[sdiy] really basic question .. tolerance

[harrybissell]

Hi Tim... (inline)

Tim Ressel wrote:

> Hey,
>
> I don't mean to be a stick in the mud,

uhhh... then dont be ??? :^P

> but I have to
> disagree. You are talking about probabilities. When I
> put a trim pot in a circuit, I don't figure what the
> mean error is going to be, I figure the worse case so
> the trimpot will have enough range to compensate. And
> yes, I have seen resistors on the ragged edge of
> tolerance. This is why Spice has Monte Carlo stuff in
> it.

Absolutely correct.  I'd be figuring here that 1% was
acceptable to start with... and two resistors makes it
(very likely) better.  The 1% actually becomes the
worst case...

...and if you care you must select resistors, buy a higher
precision (oops almost said 'tolerance' there...) or
use a trimpot..

Or if you're a shoot the dice kind of guy... you stick in what you have
and hope. In this case the extra resistors work the odds in your
favor...

...but I can see hoe that would not cut it at Agilent...

H^) harry

>
>
> I guess its all that time I spent at Agilent.
> Precision is a disease that is spread my casual
> designing...
>
> --Tim
>
> --- diysynth <diy_list at analog-synth.de> wrote:
> > Hi all, hi Tim,
> >
> > > Rules... We don't need no stinkin' rules!
> > Oooh Yes we do....
> >
> > 100 KOhm with 1 %
> >
> > to simplify, we have (with 1% tolerance) 99, 100 and
> > 101 KOhm.
> > With 3 possibilities in a 2-combination :
> > 9 Possible Combinations:
> >
> > 99 + 99 = 198  instead of 200 (i.e. 1% error)
> > 99 + 100 = 199 (0,5%)
> > 99 + 101 =200 (0%)
> > 100 + 99 = 199 (0,5%)
> > 100 + 100 = 200 (0%)
> > 100 + 101 = 201 (0,5%)
> > 101 + 99 = 200 (0%)
> > 101 + 100 = 201 (0,5%)
> > 101 + 101 = 202 (1%)
> >
> > so, we have a arithmetic mean of tolerances of :
> > (1,0 + 0,5 + 0 + 0,5 + 0 +0,5 + 0 + 0,5 + 1) / 9 =
> > 0,4444 %
> >
> > That is, of course, better than 1%.
> > This estimation doesnt take into account, that 1%
> > tolerance means, that
> > there is a gauss-distribution of the resistor values
> > with a standard
> > deviation around 100 KOhm of  (less than) 1 KOhm or
> > so. Tis would increase
> > this e-mail to some hundred lines.
> >
> > Joerg
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > > clear now?
> > sorry, no.
> >
> >
> >
> >
>
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