The present application relates to apparatus (400, 500) for measuring an impedance of an electrical load (300) that is configured to be coupled to a controlled current source (200). The apparatus (400, 500) comprises a first coupling node (402) configured to be coupled to a first terminal (302) of the load (300) and a second coupling node (404) configured to be coupled to a second terminal (304) of the load (300). The apparatus further comprises a transformer (406) having a primary winding (408) and a secondary winding (410) and a capacitance (412) connected in series between a first terminal (414) of the secondary winding (410) and the first coupling node (402). A second terminal (416) of the secondary winding (410) is connected to the second coupling node (404). The apparatus further comprises a processing unit (424) is configured to control an excitation signal that is applied to the primary winding (408) so as to cause a variation, corresponding to the excitation signal, in an input current of the load (300), measure the input current and an input voltage of the load (300), and, based on the measured input current and input voltage, determining the impedance of the load (300).