Hello Ravinder, Your question is a bit difficult to answer. You appear to have reached some conclusions on what is going wrong in the circuit, but do you have really good information on the problem? I had a similar problem in an instrument that had been designed prior to the widespread introduction of ESD tests. It had divided grounds between analog and digial, just as you are thinking here. Close examination revealed that the two grounds would bounce differently, causing logic problems and occasional lockups due to IC substrate voltages going to unexpected levels. So, dividing grounds without understand exactly what's going on can be counterproductive.
In this case, actually closing the grounds together under the critical ICs ended the problem (the separation at that point was not at all critical). Simply put, you need to either (1) get the ground to stop bouncing so much or (2) get all the associated PC lines to bounce in the same way. For example, it seems that your ASIC ground is bouncing, but the PC lines are not. Do they come from another board, perhaps? If your group is considering a board re-spin, you owe it to yourself to invest a few days with a good fast storage scope to track this monster as it moves through your system, seeking True Ground (whatever it thinks that is). Only then will you begin to gain an appreciation for how it moves and the nature of the malfunctions. You may be able to provide alternate high-frequency paths and solve the problem without a board re-spin (and be a hero). I can assure you, however, that the board re-spin has no guarantee of success unless you dig in there with a good scope. Paul O'Shaughnessy Affymetrix, Inc. -----Original Message----- From: Ravinder Ajmani [mailto:ajm...@us.ibm.com] Sent: Tuesday, February 27, 2001 1:11 PM To: emc-p...@ieee.org Subject: ESD protection Hi, My question concerns providing ESD protection to analog circuitry on a card. Currently, I have a common ground for the entire card (2S2P). For reasons, the card has to be tied to chassis. ESD discharge (air or contact) to the chassis causes ground level to move up, thus reducing noise margin and causing circuit malfunctioning. The ASIC chip in question has both analog and digital circuits, with separate decoupling capacitors for analog and digital power. Connecting a small (120 pF) capacitor directly across the analog power and ground pins seems to provide some improvement in the ESD immunity. We are considering a board redesign. Will it help if the ground plane below the ASIC is sectioned to provide a separate analog ground, connected to main ground at one location only near the decoupling capacitors, perhaps through a small inductor. Will this introduce other problems. Any other ideas !!!!! Regards, Ravinder PCB Development and Design Department IBM Corporation Email: ajm...@us.ibm.com *************************************************************************** Always do right. This will gratify some people and astonish the rest. .... Mark Twain ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall," ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
