1 What is Guarding?

Guarding is a PCB layout technique that helps shield sensitive nets from parasitic circuit elements or external influences that can cause noise or leakages which can impact the performance of an application. These leakages can be from a multitude of sources, however, applications requiring the most accurate solutions need to consider all potential leakage sources. A model of adjacent trace parasitics and possible environment influences can be seen in Figure 1-1 (top view).

As seen above, leakage paths can be formed from several sources including flux on the board, debris (surface leakages), electrical interferences, and of course the inherent board resistance and dielectrics between conductors (traces in this case). Now, while the first two can be removed by having clean manufacturing processes and operational areas or burying the sensitive traces within the PBC, the third and fourth instances can affect any PCB and can cause significant leakages to form if proper guarding is not implemented into the design.

Take the trace to trace resistance as an example. At just a 5 V potential difference between two traces on a PCB and using an estimated trace to trace resistance in the Gigaohm range, this would amount to nA's worth of leakage just from this relatively small delta alone. So one can imagine the increased leakage the net would see from any substantial increase in voltage present on the board. Hence why incorporating a guarding scheme can be so important to keep leakage currents at bay.

Guarding techniques implement active conductors that are at the same potential as the sensitive net and surround said net to create an extremely low leakage environment. With the guard ring and the net at the same potential, this effectively creates an incredibly small voltage delta between the trace and immediate surrounding area which drastically reduces the possibility of stray leakage currents leaking into this area of the PCB. Figure 1-2 outlines a typical guarding scheme.