polymorph:
The 3.3V output from an Arduino is just another DC power supply voltage. Not as stable as a reference voltage.If you are using an external ADC, especially one that is 18 or 24 bits, you need something much better than the Arduino's internal reference. Use one of the many ICs made just to supply a reference voltage, they are much stabler and more accurate. When I mentioned the internal reference, I'd forgotten that you are using a much higher bit external ADC.
With 18 and especially 24 bits, you are going to need really, really well filtered power supplies and a very, very low noise, low offset, low drift instrumentation amplifier.
Thanks you again
Now I'm using a referrence voltage Ic ( REF195 : 5.00 v) at ADC pin (V ref pin) i got a stable voltage more but i thing it isn't enough. I have some a question to ask you should i use a reference voltage to Vcc of Ina125 (amp) and Load cell ?