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Author Topic: Precision Resistors (.01% or better) - What are they good for?  (Read 1761 times)
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I specified a lot of .1 and .01% resistors for op-amp feedback networks, Very frequently and not because I had any use what so ever for the accuracy of the part which was really important but the real reason was temperature stability.
Thermocouple scaling amplifiers are one case that comes to mind. I once had a problem with a Max666 in an old design where there was a voltage divider for a battery reference comparator and both values were common E12 series resistors a 6.8 Mohm and a 3.3 Mohm resistor. Until I changed to 10 PPM/Deg C resistors none would pass a temperature test without showing a bad battery. The other part of my answer was done already... hard to make 1% accuracy cheaply with 5% parts and trimming... Believe it or not but thermistors were a frequent part of a design just to compensate for temperature changes.It's Really harder to manufacture .01% accuracy test equipment with 1% parts, It can be done and has for many years BUT not on a real production basis... like cell phones or MP3 players or television receivers, Even with digital calibration done automatically with EEprommed constants and Digipots... It's still a difficult process.

Doc
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One thing that fails with selection of cheaper lower precision resistors when really you need high precision results, is both long term and thermal stability. The two manually selected components make look perfect on the day that you match them, but will differ slightly once you solder them into a PCB, and progressively deviate from the original 'perfect match' with both passing time, and with temperature changes. That is why the precision parts cost much more than the cheaper components. Stability, both thermal, and life time, are difficult to achieve, and so come with a high price!
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I specified a lot of .1 and .01% resistors for op-amp feedback networks, Very frequently and not because I had any use what so ever for the accuracy of the part which was really important but the real reason was temperature stability.

One thing that fails with selection of cheaper lower precision resistors when really you need high precision results, is both long term and thermal stability. The two manually selected components make look perfect on the day that you match them, but will differ slightly once you solder them into a PCB, and progressively deviate from the original 'perfect match' with both passing time, and with temperature changes. That is why the precision parts cost much more than the cheaper components. Stability, both thermal, and life time, are difficult to achieve, and so come with a high price!

Thank you both, that is the answer that I was looking for.  It is stability.  I had a feeling it probably was but I really didn't know the answer there.
« Last Edit: August 28, 2012, 11:48:13 am by JoeN » Logged

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And how do you propose to do that  -  by measuring something supposed to be within 0.05% tolerance with a pice of test kit that MIGHT (at best) be within 1%  -  a difference of 20:1




 smiley-razz


* 33KResistor1.jpg (83.77 KB, 630x517 - viewed 78 times.)

* 33KResistor2.jpg (42.48 KB, 476x558 - viewed 78 times.)
« Last Edit: August 31, 2012, 07:11:57 pm by JoeN » Logged

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FoxConn alone employs 1.2 million people, almost all of them in assembly.  I guess they are paid to watch the machines.

1.0 million employees are there just to prevent next-gen iPhones from leaking.
0.19 million employees are chasing Gizmodo writers

That leaves about 10K employees watching the automated machines, packing, doing administrative jobs, HR, Mkt, Accounting, security, etc.

(sorry, but I just couldn't resist)
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Quote
FoxConn alone employs 1.2 million people, almost all of them in assembly.  I guess they are paid to watch the machines.

the place I work for doesnt even have a percentage of that kind of workforce, and is jam packed with all sorts of very nice equipment, one thing I have noticed is that no matter how awesome it is ... it screws up

so you have a crew running around adjusting and tinkering, and you have a 24/7 workforce feeding the machines parts, double checking quality, unloading machines, packaging, moving panels for different stages etc. just cause its mostly automated doesn't mean there is no use for people.

as far as resistors go, we use 1% parts, and it allows us to build pretty consistent products though millions of that product. for example I used 5% parts in a couple manual test rigs back in the lab to mirror one of our automated units, theres a +100mV on one compared to the tester out in production, the other is -100mV (rounding here btw) both of which are out of our tolerances for the part

... guess what I get to do next week
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