Beaglebone Green - Grounding and ESD protection issues

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martin_ehrenfried
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Beaglebone Green - Grounding and ESD protection issues

Post by martin_ehrenfried » Sun Nov 18, 2018 9:33 pm

Hi,

I have several Beaglebone Green's which are used in conjunction with the KiWi SDR cape.

I recently had a problem where the Ethernet ports failed on two BBG's.

I replaced the PHY chips on the BBG's and both boards are now working again.

However I've been installing the BBG's and KiwI's into metal cases and have realised that the grounding is split into:-

1. The Digital ground (DC supply 0v)

2. A chassis ground (Ethernet, USB port, and BBG fixing holes)

3. An ESD protection 'ring' around the Ethernet port, which is connected to the chassis ground by means of a small (0805) surface mount resistor (R136) of value 0.1R, which is mounted near the edge of the board under the Ethernet socket.

The problem is that when used with a KiWi, which may be connected to an antenna with an external ground system, it is possible for the various grounds to end up at different potentials.

For example my metal case is mounted by means of the fixing holes in the BBG which are at chassis ground.

The DC input is connected to the digital ground which may in turn be connected to my AC supply ground, if the DC power supply has an internal connection between the two. It also connects to the RF connector screens on the KiWi board.

The Ethernet port has a separate ESD protection ring which is used to suppress voltage spikes in order to prevent damage to the PHY chip.

In my case I have discovered that the 0.1R surface mount resistor which connects the chassis ground to the Ethernet ESD protection ground has blown open circuit (presumably due to a very large voltage spike) and as a result there is no longer any EDS protection on the Ethernet port.

I suspect this is why the PHY chips have blown.

I've been looking at the BBG schematics but I've been finding it difficult to trace any decoupling capacitors between the Digital ground and Chassis ground or a diagram showing the component layouts on the BBG (apart from an Orcad file which I can't open).

Has anyone else had similar problems ?

Is there an image of the PCB layouts showing component designations and positions ?

I'm thinking of linking the Chassis, Digital and ESD grounds together, in order to avoid future problems - can anyone see any issue in doing this ?

Regards,

Martin

bill.yu
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Re: Beaglebone Green - Grounding and ESD protection issues

Post by bill.yu » Thu Nov 29, 2018 6:00 pm

Hi Martin

Please try to remove the capacitor C162 and see any improvement. thanks.

Image

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