The invention provides an electric potential sensor including, at least one detection electrode arranged for capacitive coupling with a sample under test and for generating a measurement signal, and a sensor amplifier adapted to receive the measurement signal as input and to supply an amplified detection signal as output. An input impedance enhancing element provides a high input impedance to the sensor amplifier for increasing the sensitivity of the electrode to reduced electric potentials, and a feedback element applies a coherent feedback signal to the input of the sensor amplifier for enhancing the signal to noise ratio of the sensor.