A point to be aware of is that there are several conflicting requirements - this is engineering after all :-). As Olin notes, you can "island" the processor and feed power current to and from it via a single "highway". But, if you drive output loads that do not have their ground return paths returned to the island, then you get loop currents flowing elsewhere. This is understood and accepted. But in the process you can create common paths with analog signals also leaving the island and returning 'by another route". It can be useful for lower curren= t analog signals to have their go and return paths fed back to the "island" - either directly with dedicated go and return conductors, or as Olin notes, by trying to arrange for most signal current to flow by "directish" routes back to whence it has come, and at the same time trying to make sure that power currents do not flow over the same route. In extreme cases you can us= e "guard rings" to create a constant potential around your signal leads, but this is usually beyond what's needed in most cases. In 'good old analog days' (cardboard box, lake, uphill both ways, snow ...) where wiring was often "point to point", it was common practice to (try to= ) have a single ground point and to have ALL ground wiring return to this one point and to not use chassis, bus bars etc as ground returns. This was intended to minimise common impedance paths shared by various currents. If two signals share a common conductor that has non zero impedance and if one signal varies the voltage drop across this common impedance then the common voltage will be imposed on the second signal. If the source is high amplitude and the affected signal is low amplitude then the effect can be very substantial. Also, if grounds are interconnected at various points you can end up with one or more "loops" with non zero area that are not only transmitting aerials but also receiving aerials. The digital circuitry MAY not care but the analog circuitry usually will. This is the classic "ground loop" which causes hum in audio systems. The common point ground and the distributed ground plane are in some ways opposite ends of the continuum (connect anywhere versus connect at one point) but are also aiming at the same thing - a perfect ground plane is a "distributed single common point". The problems mentioned occur when this "distributed common point" becomes non ideal or less ideal - such as when the signals on it become large enough to produce voltage differences, and the points Olin raised are intended to attempt to push things back towards "ideallity". Murphy loves helping out when you try to achieve this :-). Russell McMahon On 2 September 2010 23:30, Olin Lathrop wrote: > Gary Crowell wrote: > > If I understand your description correctly, doesn't this mean that the > > return signal paths of all the PIC signals are confined to that single > > pwr/gnd connection point? > > Yes, for those signals that go external to the local area of the PIC. Th= e > crystal lines do not, for example. > > This means that the high frequency loop currents caused output signals > being > driven by the PIC do go accross the wide ground plane. However, note tha= t > they would in a single ground plane scheme too. The high frequency curre= nt > source in the PIC between the power and ground leads is generally much > bigger, and its current will be contained locally and not "contaminate" t= he > global ground plane. > > This kind of board design is all about visualizing the high frequency > current loops, where they are, and what effect they will have. We are us= ed > to thinking about power current flowing into the Vdd pin and out the Vss > pin > of a PIC. This is true for the DC component, but if you only look at the > high frequency components the PIC actually becomes a current source, as d= o > other digital ICs. Letting this current run accross the global ground > plane > for a bit it just another way of describing a center fed patch antenna. > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .