In this episode of the Atmel Edge, Analog Aficionado Paul Rako discusses the importance of understanding ground symbols and using the right ones in your schematic. Earth ground, chassis ground, power supply return and shield are all different. This video explains why. Earth ground, has a very precise meaning, and a very precise name, and it's earth ground. My professor, James T. McLaughlin, at Kettering University previously General Motors Institute, he pointed out earth ground is a ten foot copper-clad steel rod. And you hammer it into the dirt. And you make sure there's moisture so it has conductivity. The minute you hook a wire to it, well now you got some inductance. And 12-gauge wire all the way to get to where earth ground has to get, which is this third pin on your wall socket, well now it's got a little resistance, as well. My friend, Kirkwood Rough said, "Well actually, the copper rod, we used to do that for e-beam machines. That's not good enough. Because what would happen is the hole they drill in the concrete in this big slab of a building, would tend to be in the middle and it's dry underneath because rain doesn't fall there. So they wouldn't get good conductivity." He said, "If you want a real earth ground, find the steel structure that goes down to 100% resistance at the water table. Hook a big braided cable to that. And now you not only have an effective earth ground, because it's a lot of surface area going down multiple places, but you also have a cage, because the steel can help shield interference away from your machinery. James T. McLaughlin would flunk you for the whole semester -- because we were doing mostly car electronics -- if you used this ground symbol on a schematic. Sorry, a car isn't grounded. What you want to use is this symbol, which is chassis common. And chassis common, it's not just cars, but television, radios, PCs with metal things. Anywhere there's a metal case or a metal mounting point, that's chassis common. In America, if a human being can touch that metal, you have to connect earth ground at chassis common. Underwriter's Laboratory requires a ring terminal so it doesn't get pulled off. And that way, if there's a short of high voltage on to the chassis -- a wire or something falls down -- then it can seek a ground through this earth ground and trip a circuit breaker instead of electrocuting your customers. Now, semiconductor companies who make chips should be using this symbol, the triangle. That is power supply return. You may connect your circuit board in the corners, it may connect to chassis, and maybe you want that. Maybe you want it to connect at 100 places to get a really good RF connection between the circuit board and the metal chassis. But this symbol would be improper on a circuit board, and certainly earth ground is wrong. Henry Ott, is a famous consultant. He wrote some brilliant books you should read. He said, "Paul, where is the ground plane in a satellite?" Let's say you're designing satellite or avionics. Can't have a ground plane, because you don't have access to ground. There's no connection. It's like the car with the rubber tires. So he prefers the expression reference plane. Reference. When we get to PCB layout articles and videos we'll talk about some of Henry's principles, how to get you out of trouble. If you submit something to UL that's all full of this earth ground symbol on a circuit board, they'll say, "Well, that's not really correct. You should be using a common power supply return." Then you have to go into your schematic package and change all the symbols, and maybe the netlist is being different, and unbelievable problems can occur. So it's important to be diligent and use the right symbol first, from the beginning. And there's more to come about cutting up planes and why that's bad, and single-point grounding. We're gonna get in a big argument with the audio people who love single-point grounding. I'll tell you why it usually causes more problems than it solves. So, in future shows I have a story about a big semiconductor machinery place where I consulted. They had a machine reboot and break hundreds of thousands of dollars of wafers, because they didn't understand ground, shield, return, those principles. Experience at Ford Motor, where the hood wasn't properly grounded, let electromagnetic noise get out. We couldn't pass the truck in Canada and get it sold. So all these things in my experience -- the LAN phone -- that's what we'll talk about single-point grounding. It kinda worked until the vice president came down and put his cell phone next to it as he tried it, and you could hear all the noise from the cell phone get in. Bad grounding. So don't think we're beating a dead horse here. We're gonna talk about grounds for three or four more of these schematic episodes. Stay connected! Twitter: http://www.atmel.com/twitter Facebook: http://www.atmel.com/facebook LinkedIn: http://www.atmel.com/linkedin

• published: 13 Jan 2014
• views: 326