soldering.
Differential amplifiers and current mirrors can be useful, and they can look cute. Peer-reviewed science is performance art - the aim is to educate your readers, but you have to get their attention first, so a certain amount of smoke and mirrors (even current mirrors) can be handy. I have published in Rev.Sci. Instrum. (if only rude comments about some of the less impressive circuits) and I do have a cited paper in Measurement Science and Technology so I do know more about this than John Larkin - but probably a whole lot less than Phil Hobbs or Winfield Hill.
Barbarian, using a triangle file, think of the mechanical stress concentration begging it to snap in half. Use a round chainsaw sharpening file or round needle file at least! I'd even consider drilling a few small holes part way through it would be more humane.
OPA197 is c-load stable for small caps and for big ones. It should be OK with a tantalum... the ESR is about right. I'll test that before we release the board.
We do have some specifically c-load stable opamps in stock that would drop into the same footprint, if it ever did oscillate. But horrors, another part on the BOM!
Most rrio opamps seem to be stable with capacitive loads, certainly with big ones.
I got rid of another resistor. So now I have 8 values on a 4-sheet schematic. That includes an interesting servo to bias the distributed amplifier.
The only good use I can think of for them is absorbing large pulses of energy in a very short time, as the heat would be deposited throughout a larger mass than the mass of a thin or thick film.
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