Subject: delabs comments – Tell Me. message: why 4-20mA standard. why not 2-22mA or any thing else
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…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.