On the subject of adding caps in circuits for noise immunity. Rather than just throwing in a value of 0.1 uF, finding it works, and calling it good I like to use a different approach. Determine the lowest value that will fix the immunity problem. Then find the largest value that will still allow the circuit to function properly. Then specify a value somewhere in between; possibly the geometric mean. In this way you are not using a value "next to a cliff" that will cause some malfunction in a production run. In a high volume product just throwing in a value and calling it good is EXTREMELY poor engineering.
Dave Cuthbert Micron Technology From: [email protected] [mailto:[email protected]] Sent: Thursday, January 16, 2003 8:19 PM To: [email protected] Subject: Re: I2C bus sensitivity to EFT I have never worked on design of a product with external I2C bus, but considering the bus speed of 100 kHz to 3.4 MHz (depending on type) relative to about 60-100 MHz BW (depending on definition) of EFT, you may try killing good portion of EFT with ceramic caps. Be sure that the cable shield is connected on both ends. Also, check for the possibility that the EFT may couple to some other apparently non-critical pin of the IC and then internally cause susceptibility. The first suspect in such case would be the reset pin, but often you can be surprised that other pins may cause problems. I just had a case in which EFT would couple to the LED driver on an Ethernet device and cause problems internally in the chip, leading to packet loss. A cap on the LED driver pin fixed the problem !! :) Neven > > Hi Forum, > Has anyone on this forum worked with I2C products and maybe be able to > advise on the best method to suppress EFT noise that would alow the I2C bus, > via external cables, to operate as expected e.g. Good EFT devices? How best > to shield the I2C? Other? > As always I look forward to your proffesional advice. > History > I have a system that connects 3 products (powered from an in-line external > non-earthed power supply, SELV) using the I2C bus via a 1m shielded and 1m > shielded curly cable. I use an I2C bi-directional extender IC P82B715 on one > of the products. The I2C protocol gets corrupted when I appply the > Electrical Fast Transient (EFT) Test per the EMC Standard EN61000-4-4. > I have tried shielded cables, several EFT devices and 1nF caps on the lines > but with little affect. > > Kind Regards > Alex McNeil > Principal Engineer > Tel: +44 (0)131 479 8375 > Fax:+44 (0)131 479 8321 > email: [email protected] > > > ------------------------------------------- > 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: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] 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"
