Larry, Please see my comments in lines. Thanks. Larry Smith wrote: > Zhiping - Voltage is always with respect to (WRT) some other node. All > voltages on printed circuit boards, electronic packages or chips are > differential. Voltage is the potential of one node WRT another node. > We sometimes talk about the voltage of a node WRT the center of the > earth or WRT spice node zero (they are the same thing) but those > voltages are irrelevant for signal integrity in our products. The only > thing that counts is the difference in voltage between two nodes, for > example a signal WRT local ground. For traces in a PCB, the difference > voltage is always vertical, the voltage of a trace WRT the local > reference plane. TEM mode analysis is good for all the stuff we do. > This is very true! > > The PCB stackup in question has ground planes that surround both the > analog and digital traces. The only thing that is important is the > differential voltage between the trace and reference (ground) planes. > That difference voltage will propagate down the trace (transmission > line). Eventually, the voltage/current waveform will arrive at a via > that takes it to the surface of the PCB. There should always be a > nearby reference (ground) via. A chip will sense the differential > voltage between the signal and local ground. This will be the same > voltage that was driven into the trace at the other end of the line, > if we have done a good job of maintaining a controlled impedance > environment (50 ohms) for the whole length of the trace. The voltage > that went in on one end is the same as the voltage that comes out on > the other end a time delay later, each WRT to their local grounds. > (For the moment, let's ignore the lossy nature of transmission lines.) > > Now suppose we had a way to measure the voltage between two ground > points spaced several inches apart horizontally (I am not sure how to > do it, but let's say we could). Suppose there were several volts > difference between the two local ground points. Would that make any > difference to the waveform propagating on the 50 Ohm transmission > line? I don't think so. What ever voltage went into the transmission > line WRT it's local ground will come out of the transmission line WRT > local ground, even if the local grounds are several volts apart. The > driving chip drives WRT local ground and the receiving chip receives > a signal WRT local ground. If there happens to be a bunch noisy > digital lines on the opposite side of a ground plane, ...so what? I agree with your digital transmission line analysis, but how about the analog circuit? If the voltage difference betwen two points on the ground plane is not zero and those 2 points happen to be the reference points for a differential amplifier inputs. For my understanding, the voltage between those 2 ground points will effect the output of the amplifier. Is that true or I missed something here? Thanks. > > BTW, I agree with you that low frequency can be a killer. But signals > behave well at high frequency. The thickness of a ground plane is > all you need to isolate one signal from another. > > regards, > Larry Smith > Sun Microsystems > > > Date: Wed, 26 Sep 2001 12:27:38 -0700 > > From: Zhiping Yang <zhiping@xxxxxxxxx> > > X-Accept-Language: en > > MIME-Version: 1.0 > > To: ldsmith@xxxxxxxxxxxxxxxxxx > > CC: si-list@xxxxxxxxxxxxx, james.f.peterson@xxxxxxxxxxxxx > > Subject: Re: [SI-LIST] Re: return currents > > > > Larry, > > > > What you said about the current distribution at high frequency(>100Mhz) > > is true, but my questions is whether the current distribution is important > > or the voltage variation on the power plane is more important than current > > distribution? > > > > Let's say, there are 2 points on the power plane and the most (>93%) current > > flows > > on the surface near digital circuit. The current flow near analog circuit is > > very small (<7%), but it has big loop (large inductance) and it produces > > same > > voltage drop between those 2 points as large current on the surface of > > digital > > circuit. > > IF the anolg circuit is sensitive to the voltage noise, then it is a > > problem. > > > > Jim, another thing you need to be aware is that low frequency may be a > > killer > > for your anolog circuit. In your current stack up, it is diffcult for to > > control > > the lower frequency current return path on layer 4. > > > > Thanks. > > > > Zhiping > > > > > > -- > > Zhiping Yang, Ph. D. > > Hardware Engineer > > Cisco Systems > > 270 West Tasman Drive > > Mail Stop:SJCG/2/2 > > San Jose, CA 95134 | | > > email: zhiping@xxxxxxxxx :|: :|: > > Tel : 408 525 5690 :|||: :|||: > > Fax : 408 526 5504 .:|||||||:..:|||||||:. > > ***************************************************** > > > > Larry Smith wrote: > > > > > Jim - I don't believe that the high frequency return currents on your > > > digital traces will have much effect on your analog traces even though > > > they share the same ground plane (layer 4). > > > > > > The skin depth at 100 MHz is about 0.26 mil compared with the 0.7 mil > > > thickness of half oz copper. The skin depth is essentially the depth > > > that the magnetic field penetrates into the copper. At 100 MHz, very > > > little magnetic field (approximately 1/[e^(.7/.26)] = 7% ) will > > > penetrate through the copper plane. Even less of it will reach an > > > analog trace. At higher frequencies, the penetration will be even less. > > > > > > How sensitive are your analog signals? For digital signals, I would > > > not worry about 7% magnetic field penetration. > > > > > > regards, > > > Larry Smith > > > Sun Microsystems. > > > > > > > Delivered-To: si-list@xxxxxxxxxxxxxx > > > > From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx> > > > > To: si-list@xxxxxxxxxxxxx > > > > Subject: [SI-LIST] return currents > > > > Date: Wed, 26 Sep 2001 07:32:10 -0400 > > > > MIME-Version: 1.0 > > > > Content-Transfer-Encoding: 8bit > > > > X-archive-position: 946 > > > > X-listar-version: Listar v1.0.0 > > > > X-original-sender: james.f.peterson@xxxxxxxxxxxxx > > > > X-list: si-list > > > > > > > > > > > > hello, > > > > > > > > Stackup : > > > > > > > > 1 - gnd > > > > 2 - digital sig > > > > 3 - digital sig > > > > 4 - gnd > > > > 5 - analog sig > > > > 6 - analog sig > > > > 7 - gnd > > > > 8,9,10,11 ..... > > > > > > > > notice that the digital signals from layer 3 and the analog signals from > > > > layer 5 will probably have return currents on layer 4. > > > > > > > > question : > > > > will the digital return currents cause noise in the analog section, > > > > since > > > > they both share layer 4 for return currents? (My first guess is yes, but > > > > someone mentioned that the skin depth for the return currents is small > > > > so > > > > they can share layer 4 without effecting each other.) > > > > > > > > thanks for your input. > > > > Jim ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu