Protection Method
A circuit shielding technique eliminates the leakage current that otherwise flows through insulation by maintaining equal potential between a conductor and its surrounding shield. The application of equipotential guarding is standard in electrometer designs where currents below one picoampere must be measured. This technique removes the voltage gradient across the dielectric material to preserve signal fidelity.
Potential Balancing
A low impedance driver follows the signal voltage and imposes it onto the guard structure. Because the voltage difference between the signal line and the guard is effectively zero, the effective resistance of the insulator becomes infinite. Equipotential guarding also reduces the input capacitance of the measurement system by neutralizing the guard to signal capacitance.
Signal Integrity
External electromagnetic interference and surface leakage paths are diverted away from the high impedance input by the driven shield. Data acquisition systems for sensitive electrochemical sensors rely on this method to prevent environmental noise from corrupting the readings. Failure to maintain a precise voltage match between the guard and the signal introduces an error current proportional to the buffer offset.
Installation Requirement
Careful PCB layout ensures that the guard trace is as close as possible to the protected node without creating a short circuit. The guard must extend through any connectors and cables to maintain a continuous low leakage environment. Routine testing of the guard buffer output confirms that the tracking accuracy remains within the millivolt tolerances required for sub picoampere precision.
