[sdiy] Ratio switching!

[ASSI]

On Donnerstag, 24. Mai 2007 08:48, Louis van Dompselaar wrote:
> Although it will only be as good as the best components in there.  If
> you want around 0.01% accuracy, the resistors will have be (and remain
> at any temperature) 0.01% accurate as well.  There's the catch.

Things are not so simple.  Assuming you have normal distributed 1% 
resistors and combine them in an uncorrelated fashion, the error of the 
voltage division ratio is always better than 1%.  The numbers work out 
to about 0.24% tolerance assuming that the 1% resistors are produced 
with a cpk of 1.5, for instance.  You can halve the error again by 
replacing each resistor with a parallel-serial combination of four 
nominally identical resistors.  Further improvement can be achieved if 
you sort the resistors into two piles left and right the median and 
pair resistors from both piles in the first step, giving you better 
than 0.04% error provided the initial resistance distribution is 
neither skewed nor shifted.

I'm pretty certain that I've seen a (BASIC) program that would tell you 
the optimal choice of resistors (it checked up to six, IIRC) and their 
topology for a desired resistor value or division ratio given the 
preferred E-series.  That was before the Internet, a listing in an 
old-fashioned print publication, I don't know where to look for it 
now... no it wasn't Dr.Dobbs Journal, I have an archive CD of that.


Achim.
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