At 11:48 PM 4/30/2008, you wrote: >The difference is I think you're talking more about signal integrity >issues around high speed digital and I'm talking about sensitive analog >stuff. The key phrase in what you wrote is "any significant degree". >The particular board I had in mind had a coupling of perhaps -60 to -70 >dB which was too much for my application but would have been totally >insignificant for a digital circuit. And in this case, the coupling >path was capacitance from a signal trace to the power plane and then the >power plane back to a different signal trace. This I was able to >demonstrate conclusively in that case.
Yes, you are right, my information was in the context of digital designs, not low level analog. So I stand corrected. >Again, you have to careful of what type of design. If you have single >ended analog circuits, you have to define what the reference is >("ground") and you'll almost always find that the supply rejection is >better or worse depending on the exact circuit. Yes fully differential >circuits can help, but lots of practical RF circuits tend to be single >ended. I see how the power plane in this case can be an issue. *Any* noise injected to the power plane will be a problem. But even if you keep your signal from coupling to the power plane, how do you keep the noise from coming through the IC that is powered by that plane? I guess that is what PSRR is all about. Is PSRR effective up to the frequencies we are talking about? I know in most LDOs, the feedback loop is mostly ineffective at 1 MHz or above. I seem to recall that the output impedance of many opamp starts to rise significantly by 1 MHz. Does the PSRR also drop off? >You just can't count on ground/power noise from one chip not causing >major headaches for other unrelated circuits. I helped someone fix a >problem in an L-band receiver once where it was the 13th harmonic of a >clock oscillator coupling through the power supply over into his RF >signal chain. The solution was easy, local series impedance in the >supply and shunt bypassing with a well defined current path to the load. > Its been enough years now that I don't recall what the coupling >mechanism was on the receiver end. I also don't recall the details of >his power routing (plane vs traces, how many supplies, etc). I know that RF is tricky stuff to get right. But I have seen some really stupid stuff designed in under the guise of "it was a problem once before". In particular, we had a GPS module which needed digital and analog supplies. The RF engineer wanted an LDO on the *digital* supply (from a switcher) to keep noise from getting into the digital side of the module and coupling into the module's RF analog section. I mentioned that LDOs don't do diddly squat for isolation above 1 MHz. He replied that he used an LDO to fix an audio frequency problem from a switcher. I couldn't convince him that the LDO was not needed. If you have audio frequency noise from a switcher, the circuit is oscillating and needs to be fixed. An LDO is just a bandaid at that point. Of course bandaids are cheap, but it seems silly to use them on your hands because you once got a blister from a bad pair of shoes... :^o >About 15 years ago or so, EDN magazine had a special issue all about EMI >problems. It was a pretty good read and one of the simple facts that >they point out has proven very useful in practice. To have an EMI >problem, you need a source, a path, and a receiver. You may not always >be able to identify all 3, but if you can break at least one, you win. >In cases where you can identify more than one and improve them, you win >more. It sounds simple, but it's the first thing I remind myself of >when I have an EMI problem. That is a good way to look at it. Of course the hard part is to recognize these three elements. Rick _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user