Mohamed,
1)Please, answer to my question - with wath averaging (time long,digital filter type) You get 10mg resolution?
2)Answer about opamps.
- if You use LMP2011,LMP2012,OPA335,OPA2335 -
these are 2,7... 5V opamps. Connect +Vcc to 5V, -Ve to GND.
Use circuit as instrumentation amplifier :
- Loadcelloutput1 - to noninverting input of Opamp1
- Loadcelloutput2 - to noninverting input of Opamp2
- Two resistors R1 between inverting inputs(opamp1 and opamp2) and outputs (opamp 1 and 2).
- One resistor R2 between inverting inputs of opamps.
Amplification is R1/R2.
- Connect outputs of opamps to differential input of AD7730.

If You use single amplifier , then You can have problem , when ADC has differential input - where to connect
additional input? It is possible (i have used this) to connect
this input to one of load cell outputs. But here can be problem with excentrical loads - will be or not problem depends of input/output impedances.
For single amlifier, see opamps datasheet - both inputs are connected to load cell.



3)These opamps are "auto-zero" , with minimal tempearure drift.
For "legal-for-trade" application this is a must(i think). If application has not to be certified - opamps with lower noise/ higher temp.drift can be used.

4)If You still use AD7730 no opamp is required, ADC is good enough. Select any opamp from internet and compare noise and drift to AD7730's datasheet.
regards