Pang, I haven't done a lot with 4-20mA but will have to soon.
These links may help. Also note that sensors with voltage
outputs may be converted to 4-20mA with an LM317

http://www.piclist.com/techref/microchip/aout.htm

http://www.pacontrol.com/download/4-20mA-Current-Loop-Primer.pdf

http://www.national.com/ds/LM/LM35.pdf

Includes example of converting temperature to current (Fig 9). Does
not have a ground connection

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