A circuit for operating an amperometric sensor having a reference electrode, a counter electrode and a work electrode. The circuit comprises an amplifier having a positive input and a negative input and an output. The positive input is coupled to a reference voltage source, and the negative input and the output are coupled together via a negative feedback loop. The circuit includes means for coupling the amperometric sensor into said negative feedback loop of the amplifier wherein, in a first configuration, the counter electrode is coupled to said output and the reference electrode is coupled to said negative input and, in a second configuration, the work electrode is coupled to said output and the reference electrode is coupled to said negative input.