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I am implementing ESD protection for HDMI video input. The HDMI video goes through a custom cable of ours, into a connector board attached to our enclosure, through an edge card connector, into our main board and into the receiver chip.

I usually add the ESD protection circuit on our connector board because that's the first part where the high speed signals make contact from a cable to a board.

Would it be bad practice to add another ESD protection circuit on the main board since the signals will go from the connector board to the main board via edge card connector?

I'm worried that adding extra capacitance to the high speed signals will cause signal integrity issues and that it will be overkill.

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    \$\begingroup\$ If the connector board is just a straight through connection then place the ESD protection on the main board instead of the connector board. You just need to protect the receiver from ESD. \$\endgroup\$ May 21 '19 at 21:30
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Check the load impedance and capacitance of the ESD diodes you are adding, HDMI is 100Ω. Or the easiest thing is to not include the diodes. If the cable is shielded and it runs down the length of the cable ESD is not likely to effect the ports and ESD has been shunted to ground where HDMI enters the device (which is probably the best place, its best to shunt ESD to ground at the entrance of the cable).

I would only have 1 pair of ESD diodes if the cable is shielded and the shield is tied to chassis ground properly.

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