-- Regards, Gary L. Sanders, Staff Analog Eng., [email protected] L3 Communications, Inc. Celerity Systems www.csidaq.com Cupertino, CA USA dir. (408) 861-7325 fx (408) 873-1397 Ultra Fast Acquisition & Data Generation SystemsLarry Smith wrote:
> I don't believe in the "20-H Rule". Suppose the power plane was at > 3.3V and the ground plane was at 0V. It would be easy to reconfigure > the system so that the "power" plane is at 0 volts and the"ground" > plane is at -3.3V. Does this mean that the power plane should now be > bigger than the ground plane? > > The only difference between the power and ground plane is that one is a > 0V and the other 3.3V WRT (...thats with respect to, lest I start > another discussion...) earth ground. But even this is not true in a > battery operated system. In any modern digital system, the impedance > between the power and ground plane is much less than 1 ohm well into > the EMI frequencies. > > The ground plane probably has a path out to frame ground and eventually > earth ground somewhere. But if that path is more than an inch long, it > is going to be well over 10 nH. Ten nH is 1 Ohm at 15 MHz (Z=jwL) and > higher impedance at higher frequencies. So, above 15 MHz, the voltage > between the power and ground planes is insignificant compared to the > voltage across the earth ground connection. > > The power and ground planes should be exactly the same size. To make > one larger than the other will simply have the effect of turning nice > diffential currents into common mode current and common mode > radiation. > > regards, > Larry Smith > Sun Microsystems
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