Re: How inaccurate is a 555 or 7555 REALLY?

On 2 Dec 2006 16:29:12 -0800, bill.sloman@xxxxxxxx wrote:


mri_bob wrote:
actually the good old 555 is stable to .005% per degree C so your first
task to get .1% is to keep it in the range of +/- a few degrees C. then
you have to turn your 1.5v into the minimum operating voltage of 4.5v,
easy enough to do with a maxim charge pump chip which is tiny and smd and
needs only a couple of small caps. then you have to have your resistor and
capacitor be cermet

metal film

and polystyrene

polyphenylidene sulphide

so they will have a good enough
tempco. if you do all this you could easily get .1% out of a 555. the chip

Fat chance. You'd have to stqabilise the voltage as well as the
temperature - the divider inside the 555 is a series of three
voltage-sensitive diffused silicon resistors.

I spent a month or so back in 1974 on exactly this problem, and
discarded the 555 very early on.

is the least of your problems with an RC time constant, the problem is
temperature coefficient. you can get extremely low tempcos by combining
positive and negative tempco components to get a single one with almost
zero tempco.

For a individiual capacitor - the temperature coefficient of wound
capacitors depends on the mechanical tesion in the dielectric, and the
tolerance on te temperature coefficient is all over the place.

Polyphenylidene sulphide sufrace mount capacitors are not wound (as far
as I know) and do better, but they still don't hack it at the 0.1%
level.

i agree this is a job for a microprocessor. you can get a PIC in an SO-8
package and to get .1% stability or 1/1000 you would need something like a
watch with a stability of 1 second every 16 minutes. a pretty easy thing
to do, as you can see. when you want precise stable timebase related
functions a computer is the easiest and best way to do it.

10MHz crystals are sold with a +/-0.1% absolute tolerance.

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Rubbish.
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32768Hz watch crystals aren't that good (if I remember my Farnell catalogue
corectly.

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Instead of trying to remember your Farnell catalog, you ought to
visit crystal manufacturers' websites to fined out what's current.

20ppm at 25C for tuning fork crystals is easily achievable nowadays.


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JF
.