%>Hi Max, %> %>Multi-point grounding of PCB to metal chassis is a good idea. %> %>The beneficial effect of the multi-point connection of the PCB to a %>metal chassis is to reduce the net impedance associated with the %>return currents for signals flowing on the PCB. The impedance decrease %>provides a lower voltage generated across the printed circuit board, %>thus reducing the common mode present on the various circuit nodes on %>the board. %> %>This is especially critical as frequencies rise, regardless of the %>concern often raised about (the red herring of) "ground loops".
[snip] Mike, Thanks for the great info. I have had similar experiences and problems in trying to avoid the red-herring of ground loops. We had one product, for example, in which I used BNC's with built-in 10,000 pF decoupling capacitors. I'm sure these connectors were well designed, but we had to remove them and replace them with regular "grounded" BNC's in order to pass EMI tests. In those days we didn't have to do the susceptibility testing, so I don't know how well these decoupled caps work for the new 61000 tests. I expect they could be a problem since discharges placed on the shield could go directly to the circuit ground plane (or traces). YEE GADS! I WONDER IF THAT COULD REALLY HAPPEN? In the case of high-frequency ground loops, like with video, BTW, I don't think there are any ground loops. High frequency returns always follow the signal path (if they can) because this is the path with the least impedance. Therefore, I don't think the familar argument that currents will return through the power cord to the driver are pertinent. Low frequency signals (kHz range) can return via the power cords, but I doubt if this is a problem with computer systems. If there is a potential problem here, though, I would be very interested in knowing about it. Max Max Kelson [email protected]
