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Hello~
My product is powered by external AC/DC power so there are two wires (+/-) in the input.
There is a isolated RS-485 circuit in the product like below :
The question is :
My system is DC system and no Earth Ground in it.
How will I protect Data Line A/B even if I put TVS on them? What should TVS be connected to ?
Thanks in advance.
CC,
You could add cap between isolation barrier to create a current path during ESD events. This note http://www.ti.com/lit/an/slyt725/slyt725.pdf detailed ESD results with 100pF cap. You can find how the parasitic cap is made by PCB layers in this guideline http://www.ti.com/lit/an/slla368a/slla368a.pdf.
Regards,
Hao