I have a side-project to read temperatures from 4 thermocouples (K type, around 500F for coffee roaster data logger), and no previous analog experience at these levels.
I've identified two possible solutions, but I'd like to hear feedback / suggestions / alternatives.
The first method would be to feed the four thermocouples through an analog mux (say HEF4052, though haven't done as much research on this- is finding a mux for low-freq uV signals), and then to a typical thermocouple IC (MAX6675).
The second method would be to feed the thermocouples into a 4 channel differential ADC, probably with an integrated PGA (AD7708). Then have an on-PCB temp sensor to handle cold junction compensation. Another option'd be analog mux to a single channel ADC, etc.
Are there advantages to one approach over the other? It seems like the first would be fewer components, and a bit more reliable, though a bit more expensive then the second option.
I've also seen circuits using an RC filter on both thermocouple lines- is that worthwhile? What kind of TC would be worthwhile on these kind of signals? Something for blocking 50/60Hz? Just installation specific?
My primary concern is that this is the first time working with signals measured in uV, so I'm not sure what is and isn't important (beyond basics like keeping digital and analog seperated).
EDIT: One more question for the ADC approach: is it important to measure the signals differentially, or could the low end of all 4 thermo couples be tied to ground, then be measured in a single ended fashion? ADS1118 seems like it has alot of what's wanted, but is only two-differential channel (and the datasheet shows differential measurements at that). More plainly: what's the advantage of doing differential measurement vs. single ended with one end tied to ground? Just isolation from the rest of the system?