An apparatus for removing a periodic perturbation signal from a first bit stream where the first bit steam represents a digital version of an analog signal which comprises the periodic perturbation signal and a measurand, and where the first bit stream is provided at a first bit rate, the apparatus comprising: a perturbation signal processor adapted to: receive the first bit stream; identify the periodic perturbation signal, wherein the perturbation signal processor is provided with knowledge of the frequency of the periodic perturbation signal and uses this to identify parameters of a component in the first bit stream that is attributable to the periodic perturbation signal; generate a replica of the identified periodic perturbation signal based on the identified parameters; convert the replica of the periodic perturbation signal to a second bit stream at the first bit rate; and a digital processor adapted to receive the first bit stream and the second bit stream and to form an output bit stream as a difference between the first bit stream and the second bit stream, wherein the apparatus is characterized in that it comprises: a further processor configured to monitor the output bit stream to identify residual components of the periodic perturbation signal, wherein if residual components have been identified, the perturbation signal processor is configured to modify the second bit stream so as to shift the second bit stream in order to minimize the residual components in the output bit stream. a perturbation signal processor adapted to: receive the first bit stream; identify the periodic perturbation signal, wherein the perturbation signal processor is provided with knowledge of the frequency of the periodic perturbation signal and uses this to identify parameters of a component in the first bit stream that is attributable to the periodic perturbation signal; generate a replica of the identified periodic perturbation signal based on the identified parameters; convert the replica of the periodic perturbation signal to a second bit stream at the first bit rate; and receive the first bit stream; identify the periodic perturbation signal, wherein the perturbation signal processor is provided with knowledge of the frequency of the periodic perturbation signal and uses this to identify parameters of a component in the first bit stream that is attributable to the periodic perturbation signal; generate a replica of the identified periodic perturbation signal based on the identified parameters; convert the replica of the periodic perturbation signal to a second bit stream at the first bit rate; and a digital processor adapted to receive the first bit stream and the second bit stream and to form an output bit stream as a difference between the first bit stream and the second bit stream, wherein the apparatus is characterized in that it comprises: a further processor configured to monitor the output bit stream to identify residual components of the periodic perturbation signal, wherein if residual components have been identified, the perturbation signal processor is configured to modify the second bit stream so as to shift the second bit stream in order to minimize the residual components in the output bit stream.