Wow sir, you have just got me confused even more now. I know the problem is hard, and can't be solved by just high bit ADCs. Btw how did the factor became 90, it should be 80, (4M+50k)/50K = 81 actually.
Now, going through my calculations in reverse, the 22 Bit ADC at 5V reference can measure 1.2uV theoritically.
Now multiplying it with the factor of 81 is 97.2 uV (The min drop across R1 that can be measured). So at 2A current from the supply the least possible value of R1 that can be measured is around 50 uOhm(48.6) without any buffer amplifier analog design. Am I doing anything wrong here?? These calculations are for the MCP3551 22 Bit ADC. Please consider this ADC as well - I already have this.
I know in practical these values will not be near enough. That is why I am here asking for help. But I believe this can be done, combining with good analog design, high bit adc, good pcb and noise filters. So please keep helping me. Oh and I will be satisfied if I can achieve these numbers the 50uOhm and 1.2uV.
Thank you.