Electrical Isolation
Electronic test technique that eliminates the effects of leakage currents by surrounding the signal path with a conductor at the same potential. Implementing a guarded measurement requires a three terminal configuration where the guard shield shunts unwanted current away from the sensitive measurement node.
Voltage Feedback
Precision instrumentation uses a unity gain amplifier to drive the guard conductor. Because the voltage on the guard is kept equal to the voltage on the signal line, no current flows through the insulation between them. The guard shield intercepts stray currents from higher voltage sources and directs them to a low impedance return path.
This arrangement effectively cancels the capacitance of the test leads and increases the response speed of the system. Effective guarding requires careful routing of the guard trace around the high impedance input on the board layout.
Error Mitigation
Surface leakage on printed circuit boards often degrades signal integrity in high impedance nodes. A guarded measurement prevents these parasitic currents from entering the input of the transimpedance amplifier. In a typical in-circuit test environment, the guard terminal isolates the component under test from parallel paths in the surrounding network.
Accurate characterization of femtoampere level signals is impossible without this shielding.
