%> %> %> Hi Ladies/Gentlemen, %> %> I have been reading with interest the discussion articles on this %> subject. Since Mike Violette 04/15/96 presented his opinion on VCC/GND %> plane layout in multilayer board, the discussion seems to be focused %> on PCB EMC design. Max Kelson 04/16/96 wrote: %> %> [snip] %> What this ferrite/cap configuration would do is to force the %> oscillator to draw all transient current from the capacitor. Or, in %> otherwords, the rest of the caps on the board would be unable to help %> provide fast-transient current because of the ferrite. This would %> keep the current loop (power AND GROUND) small and prevent it from %> infecting the rest of the board. The path for the transient current %> surges would be from the capacitor to the IC's power pin, out the %> IC's ground pin and back to the negative side of the capacitor (a %> relatively small loop). %> [snip] %> %> %> It might be worthwhile to pay attention to research work done by %> professors at the Univ. of Missouri-Rolla. In the article "Power Bus %> Decoupling on Multilayer Printed Circuit Board", IEEE Trans. on EMC, %> vol. 37, pp. 155-166, May 995, they wrote: %> %> [snip] %> VI. Conclusion. %> Unlike boards without internal power and ground planes, multilayer %> boards have a built-in capacitance that is a more effective source of %> current than surface decoupling capacitors at high frequencies. In the %> time-domain, this means that most of the initial current supplied to a %> fast switching device is provided by the interplane capacitance. %> [snip] %> %> %> Regards, %> Barry Ma %> %>
I attended an EMC seminar conducted by Dr. Tom Van Doren on June 8, 1992 and then another one on March 23, 1995 by Dr. Van Doren, entitled "Circuit Board Layout to reduce Electromagnetic Emission and Susceptibility. It was amazing how much progress Dr. Van Doren had made on the general subject of controlling EMC on the board level between the two dates. In this seminar the importance of a low Zo with the DC power bus was discussed extensively and there was a lot of information on bypass capacitors also. These two subjects are really the leading edge topics for EMC control in the 90's, I think. One idea that Dr. Van Doren suggested during the seminar was to connect the bypass caps directly to the VCC and ground pins. This suggestion was given with the caveat that the physical geometries be within some critical spacing distances that he recommended. With the cap connected directly to VCC and ground, he also suggested a layout where traces go some distance before they are connected to the DC power and return planes. So, in otherwords, Dr Van Doren, an EMC professor, and Earl McCune, an RF communications consultant [1] have essentially arrived at the same conclusion, I think. They are both suggesting that the loop be minimized with a bypass capacitors and that a series impedance be added in the tap off the power plane. The only difference is that McCune recommends the use of a resistor, a low-Q inductor or a ferrite while Van Doren suggests using the natural impedance of circuit board traces to provide the impedance. The question that remains unanswered, I think, is at what frequencies is this method effective and will the capacitors be fast enough to provide the current, etc. This is an issue that needs a lot of research and one of the big problems is the dynamics in the capacitor industry and the lack of appropriate capacitor specifications. Dr. Van Doren, for example, provides some sample ESL figures, in one of his calculations, of 30 nH. In other calculation, he uses 100 nH. However, I don't think he is aware that there is at least one leading edge manufacturer (AVX) that is providing SMT MLC's with very low ESR's and ESL's. As I recall AVX makes some of these capacitors with ESL's as low as about 0.7 nH. Ultimately, I think the answer has to be to use these low inductance caps to provide for an IC's high (relatively speaking) transient current requirements and use built-in caps to provide the higher frequency, lower current requirements (as mentioned previously). I highly recommend Dr. Van Doren's seminars, BTW. The information that he provides is new, and interesting and exciting. Dr. Van Doren, incidently, provides his email address in his seminar book and, as I recall, I did send him one question once and got a polite and prompt response. Max [email protected] Max Kelson Peripherals Engineer Evans & Sutherland Computer Corp. [1] "Ground-Current Control Enhances Dynamic Range in High-Speed Circuits, Earl McCune, MSEE, RF Communications Consulting, EDN, January 19, 1995.
