PicoAmmeters/Electromeers/Sensitive SMUs -What is Guard?

The concept of guarding is to surround the input high node or DUT with a guard shield that is at the same potential. Current cannot flow through a leakage resistance that has a 0 V drop across it.  Guarding is used to greatly reduce leakage current in a high impedance test circuit. Leakage resistance exists in the input cable (between conductor paths) and in the test fixture (at connectors and insulators).

Guard is mostly used in low current and high resistance measurements.

Electrometers (Model 6517B, 6514) utilize guard in the current and charge functions.

PicoAmmeters(Model 6482, 6485, 6487) utilize guard in the current measurement function.

In the voltage and resistance functions of electrometers the guard is a driven guard. This keeps the guard potential at the same magnitude as the high impedance node.

Below is an excerpt from the Model 6517B electrometer reference manual about guard.

Notice in the Guarded configuration the LO of the ammeter is connected to the Metal Guard Plate. Because this is a feedback ammeter the LO terminal is within 100uV of the HI terminal. This is essential the condition for guarded measurements.