A recent edition of Design News had a nice story about ground bounce causing problems in LCD panels. Poor or incorrect grounding causes all kinds of horrible problems in electronic systems. The first thing you need to understand is that silly little symbol on your schematic does not magically create an ocean of zero impedance. The ground symbols are just a convention so we don’t have to draw all the separate return paths in our electronic circuits. Many days I think it would be better if we did draw all the grounds as separate wires on our schematics.
The article above bemoans that LCD panel suppliers are connecting their power supply returns to the chassis of the display. The author seems to think this is bad, and I tend to agree, if I understand the problem correctly. He says the LCD panel people do this to lower EMI radiation out of the panel. I have to assume what is going on is that the ITO (indium tin oxide) transparent electrodes on the panel need to be at least ac referenced to earth ground, so they can serve as a shield for the EMI caused by the digital signals inside the panel. But he points out that these fast digital signals can cause the ground to bounce up and that causes memory erasure and all kinds of other problems.
Now a Ham radio person would know the difference between a ground, a shield, and a power supply return. Those RF folks really understand EMI and radiation and low-impedance, even if they are not engineers. Ideally you would have an ITO layer on the display that was continuous and connected to the chassis of the product. That would serve as an EMI shield for all the fast edges inside the LCD panel.
To reduce EMI you want the tightest shortest loops between current carrying conductors. So if there is a ribbon cable to the display, you would want a return line next to each and every signal line. If the ribbon is that twisted pair type that is even better. In addition to putting in power supply returns for the signals, what you folks love to call “ground,” you could also shield the cable by running it a conduit or wrapping it with copper tape. But you have to be very careful where you connect that shield to the power supply returns (aka ground) and also to earth ground, which is that third round pin on your wall plug.
If you connect that shield in multiple places, it will start sharing current with the power supply returns. Now you have changing currents in space, and EMI. I am starting to film a whole YouTube series on schematics, and the first 6 shows are all on the humble ground. So remember, that upside-down Christmas tree that everyone calls ground—that is earth ground. Linear Tech has routinely used it as a signal ground on their datasheets and app notes for 30 years. It is absolutely wrong and sloppy to do this. They are chip guys, maybe brilliant chip guys, but they don’t do system design. If you try to take a product through UL or CE they would like you using earth ground symbols all over the place.
The middle symbol above is chassis ground. That is what you use for a chassis of a car or radio. Unfortunately car makers do use the chassis to return electrical signals, but they are getting smarter and putting in copper wires to make sure the return currents really do return. What we should be using for most all our circuits is the little triangle symbol. And yeah, the power supply common does connect to the chassis common, and you should show that on your schematic. And if your product plugs into a wall, you have to connect the metal chassis to earth ground, unless it is a double insulated product, in which case the plug need not carry the earth ground.
Stay tuned, I will start filming these shows in our new studio here at Atmel and will back-post to them on this blog once I start getting them up.