Active Guarding of a Four-point Impedance Probe with one Common Guard Electrode for Maximum Readout Bandwidth

Active guarding of the front-end of a four-point impedance probe is analyzed for use in a set-up that is characterized by a conductive layer or medium, with one contiguous conductive layer in close proximity that can be used as an active guard electrode. It will be shown that active guarding of this single electrode provides effective shielding of the differential signal from EMI and enables a higher signal level and bandwidth as compared to grounding of the guard electrode. Applications are in impedance spectroscopy when using a 4-point probe in a poorly conductive fluid, or on a thin-film impedance measurement in the presence of a conductive substrate. The performance was validated in a simple set-up.