Investigation of a low frequency measurement anomaly encountered with measurement systems utilizing a common shield

Theory, verification data, and suggested solutions are discussed for a low-frequency measurement anomaly encountered with measurement systems where the excitation and detection circuits are referenced to the same ground potential via a common shield. Drive return current is allowed to flow on the imperfectly conducting shield of the measurement circuit cable, introducing an error voltage in addition to the desired voltage. The inductance of the shield reduces this current at higher frequencies, thereby causing the error to roll off. This measurement error, caused by the transfer impedance of the shield, is most evident when measuring the small impedances commonly characterized in electromagnetic-pulse/electromagnetic-compatibility testing