DDC264: Guard shielding

Hi,

Guard is simply a trace/plane at the same potential as the trace that you want to protect from leakages, that surrounds the trace that you want to protect. Basically the input traces potential is "ground" (as the DDC amplifiers sets it through its feedback loop). If you have a DC line near an input, there is a risk that that it leaks a current into that input equal to (DC-ground)/R, where R is the resistance between the DC line and the input (usually a parasitic, not infinite). If you place a trace (called guard) between the two, at the same potential as the input (i.e., ground), then any R between that trace (guard) and the input will not matter as the difference of potential between the two is zero (i.e., no current is created). The current created between the DC line and the guard is irrelevant and sank to ground through the guard...

Notice that you have to think of this in 3D, so, not only laterally, between conductors on the same plane, but also across planes. I.e., watch with having a DC plane under/above your inputs too, for instance...

In any case, in many applications, DC traces are quite far from inputs, or isolation is big enough, hence the resulting leakage currents are negligible/smaller than, for instance, the input bias current of the DDC itself, so, may be a don't care. If you still need better accuracy, you may have to implement some kind of calibration mechanism to remove all of them.

Regards,

Eduardo