Floating "grounds" on PCBs tend to be problematic, especially at high frequencies. Minimizing the effective capacitive reactance between the floating "ground" and the real ground will ensure that the floating ground is "AC Cold". I don't just mean bypassing it with (say) a few thousand uF of electrolytic capacitance, but instead ensuring minimal capacitive reactance to ground across the entire frequency band of interest. This usually entails (as an example) paralleling something like a 0.1uF cap, and with an NP0 1000pF cap (or similar).
If the floating "ground" not properly decoupled to the main ground, and it is a significant proportion of a HF wavelength, then it can have very high AC voltages superimposed and act as a marvelous antenna. Reducing the size of the floating "ground" is always a good plan. Increasing it merely means that you have a larger potential antenna. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Paolo Peruzzi [mailto:paolo.peru...@esaote.com] Sent: February 25, 2002 7:34 AM To: emc-p...@majordomo.ieee.org Subject: PCB floating area layout Hi all, I'm dealing with a PCB that has a floating section isolated from the rest of the board for safety purposes (patient applied part). I found out some problems with emissions, due to the coupling between the floating part and of the PCB and the earthed one. My questions are concerning the layout design of the floating area: 1) Is it best to minimize the HF capacitive coupling between the earthed ground and the floating ground or to maximize it? 2) Is it best to reduce the amount of the floating ground or to increase it? Does it depend on the goodness of the "main ground", i.e. how much it is "cold" ? (I see the board as a dipole with one end connected to earth, and the other floating). Thanks, p.p. ------------------------------------------------------------- ESAOTE S.p.A. Paolo Peruzzi Research & Product Development Design Quality Control Via di Caciolle,15 tel:+39.055.4229306 I- 50127 Florence fax:+39.055.4223305 e-mail: paolo.peru...@esaote.com ------------------------------------------- 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: Ron Pickard: emc-p...@hypercom.com 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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- 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: Ron Pickard: emc-p...@hypercom.com 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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
