An integrated circuit (140; 240) comprising: an output circuit (160; 260) having a first terminal (160a; 260a) adapted to couple to an external power supply that provides a supply voltage that is ramped through a predetermined voltage range, a second terminal (160b; 260b) adapted to couple to a reference potential, and a third, control terminal (160c; 260c), wherein the first and second terminals provide output terminals of the integrated circuit (140; 240); and an impedance measurement circuit (150; 250) responsive to the external power supply and arranged to generate a control signal for coupling to the control terminal (160c; 260c) of the output circuit (160; 260) for controlling a current level associated with the output circuit, and to monitor an output signal of the integrated circuit for an oscillation, characterised in that the integrated circuit is configured such that the controlled current level of the output circuit selectively causes an oscillation in the output current of the integrated circuit in dependence of the voltage change across an external impedance of the integrated circuit due to the change in output current, wherein the external impedance is representative of resistance(s) between and associated with the integrated circuit and the external power supply, and wherein (i) a time of occurrence of the oscillation; (ii) a duration of the oscillation; or (iii) both a time of occurrence of the oscillation and a duration of the oscillation is indicative of the external impedance of the integrated circuit. an output circuit (160; 260) having a first terminal (160a; 260a) adapted to couple to an external power supply that provides a supply voltage that is ramped through a predetermined voltage range, a second terminal (160b; 260b) adapted to couple to a reference potential, and a third, control terminal (160c; 260c), wherein the first and second terminals provide output terminals of the integrated circuit (140; 240); and an impedance measurement circuit (150; 250) responsive to the external power supply and arranged to generate a control signal for coupling to the control terminal (160c; 260c) of the output circuit (160; 260) for controlling a current level associated with the output circuit, and to monitor an output signal of the integrated circuit for an oscillation, the integrated circuit is configured such that the controlled current level of the output circuit selectively causes an oscillation in the output current of the integrated circuit in dependence of the voltage change across an external impedance of the integrated circuit due to the change in output current, wherein the external impedance is representative of resistance(s) between and associated with the integrated circuit and the external power supply, and wherein (i) a time of occurrence of the oscillation; (ii) a duration of the oscillation; or (iii) both a time of occurrence of the oscillation and a duration of the oscillation is indicative of the external impedance of the integrated circuit. (i) a time of occurrence of the oscillation; (ii) a duration of the oscillation; or (iii) both a time of occurrence of the oscillation and a duration of the oscillation is indicative of the external impedance of the integrated circuit.