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op-amp output to 16F876A

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about the zero pot, is it just an add-on circuit of the non-inverting amplifier when tuning the the voltage to 0V or i need to build the 0 pot and tune to 0V before i build the non-inverting amplifier?

hi,

The ZERO pots are part of the circuit and are always there.
The OPA that you are using has a low specification, for mVolt operation you need to have a modern OPA, I like like the CA3240 dual OPA, its not expensive and its a good general purpose amp.

when the input is shorted, the output is fluctuated from 33mV to
36.4mV

A lot of this noise is from the OPA.!

I should point out even if you get the MC1458 working it will not give the accuracy or performance
you must have for the thermo-couple.
Also you dont appear to have included the 'cold junction'.

Its OK to use an 'ambient junction' as reference but this will only give reasonable accuracy over say 10C thru 35C.

I would suggest that that you use the LM335 temperature sensor, it will do all you want for the least cost and hassle.
 
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:) thanks alot eric, after i replace the amplifier with a low noise amplifier, the problem on the input is solved.
thanks everyone :)
 
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