Driven guard

A driven shield is a method of electrical shielding used to protect low-current circuits against leakage current, a driven shield is often referred to as a guard, especially when applied to PCB traces. It is used in situations where the tiny leakage of current through the insulating surfaces of a wire or PCB trace would otherwise cause error in the measurements or functionality of the device. The basic principle is to protect the sensitive wire with a surrounding guard that is held at the same voltage as the wire, if they are at the same voltage then there will be no current flow. This is typically achieved using a voltage buffer/follower that matches the guard voltage to the wire voltage. The leakage from the shield to other circuit elements is of little concern as it is being sourced from the buffer which has a low output impedance.