Driven guard

It is used in situations where the tiny leakage of current through the insulating surfaces of a wire or PCB board would otherwise cause error in the measurements or functionality of the device.

The leakage from the shield to other circuit elements is of little concern as it is being sourced from a buffer which has a low output impedance.

[2] Any situation in which the source to be measured has a very high output impedance is vulnerable to leakage current and if sufficient insulation is not practical then a driven shield may be required.

Coaxial cable is well suited for use as a guard; if electromagnetic shielding is also required then triaxial cable may be used as depending on the type of buffer circuit any noise on the guard may be amplified in the output.

Care must also be taken to ensure there are no unexpected paths by which leakage current may bypass the guard as this will defeat the system, and extra care must be taken in the design of the amplifier/buffer circuit to prevent oscillation as the guard, especially if it is used over a coaxial cable, may have a strong capacitive coupling to the sensing wire.