Re: How inaccurate is a 555 or 7555 REALLY?

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

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.

-bob


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