Hi Eric/Ubergeek,
Thanks for help of both of you. I tried the circuit with two instrumentation amplifiers.
Eric: you are correct about 10bit resolution, I don't know how did I missed such a simple thing at that time.
Ubergeek: You are quite correct about AD623 but it worked with less accuracy, so I found another IC INA128P which with 10uV offset, and I could measure 4grams resolution, for 1000grams span.
Still I didn't use any filter all the circuit if affecting ordinary items so in that scale output is varying by - or + 1 value from ADC units.
I draw the graph with some weights (vegetables from super market with written weight), and it is linear.
I used the reference voltages of PIC to bring values to effective range.
Now I change the plan to AD7730, which is a built in solution for this project.
And mean time I found 12bit ADC, SPI interface MPC3201 and I'm going to give a try and also same series they have 22bit ADC also.
I will attached pictures of the project at the moment, And next week I will come back to you with AD7730 results.
Thanks again for both of you guys and I think my result would helpful for others too.