Steve:
Like I said, you could do all that in software too, just a question of offsetting your zero.
Prahlad:
PCF8591 + precision Op-Amp is meeting almost all requirements.
Zero offse in any circuit with an OP-amp is a breeze. To do zero offset correction you can use a "plain vanilla OP-Amp, no need for a "precision Op-Amp "

But I know there exists a simpler method where in I can simply make the coils interchangeable by puttin a resistor but I couldnt find more information about this technique what resitor where to put it. I am looking for more information on this technique.

Of course you can replace the EEPROM with a resistor and measure it's value with the A/D and that way get the calibration value we suggest obtained from a EEPROM. This however is quite cumbersome, first you must calibrate using a variable resistor, then you must replace it with a fixed (the variable WILL change in transport) and then you must check that the fixed resistor is correct. Also a high quality trimpot probably will cost you more than a small EEPROM. Why not just use a 2 bit EEprom and KISS PLUS you get stability, and reliability.

A bit of editorializing: You are dealing with thousands of amperes, that is the wrong place to think anything but reliability.

Erik

Oh, I just saw it: 2 bits = Slang for 25 cents
KISS = Keep It Simple Stupid.