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AD8221 to measure thermocouple with common power pin

Product Number: AD8221


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?