PRB_tech:
and if so Are analog switch capable of passing micro-volt from thermocouple and rtd's, can they pass 4-20ma dc current exactly without distroting .
I lke your honesty.
Analogue swithes can handle 4-20ma, but the problem is not there.
Thermocouple is producing low signal in volts (30mV ? ) which can be easily interfered by digital noise.
Use seperate voltage regulator for analoge part and seperate for digital part.
Consider amplifiyng thermocouple voltage before switch.