The circuit comprises:- an instrumentation amplifier (Amp1) with:- a first input (Amp1+) connected to a first receiver electrode (E1) in contact with a first area (A) of a medium (H), and- a second input (Amp1-),- a voltage generating device (D) connected to a voltage supplying electrode (E3) in contact with a second area (C) of the medium (H) for applying a direct current reference electrical signal with a fixed value to it,- compensation means, electrically insulated from the device (D), for compensating the direct current offsets of a weak electrical signal (SE1) received by the first electrode (E2), generating a reference voltage signal (Vref1), with a value which can be modified by means of a control system, and supplying it to the second input (Amp1-).The method is intended for controlling the circuit.
