A sensor arrangement comprising: a high impedance electrometer including a detection element for detecting electrical potentials from an object under test and for generating electrical detection signals, and an amplifier connected to the detection element for receiving the electrical detection signals from the detection element and for producing an amplified measurement signal as output; the amplifier having at least one ancillary circuit providing a positive feedback arrangement for enhancing the sensitivity of the electrometer to the detected electrical potentials, the at least one ancillary circuit including a bootstrapping circuit for providing bootstrapping to an input bias resistor for the amplifier characterised by the amplifier further having an additional feedback arrangement for controlling the sensitivity of the electrometer to signals of different frequencies in order to tailor the frequency response of the electrometer, the additional feedback arrangement comprising a feedback path from an output to an input of the amplifier and a narrow band filter (14, 92) situated in the feedback path of the additional feedback arrangement and tuning means (32, 34; 44, 46, 52, 64, 66) for tuning the filter to a selected frequency or frequencies, whereby to tune the frequency response of the electrometer with respect to the detection signals generated by an object under test and unwanted signals representing interference so as to increase the amplitude of the detection signals relative to the amplitude of the unwanted interference signals in order to increase the effective dynamic range of the sensor arrangement. a high impedance electrometer including a detection element for detecting electrical potentials from an object under test and for generating electrical detection signals, and an amplifier connected to the detection element for receiving the electrical detection signals from the detection element and for producing an amplified measurement signal as output; the amplifier having at least one ancillary circuit providing a positive feedback arrangement for enhancing the sensitivity of the electrometer to the detected electrical potentials, the at least one ancillary circuit including a bootstrapping circuit for providing bootstrapping to an input bias resistor for the amplifier characterised by the amplifier further having an additional feedback arrangement for controlling the sensitivity of the electrometer to signals of different frequencies in order to tailor the frequency response of the electrometer, the additional feedback arrangement comprising a feedback path from an output to an input of the amplifier and a narrow band filter (14, 92) situated in the feedback path of the additional feedback arrangement and tuning means (32, 34; 44, 46, 52, 64, 66) for tuning the filter to a selected frequency or frequencies, whereby to tune the frequency response of the electrometer with respect to the detection signals generated by an object under test and unwanted signals representing interference so as to increase the amplitude of the detection signals relative to the amplitude of the unwanted interference signals in order to increase the effective dynamic range of the sensor arrangement.