Measurement of Remote Floating Capacitive Sensors: Limitations of Passive Shielding and Advantages of Active Shielding

The measurement of remote capacitive sensors requires special shielding techniques to reduce the effects of the parasitic capacitance of the interconnecting cable. According to the literature, the passive-shielding technique is suggested for floating capacitive sensors (FCSs), whereas the active-shielding technique is for grounded capacitive sensors (GCSs). This article focuses on the measurement of remote FCSs by analyzing the limitations of conventional passive shielding, then proposing the application of active shielding, which is typically restricted to the measurement of GCSs, to FCSs. The analysis is carried out theoretically, by simulations, and experimentally considering: a capacitive sensor of units of pF, cable capacitances from 100 pF to 1 nF that correspond to interconnecting cable lengths from 1 to 10 m, three commercial operational amplifiers (OAs) with different values of open-loop gain, and two operating frequencies (10 and 100 kHz). According to the results reported herein, the passive-shielding technique has significant limitations when the cable is long and the operating frequency is high. For example, at 10 m and using the best OA in terms of open-loop gain, the cable introduces an error (relative to the full scale) of around 2% at 10 kHz, but 60% at 100 kHz. The application of the active-shielding technique significantly reduces such errors, although an additional OA is required to drive the cable shield. For instance, under the same conditions indicated before, the resulting error is around 0.01% at 10 kHz and 1% at 100 kHz. Therefore, for scenarios with long interconnecting cables requiring high operating frequencies, the application of active shielding seems to be really necessary to avoid large measuring errors.