proper use of a guard/ESD ring - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: April 03, 2017, 02:18:02 pm
content: What is the proper implementation of a guard ring? I'm starting some new products and wanted to be sure that I start the design from the best possible stand point in regards to emissions.The biggest question is to verify that the guard ring needs to connect directly to the shields of the USB cables and other IO connectors (I have a USB, DB15, and a GPIO connector). The plan is to have all of these connected directly to the shields and then the ring connected to the ground plane in a single location with a high value R and a low value C, correct?If anyone has an accurate design guide for this, I would appreciate it.

author: tszaboo
author_rank: Super Contributor
timestamp: April 03, 2017, 03:07:00 pm
content: Guard ring is a precision analog stuff, it has nothing to do with ESD USB or any of the other stuff that you are talking about.ESD ring is not a technical term AFAIK.Maybe you are talking about "chassis ground". RF gurus will tell you that chassis ground should be connected to the system ground with as low inductance as possible, meaning direct connection on the PCB, no components in-between.

author: Pack34
author_rank: Frequent Contributor
timestamp: April 03, 2017, 03:23:26 pm
content: I've seen it implemented and referred to that way in a couple designs. Noteably the open source schematic on the BeagleBone Black.With USB shield directly connected to ground, I've seen that cause some earth ground loops in specific situations. I'm planning on adding the resistor and capacitor so when a product is shipped for that application they can be isolated but everything else have it shorted.

author: tszaboo
author_rank: Super Contributor
timestamp: April 03, 2017, 03:54:52 pm
content: The BBB schematic has the term ESD_RINGused on the schematic on the Ethernet. Ethernet is different, becuase it is isolated with magnetic coupling, and the grounds are not to be connected together.They curiously connected the Ethernet shielding ground to the USB Shield and even the HDMI shield. I'm quite sure that is not ok.

author: ap
author_rank: Frequent Contributor
timestamp: April 03, 2017, 04:19:55 pm
content: The issue is that when you touch a connector case, any ESD charge will flow to the case and from there to the next possible other conductor (and be it through capacitive coupling). Therefore, if the connectors are not properly gounded (actually earth, thats where the charge is referred to), you will cause all kinds of disturbances on the board with ESD. So you could ground the connector frame, and use an RC plus sparc gap combination between ground and board negative (board reference potential), or connect board ground and earth directly (at an appropriate position).I have seen several designs, where the connectors were not propery grounded, cauing the system to fail ESD testing.A guard ring usually is a conductor that prevents charges and leakage currents cause erroneous readings in e.g. a sensitive analog circuitry. Precision transconductance / current amplifiers e.g. use that.