i am trying to read a thermocouple which has a shared pin with a heater.
The heater is suplied with pwm from another isolated galvanicaly circuit.
I have attached a schematic, where you can see how things look like.
R5 - is the heater
R6 - is the thermocouple
V4- is the pwm source for the heater
The gain for the amplifier is set at x100.
Of course, in simulation everything works :)
But in real life, when i apply power to the heater, the values go crazy.
The PWM frequency is 40khz at 50% duty ( 25us period).
I read the amplifier output using a 1msps adc in the "off period" of the pwm signal... this gives me a window of roughly 12.5us.
What do you think of this aproach? Is it posible to read the voltage accross the thermocouple in this manner. Or do i need to drastically change the design aproach?