A Comparative Study of the Preamplifiers in Electrostatic Sensors through Analytical Modelling and Experimental Evaluation

Electrostatic sensors have been used to monitor a diverse range of processes and systems that involve movement of charged objects. Several types of preamplifiers are available for signal conditioning of electrostatic sensors, which differ in many aspects. This paper presents a comprehensive comparative study of four types of preamplifiers, namely trans-resistance amplifier, charge amplifier, current sense amplifier and potential amplifier, in order to clarify their differences for correct selection and facilitate proper cabling and electrode design. An equivalent circuit model that quantifies the interaction between the charged object and the electrode using coupling capacitance is established. The voltage outputs of the four preamplifiers are expressed analytically using the potential of the charged object and the object-electrode capacitance that varies as the object moves. In order to produce repeatable and controllable inputs to the electrostatic sensor, a fluctuating electric field is actively generated by applying an excitation voltage signal on an emitting electrode. Under impulse excitation, similarities in the signal waveforms are found between the trans-resistance and current sense amplifiers as well as between the charge and potential amplifiers, apart from the opposite polarities. Both impulse and frequency responses show that the current sense and potential amplifiers are significantly affected by the parasitic capacitances that depend on electrode structure and cabling and measures should be taken to reduce their effects.