The MAX420 is operated at a gain of 191 to convert the 52 p.VI o C output of the type J thermocouple to a 10 mV/Â°C signal. The -2.2 mV/Â°C tempco of the 2N3904 is added into the summing junction with a gain of 42.2 to provide cold-junction compensation. The ICL8069 is used to remove the offset caused by the 600-m V initial voltage of the 2N3904.
Adjust the 10-KO trimpot for the proper reading with the 2N3904 and isothermal connection block at a temperature near the center of the circuit" s operating range. Use the component values shown in parentheses when using a type K thermocouple.