Subject: delabs comments – Tell Me. message: why 4-20mA standard. why not 2-22mA or any thing else
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they knew you would come up with a question like this. so see this page 4-20mA Current Loop Primer and also Intrinsic Safety Circuit Design.
…transmitting 0mV or 0mA will be lost in the noise too. 4mA is enough power to go over long wires with low error due to leakages. and current ensures RFI-EMI immunity as the picked noise does not have any juice just some voltage.
4-20mA helps in Noise Immunity, Providing Power for efficient signal conditioning circuits making two wire transmitters possible.
See more at my pages Industrial Process Control Circuits
Hence it could be 4-20mA not mV. 0-20mA is not good enough, but is used when you just have to interface two cards or instruments close by. 4-20 mA can go from your street to the next or even much more depending on cables and voltage.