ESD Protection Device ground

I have a bus powered USB device in a plastic casing. As it is a bus powered, power to the device is supplied through the 2mtr long USB cable. I have the following questions regarding ESD protection. This device is required to pass only ESD testing and will not be required to undergo EMI testing.

Almost all ESD protection device datasheets recommend to connect the ESD protection device ground to chassis ground but my device has a plastic casing.

Please see the draft schematic at the link

3) C17 is recommended as additional discharge path. Where should C17 ground go, chassis ground or power ground like all other bypass caps?

Thanks in advance mj

Plastic? Then, it is NOT an ESD protectible device.

You have to open the case and insure that there is a ground connection between the input and output shield shrouds on their connectors.

Task done. An ESD event on an actual data conductor will cause a failure anyway, and even a metal case device would not stop that, so the grounded connector shield pair is all you need to certify is in place.

A continuity tester may be all you need for that one you open one and insure that there is a hard connection in place.

Otherwise, you'd have to open it up, remove it, and place it in your own, in house refitted metal chassis (case).

Use VERY careful PCB layout. Protection inside plastic case for small unit/devices is doable.

Are you analog or digital?

As you know for analog filtering where you are keeping BIG signals out of your electronics, filtering to reject ALWAYS alternates impedance, high to low, low to high, etc. that kind of idea. For robust protection, use several stages.

Keep in mind your spectrum, you have DC, signals, and ESD [essentially lightning]. ESD's spectrum easily extends from MHz up through GHz [centers around 3 to 30MHz] You are trying to let through DC and signals, but not the ESD. Also watch out when trying to protect by shorting, that creates some very large high speed currents that can 'induce' themselves into adjacent circuitry, just like a transformer.

Also, think in terms of common mode and differential mode. Common mode is like being in a Faraday cage where all is at the same potential. Differential mode is just that and kills electronics. Common mode chokes help in that they let high speed signals through, but do NOT allow common mode ESD to change into differential mode on your PCB.

Obviously follow the trace routing rules: go from to and then onward. NEVER go from to and then tap into the trace somewhere earlier in the routing.

Rule of thumb for connections: connections wider than long are decent. connections longer than wide are inductors. Inductors ALWAYS are open at some higher frequency.

Study AppNotes from USB interface chip manufacturers. Many even supply PCB patterns.

Regards, Robert

You might want to have an ESD guard ring around the device. A lot of this is FM and is highly dependent on physical properties and remember, anything you do to keep out ESD can also become a path to let it in. In this case, I'd try to keep everything away from the USB signals. Let the shield take the hit. Of course, this assumes there isn't another way for ESD to enter the device, other than the USB cable. If there is, it gets a lot more complicated.

I solved one particularly nasty ESD issue by using a conductive paint on the inside of the plastic (a Faraday cage). An expensive solution, to be sure, but the alternative was no product.

I wouldn't. There is no guarantee that the shield is at the same potential as the ground. R12 is there to leak off any stray charge but I wouldn't trust it alone. Since your data lines are essentially clamped to USB power (and ground), that's the way I'd keep it. If you were using TVS diodes, instead, I might have a different opinion (but perhaps not even then). For a similar reason, Pin-1 of U2 should be tied to signal ground, rather than the shield, I believe.

That's what I would do. I might cheat and add lands for different options, though. ;-)

Yes, particularly R12. It's there to leak any potential difference between the shield and ground.

Power ground, IMO.

AlwaysWrong strikes again.

Please sigh one last time for the sake of mankind. You sully the gene pool as it stands.

Do us a favor and take any spawn you may have shoved upon the world with you.