And the problem with using two resistors (one after measurement) is... ?
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Well, I laid out the board with resistors, in the places that it needs resistors.
I used 1% resistors, which we had in stock, and worst-case tolerance stackups are a concern in production. We're in spec on about 10 units built so far, but I wouldn't want to push our luck in production. Bell-shaped curve and all that.
Mouser has 0.5%, but I'd like better, if they exist.
If you never produce and sell stuff in volume, things like this are not a concern.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
That would be physically messy, and labor intensive. The board layout is done.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
OK, pinged them, thanks.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
But you have all the parts spec'd before layout, let alone building the board. Hence the confusion.
I'm not confused.
As I said, we used resistors where appropriate. And we used what we had in stock.
It turns out that it's hard to find these values in better than 1%. It's appropriate to look, now, before we get to volume production, where the probabilities might occasionally push a unit to the spec limits.
What's wrong with improving a design to reduce risk and make it more producable?
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Try these guys...
Cheers
I did ping them, waiting for response.
Thanks
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Actually you do the sensitivity analysis before you commit the product to design. So it makes little sense to put 0.1% resistors where you decided, after careful design analysis, that 1% was adequate.
Mechanically accurate stuff tend to weigh a lot. There's a company nearby that makes 3D scanners for priceless museum artifacts- they use a gantry CMM machine as the base, requiring a pretty hefty floor rating.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
No, I'm re-evaluating the sensitivity before I put the product into volume production. Building ten 1st articles was easy; the probability of any one having a worst-case stackup was low, and I could always tweak one, or throw it away, if it was unlucky.
0.1% resistors weren't handy when I did the 1st articles. Now I have more time to shop around. Which is what I'm doing.So far, the Mouser/Vishay 0.5% parts look like the only game in town, short of something full custom.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
After all your rev A bragging you have done it to yourself with unobtainium parts, I laugh. BTW i have already read the thread to date (23 posts so far).
?-)
-- It's not a question of luck, it's a question of making unwarranted assumptions, and if you had considered the "bell-shaped curve and all that" in the beginning, you probably wouldn't be having to figure a way to get the problem's teeth out of your ass.
-- Absolutely nothing, and competent designers don't relegate those design details to futzing with production rejects in order to get them out the door.
Standard method is to heatsink the leads whole soldering..from the old daze ~40 years ago...
This is rev B. Rev A worked, and we sold some, but the customer changed the specs and changed some mechanical requirements. My current offset and resistor tolerance concerns, on rev B, is a direct result of some spec changes. The rev B units work, too, but I'd like to tighten up the statistics before we build thousands of these things. What's wrong with that?
That happens, when you have actual customers.
It is interesting that 2M, 0.1% 0603 resistors are, so far, impossible to find. I wonder why.
What are you working on lately?
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
It's not a problem so far. I just don't want it to be a problem when we make thousands of these things.
The real problem is that the customer changed some specs. Before that, I had no problem using 1% resistors. I might not, even now, but I don't like the risk that some units might have a worst-case stackup.
It's essentially a common-mode-rejection problem, involving the tolerances of 5 resistors and the betas of three transistors. It's fairly complex.
You are senselessly crabby. Sounds like a hell of a way to live.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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And the curves are not all bell-shaped either. Every now and again you got batches of resistors from which the manufacturers had selected a tighter-to lerance range, or bunches of what should have been 0.1% E96 parts which mis sed the target and got dumped into a 1% bin where they fell within toleranc e.
It does look like a drop-off, but who hasn't missed something somewhere in a design? The real art is in catching as many drop-offs as possible before they get expensive to fix.
-- Bill Sloman, Sydney
--- You confuse crabby with critical, 'critical' being sensible to the point of being de rigueur as far as technical matters are concerned.
But YMMV.
No, it's crabby. I'm doing sensible engineering and you're whining.
Design something. You'll feel better.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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