USB ESD protection

[FrankBuss]

In a datasheet for the ATSAM21 family, they recommend to connect shield of an USB connector with a RC element to the board ground. But they also connect the diodes of the additional ESD protection device to shield. Wouldn't this prevent some of the protection, because if an ESD event happens, it has to go through the RC filter as well, which could cause high negative voltage? Or is this better for EMI?

[ataradov]

Yeah, this does not look right at all. I would connect the diodes to the hard ground, for sure.

And all that antenna effect discussion is pointless most of the time. If it is just a regular consumer device, then don't bother and connect everything to the ground. If you are in a high-EMI environment, then you will likely have to do something special anyway.

[nctnico]

Yeah, this does not look right at all. I would connect the diodes to the hard ground, for sure.

Indeed. In my experience the best way to start is to connect everything at 0V level (shields, 0V) to ground (= a ground plane of the board and any metal casing).

ESD diodes are supposed to avoid a high potential across the pins of a chip so they must be connected between the ground of the chip and the pins. ESD diode connections should be routed so that the signal must pass along the ESD diode before going to the pin of the chip. No stub-like connections!

[wraper]

Yet another datasheet written by some clueless dumbass. Much worse for both ESD and EMC. ESD protection made in this way will actually inject ESD from the shield into the data lines, not protect anything. That "low cost" version is actually a proper way and is 100x safer than 'protected' version. Below is what you should do to actually improve protection.

[FrankBuss]

Much worse for both ESD and EMC. ESD protection made in this way will actually inject ESD from the shield into the data lines, not protect anything.

Thanks, sounds bad. BTW, was from this datasheet, page 1122: https://ww1.microchip.com/downloads/aemDocuments/documents/MCU32/ProductDocuments/DataSheets/SAM-D21-DA1-Family-Data-Sheet-DS40001882H.pdf Maybe someone should tell them to correct it? I'll connect then just all to ground, as always.

[wraper]

^I doubt anyone will bother to fix it. This USB shielding snake oil with random components in series with shield is floating around for many years with barely any scrutiny.

[ejeffrey]

That's one of the worst I've seen, that 1 MOhm resistor does nothing at all.  I've seen this variations with a couple hundred ohms, which at least could plausibly make sense in some situation.  But when in doubt just connect all the grounds together.  And yes, the ESD diodes need to connect to the same ground as the chip or they won't work correctly, and can potentially conduct ESD to your chip.

There are some really good application notes out there, but a lot of them are written by people 2 years out of university so they know the fundamentals but have very little practical experience yet. They have gotten some incomplete guidance from more senior engineers (who might also not really understand the issue, but may have better developed "instincts") and they search around on the internet or other datasheets, or their companies internal designs and try to make up a good story about what they see.  So often what you get is very similar to the output of a game of telephone or a large language model.

So if you are reading an application note or application section of a datasheet if something doesn't make sense, don't assume the problem is just you.  The AN could be completely wrong.  It could be right but the explanation is wrong, or it could be right but only in a situation completely different from what you are interested in. 

Textbooks are typically better vetted than application notes, since they are more often written by experts who have both training and experience.  But they are updated less frequently and usually written to be more generic, so you may need to do some more work to adapt the information to a particular situation.

[nctnico]

I just tried asking ChatGPT OP's question and the answer is way more useful (as in being correct) than the datasheet  I'm not a fan of ChatGPT but I had to try just for kicks.

[Nominal Animal]

That's one of the worst I've seen, that 1 MOhm resistor does nothing at all.  [...] There are some really good application notes out there, but a lot of them are written by people 2 years out of university so they know the fundamentals but have very little practical experience yet.

I for one have been fooled by such datasheet recommendations.  I've got (some) formal uni education on electronics, but not much practical experience, so was particularly susceptible. 

(I've since learned better, from more experienced members here.)

[peter-h]

The purpose of the 1M resistor is to discharge "static" which would otherwise build up on that capacitor.

It is a standard problem if you have e.g. two RS232-RS422 opto isolated converters connected back to back, to create a RS232 line extender. The bit in the middle will freely float, possibly by hundreds of volts. In appnotes I wrote, I tell people to connect the isolated ground to earth at some point (doesn't matter where).

Whether this is applicable to this, I don't know.

In my last product I use a USBLC6-2, and the bottom goes to the digital ground which is the whole product ground.

[Siwastaja]

I just tried asking ChatGPT OP's question and the answer is way more useful (as in being correct) than the datasheet  I'm not a fan of ChatGPT but I had to try just for kicks.

It's average-then-hallucinate algorithm so it's probably less prone to certain types of extra stupid cargo cult ideas that only live in bubbles. Probably it has seen enough text that opposes this design pattern, e.g. on internet forums etc.

[sslupsky]

I found the following video quite helpful to understanding "grounding".  You may find it helpful as well.

[Aheld]

For ESD protection topics I can recomment the application handbooks from Nexperia:
https://efficiencywins.nexperia.com/efficient-products/nexperia-design-engineers-guides.html
If I remember right, USB2.0 and following families are covered in this documentation. Really compact knowledge and well described.