Thanks for the replies. Let me clarify the 10%. The circuit creates a 0 to 5v signal that varies over time -- sometimes slowly and sometimes quickly. By 10%, I meant anytime it goes up by more than .5 volts within 50ms. I'll need to do a little trial and error to pick those values in reality (so maybe .4v within 85ms, or whatever) jut that's the idea.
When I said no timeframe or high/low limits, I just meant there was no particular time to start the stop watch, nor any place within the 0-5v range when it reaches a trigger point.
I was hoping to avoid storing a log of values and comparing each new one to the lowest within the last 50ms, but that's the only thing I've come up with so far too...