HDMI? More like HDM-WHY? Thoughts on Cable Shield Grounding

Ladies and gentlemen, I present this week’s episode of “Crimes Against Cables”

 

Example 1: “I had some leftover components to use”

I’ve seen plenty of interesting EMC “solutions” over the last several years to deal with radiation from cables.

A common one is to separate the shield ground from the signal ground with some combination of components (beads, capacitors, resistors). This approach appears to be particularly common on industrial touch screen display modules for some reason.

poor USB cable grounding suggestions

 

This is (in 99% of cases) a bad idea. I’m not sure what you are hoping to achieve by this and, probably, neither are you 😉

In fact I dedicated a small part of a recent talk to discussing grounds and grounding – you might want to check it out.

 

Example 2: How to Break a Shield

Another notable poor example was an otherwise well crafted piece of military equipment. Shielded connectors and cables all over, it looked like it would be survive some serious electromagnetic abuse (as anything being tested to MIL-STD-461 should).

However,

insulating plastic insert bad idea

insulating plastic insert cross section detail

 

This ends up being not only an emissions problem but an immunity one as well as the cables are just as capable of conducting noise into the shielded case.

This sort of thing can be solved with something like an EESeal type component or by a secondary external screen over the entire assembly.

 

Example 3: Plastic Fantastic

I’ve even seen ferrite cores that were just a moulded plastic lump to appear like cores. Maybe it was a “special” plastic? I never found out, it didn’t help the emissions either.

vga cable ferrite just plastic

But this next one was a first even for me.

 

Example 4 – The Strangest Decision Yet

I was performing a full set of EMC tests to EN 55032 and EN 55035 for a customer. The product had a HDMI interface so obviously there were radiated emissions problems.

The first step as a diagnostic was to use some copper tape to make a connection between the connector shell and the metal back plate – the anodised chassis and EMI gasket material provided was not making a good contact.

This didn’t help so I buzzed the connection with the multimeter to make sure I had some continuity and… nothing.

No connection between the connector shell and PCB Ground.

OK, so there must be a capacitor in series with the shield connection. Fetch the capacitance meter and… 1.2pF.

The board designer had neglected to connect the shield of the HDMI to PCB Ground. It’s a new one for me!

The addition of copper foil to bridge the connector pins to nearby solved the emissions problem but left me wondering why someone thought that was a good idea.

 

I’m going to leave you with this closing thought:

I’ve yet to come across an EMC problem where floating or not connecting a shield ground has improved the situation.