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I've designed an RF device that runs via a USB connection to a PC. It needs to be connected to an outdoor antenna to receive a telemetry signal.

On the antenna input PCB trace i've added an ESD diode rated to 15kV, as well as a gas discharge tube and a clipping diode to protect the sensitive RF input MMIC. As far as I know all these ESD protection devices work by shunting ESD pulses to ground, and they require a good ground (earth) connection to actually work.

But what happens if there is no ground? For example one scenario might be that someone inexperienced connects an ungrounded outside antenna (which we are assuming has built up a large ESD charge), BEFORE connecting the USB cable (meaning that the USB shield/GND is not connected).

In this case there is no ground for these ESD protection devices to shunt to. Is the MMIC probably going to be damaged in this case?

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Your internal ESD devices will still take the discharge current themselves instead of letting it through to the sensitive MMIC input node. This assumes that there is some charge built up between the inner conductor and the shield of the coaxial cable. Some kinds of antennas have a naturally low impedance which pretty much precludes this kind of charge buildup.

But, as you say, if there is still a charge relative to earth, then it is not discharged until there is a ground path (like the USB plugged into the computer).

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  • \$\begingroup\$ Does that mean the shield is storing the ESD energy? So I guess if the charge builds for long enough it could reach a voltage high enough to destroy the MMIC? \$\endgroup\$ – Projectile Fish Nov 6 '16 at 6:59
  • \$\begingroup\$ No. The shield is not "storing" the ESD energy. And your circuit is not "storing" the ESD energy. But if the system is not discharged to ground, your circuit may be floating up at the ESD charged level until it is discharged. Your MMIC needs "relative" protection between its input and its ground. It doesn't know or care if the whole circuit is floating up somewhere above ground. \$\endgroup\$ – Richard Crowley Nov 6 '16 at 8:35

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