Electrostatic Shielding
Ultra-low current sensing requires active shielding to prevent parasitic leakage currents from entering the measurement node. The application of triaxial guarded measurement uses a three-conductor cable where the inner shield is driven to the same potential as the force conductor. This configuration eliminates the potential difference between the signal line and its immediate shield, suppressing leakage.
Leakage Suppression
Guarding improves the accuracy of femtoampere-level measurements by routing parasitic currents away from the sensitive input of the electrometer. In a typical triaxial guarded measurement setup, any current flowing through the outer insulator is collected by the outer shield and returned to the common ground of the instrument. The inner guard conductor stays at the same potential as the center signal conductor, ensuring that no current flows across the inner insulator.
Instrument Configuration
Implementing this technique requires compatible triaxial connectors and specialized cabling designed to maintain shield isolation. High-resistance electrometers must feature a dedicated guard output terminal to drive the inner shield of the triaxial cable. Proper physical routing of these cables is essential to prevent triboelectric effects, where mechanical movement generates static charge that interferes with the measurement.
Calibration Protocol
Metrology laboratories verify the isolation resistance of the measurement system before executing sensitive tests. This process ensures that the guard circuit is functioning correctly and that the background leakage remains within acceptable limits, typically below a few femtoamperes.
