[sdiy] Ratio switching!

[Needham, Alan]

I'm getting confused here...
Surely the composite circuit will be as good as the >worst< component ?

We keep talking of ppm errors but is this absolute resistor error - selecting resistors by hand would reduce this.
Are we talking ppm/degree C - if a potential divider uses two resistors, both with +100ppm/C characteristic then would that not cancel out ? if so, then simply selecting both resistors as "metal oxide" or even "carbon film" (both the same technology) then any change in one would be compensated by a matching change in the other ?

Calculating the values required isn't difficult, a BASIC program might remove the tediom but it's mostly just product over sum for parallel 'correction resistors'.

	Alan

## -----Original Message-----
## From: synth-diy-bounces at dropmix.xs4all.nl
## [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of ASSI
## Sent: 25 May 2007 7:05 PM
## To: synth-diy at dropmix.xs4all.nl
## Subject: Re: [sdiy] Ratio switching!
##
##
## 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.
##
## ##snip##
##
## 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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