4 to 20 ma current transfer and sampling

There are many natural variable that changing very slowly, like force, temprature,distance, all this signal can be sensing by analogue signal, after amplying,filter,signal conditioning, normally it become the standard signal, he most commom is 4 to 20 ma signal, there are two type 4 to 20 ma signal 1 The signal come from instruments, so that means the positive and negative signal all is hanging, very easy got interference.could not transfer longer.can connect to printer, logger instruments. 2 The signal come from transducer,it have same common with the power,so it is anti-interference, can transfer longer, it is very common to connect to PLC or DCS. Note: all the grounding only can be one point.