A conversion circuit is arranged for converting an ion detection current (id) produced by an ion detector into an ion detection signal (P). The conversion circuit comprises an input stage for converting the ion detection current (id) into an ion detection voltage (Vd), an output stage for converting the ion detection voltage into the detection signal (P), the output stage being arranged for drawing a first current dependent on the ion detection voltage, and a supplementary stage for providing a second current (is) dependent on the ion detection voltage to the output stage. The second current may be substantially equal to the first current.