hi,
i am confused in selecting constant current source using FET or Op-amp.I just want to know that which would be more stable,because i am using that current source for excitation of the RTD sensor.Please help.

 

I suggest a linear regulator might be more stable than either, as well as easier to impliment.

 

Or even one of the 3 terminal adjustable current sources such as the LM334. If the adjustable current range meets your requirements.

 

A well-designed op-amp current souce will be much more stabl than an FET connected as a constant current diode.

 

It depends on the lengths you are willing to go to. There are many things that affect stability of a circuit. After you eliminate the big influences you can go after the smaller ones. There is a point of diminishing returns where you expend enormous resources for minuscule improvements.