high value 0.1% resistors

On a sunny day (Wed, 26 Mar 2014 10:38:25 -0700) it happened John Larkin wrote in :

Too late, the board is done.

Canted tombstones, touching at the top? George H.

Oh I did some hand selecting of 10Meg 1% ers.. but one gotcha I found was that soldering them into place changed the resistance.. (by 0.1-0.2%) (though hole metal films)

can you squeeze an 0805 in?

George H.

No, the board is under 2 square inches and it's really dense.

(I mean "dense" in the sense of lots of parts!)

Colorful web site! Do foils go that high? Anyhow, I pinged them on the general requirement. Thanks.

The vishay metal foil resistors are not available in the megaohms range AFAIK.

See e.g.

Do you have a laser trimmer handy? If so, buy 1M or lower value resistors and trim them up to the desired value. Available in 0603 size: App note:

I used them in various filters but at lower resistance values when we switched from hybrids to PCB construction. Not having to buy or stock a mess of non-standard resistor values was the major benefit. Already having the laser trimmer on site from the hybrid operation was a big plus[1].

You'll probably need some conformal coating to protect the cut after trimming. I suggest an acrylic.

They got right back to me. They can't supply those values.

Mouser has some 0.5% parts, maybe the best I can do.

(Rummages around in his desk drawers)

No!

Maybe I could buy a couple of K of the trimmable parts and get some shop to trim them up for me. I could start with 1.5M or something like that.

Actually, I could start with 2M and/or 2.5M and trim *up* a bit. Ratios are what matters in this circuit, not so much the absolute values.

Hey, Jim claimed to have an analytic solution to resistor trimming. I wonder where that is.

On a sunny day (Wed, 26 Mar 2014 13:19:47 -0700) it happened John Larkin wrote in :

So.. how about 2 on top of each other, the second a much higher value to get the accuracy?

That would require hand selection. Production would lynch me.

Put them on edge, diagonally. ;)

Cheers

Phil Hobbs

To get 2 megs from 1 meg resistors, we'd have to make a teepee. 2.5 megs is Stonehenge construction. 5 places on the board.

I'll find out if our pick-and-place machine can do that.

Nah, I meant fitting in the 0805 that way. I do it manually all the time, because it's much easier to hold the parts edgeways.

Pretty fancy.

Cheers

Phil Hobbs

It's interesting that the PnP weighs 5000 pounds, to place milligram parts. We've rented a *big* forklift. Liz and Paulo are planning a forklift race.

I once ran a car into a forklift tines. Slit that Chevy open lengthwise like a can of sardines. That's a whole nother story.

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