The OP mentioned 'slowly changing signals'. This is a hint that the problem 
can be addressed by using a time varying signal. See:

http://www.edn.com/archives/1996/101096/21di_01.htm

which does just that using a simple triangle wave generator, a comparator, 
an analog switch, and a couple of discretes.

Really rather elegant.

Bob Ammerman|
RAm Systems 

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