Hi,
A thermocouple type K has about 0.04mV/°C.
Your LM324 has a typical input offset voltage of about 2mV
--> this makes an offset in temperature of about 2mV/0.04mV/°C = 50°C
The offset voltage drift of your LM358 is about 7uV/°C.
Compared with the voltage of the tyke K element it generates only a drift of 7uV/°C / 0.04mV/°C = 0.16°C/°C
This is all we can tell because you don´t give more informations.
**********
For further posting please give all informations:
* You circuit/schematic with part values
* What (ouput) do you expect? In values with units.
* What does happen instead? (Best also with values and units).
* How do you observe your signals? DVM, scope, ADC, microcontroller?
* Do you have filters installed? (Filters for supply voltage, for temperature signal, analog filters, digital filters ....)
--> fluctuation of an analog signal is quite normal. But what size does it fluctuate? Only some 0.1°C or does it fluctuate about 100°C?
And in what itme? Does it fluctuate 10°C within milliseconds or 0.1°C within hours?
And how do you measure the fluctuation? DVM.
Klaus