Hello Praveen,
The topic in which your question addresses is "Grounding". Grounding is one of the most complex topics EMC engineers have to deal with especially in a design that has both Analog and Digital circuitry. Most every case is different and there are thousands of variables to consider. I will address the most basic points and hope they will help you out. The concept of Ground Loops or Earth Loops is more of an analog issue than digital. Lower frequency and usually low voltage analog signals can be very sensitive to Ground Loops. On the other hand, digital circuitry is less effected by ground loops and in most cases it is desired to keep the loop areas as short as possible. This is usually accomplished by grounding digital circuitry as often as possible. Bypass caps and other de-coupling components are also reducing loop area. It is still a debatable topic on whether Digital PC cards should have a single point ground or multipoint. I think in most cases it just depends on the application. But, I think most everyone will agree that getting a good ground connection on both ends of a digital I/O cable is a must. You also need to have a good PE ground connection between equipment and its peripherals or it will have trouble passing the immunity tests you listed below (not to mention possible radiated emissions problems). My advice it this.... internal to your design you should do a good job of isolating Digital and Analog circuitry to keep digital noise out of your analog. Digital I/O should be grounded well at BOTH ends of the cable providing a good return path for signals and providing a common Earth Reference to between the main unit and the peripherals. Like I said, these are only the basic concepts that apply most of the time. Good luck, Brian ____________________Reply Separator____________________ Subject: Grounding Author: Praveen Rao <[email protected]> List-Post: [email protected] Date: 12/6/00 6:52 PM Hello Group, We currently have a system which has modules in a box(metal) with floating digital and analog grounds.It is a Class 1 (safety) equipment. We intend to connect the 0V of each board to protective earth (chassis) at a common single point at the power supply. By doing this, the conducted emissions, ESD, Surges, etc show better results. We have a serial port (DB9)on the device which is only used for configuring Our concern is when a PC is connected to this port, cause in PCs the signal ground (pin 5) is normally connected to PE. So, when a PC is connected to the serial port of our system we may end up with signal grounds on each end of this serial cable connected to it's own device PE. This may cause earth loops. Since this is only a config port, approvals may not be an issue but functionally in the field this may create problems. Is it common to connect digital grounds to a single point chassi ground directly at the power supply ? Should there be any concern from functionality point of view ? especially when a PC is connected to our serial port. Your comments will be greatly appreciated. Praveen Rao ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
