An apparatus operable to reconstruct a biopotential signal from a signal received at a surface of a body, the apparatus comprising: a sensor device which forms a first signal connection for the biopotential signal, the first signal connection having a first sensor impedance, and which forms a second signal connection for the biopotential signal, the second signal connection having a second sensor impedance, wherein the sensor device is arranged such that the second sensor impedance is linearly related to the first sensor impedance by an impedance relation; sensing circuitry which provides i) a first channel wherein a first sensor signal is related to the biopotential signal by a first channel relation which is dependent on parameter values for the sensing circuitry providing the first sensor signal, dependent on the first sensor impedance and dependent on the biopotential signal, and ii) a second channel wherein a second sensor signal is related to the biopotential signal by a second channel relation which is dependent on parameter values for the sensing circuitry, dependent on the second sensor impedance and dependent on the biopotential signal; a processor operable to read data carrying information on parameter values of the circuitry, operable to read data carrying information on the first sensor signal, operable to read data carrying information on the second sensor signal and operable to generate data carrying information on a reconstruction of the biopotential signal said reconstruction using a derived biopotential relation which is derived from a set of relations comprising the first channel relation, the second channel relation and the impedance relation, wherein the derived biopotential relation used to reconstruct the biopotential signal is independent of the first sensor impedance. a sensor device which forms a first signal connection for the biopotential signal, the first signal connection having a first sensor impedance, and which forms a second signal connection for the biopotential signal, the second signal connection having a second sensor impedance, wherein the sensor device is arranged such that the second sensor impedance is linearly related to the first sensor impedance by an impedance relation; sensing circuitry which provides i) a first channel wherein a first sensor signal is related to the biopotential signal by a first channel relation which is dependent on parameter values for the sensing circuitry providing the first sensor signal, dependent on the first sensor impedance and dependent on the biopotential signal, and ii) a second channel wherein a second sensor signal is related to the biopotential signal by a second channel relation which is dependent on parameter values for the sensing circuitry, dependent on the second sensor impedance and dependent on the biopotential signal; a processor operable to read data carrying information on parameter values of the circuitry, operable to read data carrying information on the first sensor signal, operable to read data carrying information on the second sensor signal and operable to generate data carrying information on a reconstruction of the biopotential signal said reconstruction using a derived biopotential relation which is derived from a set of relations comprising the first channel relation, the second channel relation and the impedance relation, wherein the derived biopotential relation used to reconstruct the biopotential signal is independent of the first sensor impedance.