3z33:
(I know they sell purpose built amp ICs to do this, but they are $12 per chip. I'd like to try to do it with a less expensive per unit item like an op amp...but I have no idea which one would work.)
These work quite well for me. $17.50 > $12.00, I know ;). But well worth it IMHO. Gets out of the analog realm, no conversion table to take up memory space, built-in cold junction compensation. But if you're dead set against it, or just in it for the academic exercise I might try an LF353. Nice high input impedance so as not to load the thermocouple and < $0.50 in single quantities. Actually I'd want to research the proper way to interface to the thermocouple, there may be a standard input impedance that may need to be provided with a resistor or other considerations, I have no idea, which was another reason I went with the MAX6675. If you do try that route, I have a simple little library, not much to it, but nice for multiple TCs.