Connect k-type termocouple using o-amp

first of all I have to say that this is my first Arduino thing. I’m trying to connect a k-type termocouple to my Arduino using an LM2904 Oamp, Yes I know that this is not the best solution but is the only that I have right now due to the pandemic situation, that I can’t get a dedicated module as an MAX6675

See te picture for .y circuit. The Grnd symbol next to 2.6k resistor is a handwritten error, its not connected in the circuit

my problem is that with this I’m having 3.7V on the arduino analog input even before connect the termocouple (so there’s no input, this is correct?). After connect the termocouple, the read is about 2.6V and it starts to increase as expected when heat is applied. But I expect to have near .5V not 2.6V

My problem is if I have ~2.6V for ~32°C, for temperatures over 200°C I will have more than 5V. Any help with this?

The standard type-K thermocouple has about 40µV/degC, and the LM2904 has a worst case
offset voltage drift of 30µV/degC and a worst case offset voltage of 7000µV.

Its not suitable for precision work with a thermocouple really, that voltage offset is equivalent
to a 200C temperature error.

So you will need to trim the input offset somehow, and keep the opamp temperature stable
if possible, or at least measure it and calibrate it out. This could be combined with the
cold junction sensor. You have a cold junction temperature sensor?

MarkT:
So you will need to trim the input offset somehow, and keep the opamp temperature stable
if possible, or at least measure it and calibrate it out. This could be combined with the
cold junction sensor. You have a cold junction temperature sensor?

I have plan to calibrate it using a REX-C100 temperature sensor using the same thermocouple, but I can figure out why the LM2904 give that values without any input, have to be a reason for that, maybe with no input its amplifying "noise", but when the thermocouple is connected I would expect ~0.4V at 30°C( 1.2mV thermocouple table) due to my amplification gain, and not 2.5V