On Sat, Mar 25, 2006 at 09:55:55AM -0700, John Doty wrote: > > On Mar 24, 2006, at 7:41 AM, Steve Meier wrote: > > >Ideally true but in reality not true. Put you oscilloscope probe > >tip to > >the ground plane at a point further then the probes ground. More often > >then not you will see some sort of noise. If the ground had zero > >resistence and an infinite supply of available electrons this noise > >wouldn't be there. > > Not true. This usually isn't due to resistance. Taking skin effect > into account, at 100 MHz a copper ground plane has a surface > resistance of ~3 milliohms/square. On the other hand, a 3 cm diameter > loop has an impedance of ~50 ohms. Now this is just dimensional
What is attenuation in decibels of an infinite copper plane 0.2mm thick @100MHz (driven by a plane space wave perpendicular to the plane)? CL< > analysis, but the mutual impedances that cause the crosstalk tend to > be proportional to these numbers. So at this frequency you can expect > crosstalk due to mutual resistance to be something like 80 dB below > the mutual inductance crosstalk for circuits of typical dimensions. > > The reason you see this on the scope is that the ground lead -> probe > -> probe tip -> ground lead loop picks up the induction from loops in > the circuit. > > >Since ground is the return path of most circuits > >(obviously not differential circuits) and two or more circuits can > >have > >return paths that cross each other on the ground there will be cross > >talk on the ground plane. In the case of digital circuits which often > >have high frequency clocks that clock itself can propigate through the > >ground and power and into the analog circuits where it can then be > >amplified causing distortion. > > Propagate through ground? No! The fields in a copper ground plane are > very low. The interference propagates through fields surrounding the > circuitry. The most common problem is coupled loops, as in your probe > experiment. > > I wish EE profs would teach their students that the user of > Kirchoff's Voltage Law is pledging to account for all of the > induction in the circuit. > > > > >So it is often recomended using two grounds which are connected at a > >single point. One for analog one for digital. The question is often > >where and how to connect the two grounds. > > Wherever you have net current flow between the circuitry on the > separate grounds, you want them connected at that point to provide a > return current path. Wherever wish to accurately carry a single ended > voltage between the ground systems, you want them connected at that > point to provide a return reference. If you're serious about EMI, > only differential signals may be allowed to cross between the ground > systems without a return connection between the grounds at the point > where they cross. Even there, you should be careful about common mode > excitation and CMRR: it may be better to provide a common mode return > connection. > > The purpose of this is to collapse the loops that are the source of > the trouble. The return current has a strong tendency to flow as > close to the signal current as it can, so giving it a way to be > really close to the signal can make the effective loop area very > small. By reciprocity, similar considerations apply to voltage > transport. > > Alexander Graham Bell attempted to patent the "return next to signal" > idea in 1878, but in 1881 the US Patent Office ruled that David > Brooks had beaten him to it (although Bell was successful at > obtaining a patent in England). > > If coupling is through mutual resistance, the "single point ground" > idea is a good one, because in this case the coupling must occur > through a common conductor. For precision measurements below ~1kHz, > this may be the main story. But for mutual inductance coupling, no > contact between circuits is required. It's usually better then to > "let ground abound": you're much more likely to get into trouble by > being too clever. > > John Doty Noqsi Aerospace, Ltd. > [EMAIL PROTECTED] >
