Hi Raghu ,
In industrial PLCs , usually it is the practice to have separate grounds for the CPU and the AI card , because the CPU is generally powered up by an SMPS which is a source of much noise, same would happen for a PC I assume.
Try this site http://www.omega.com/literatur...gnal2.html
Generally in hydraulic systems (@250bar) if it is only for display +/- 0.5 bar does not make so much of a difference , if you are using it in lieu of a pressure gauge.I assume you are not using it for this as this is much too expensive.
What is much more important if you are using it for diagnostics, is to be able to capture pressure peaks which can damage the pump,if some valve is turned on out of sequence,or in your case some actuator is stuck and relief valve opens late(tractors ? ).In this case also the quantum of the value is not as important as missing the reading altogether,which is more likely to happen,if you average / update less frequently.

Regards
Thomas