An apparatus for measuring parameter values from an electrical input signal comprises a signal sampling device operable to sample the input signal and to produce a corresponding output comprising a measured parameter value; a signal level detector configured to detect changes in the level of the input signal by more than a threshold amount; and a controller co-operable with the signal sampling device and the signal level detector. The controller is configured to operate the signal sampling device to sample the input signal in response to the signal level detector detecting a change in the input signal level by more than the threshold amount.