An apparatus for compensating for a differential error between a plurality of body surface sensors for acquiring respective physiological signals from a human or animal patient, each such sensor respectively comprising a transmitter, a receiver, and connected to an output of the respective receiver, a charge-to-voltage converter, the apparatus comprising: a timing signal generator (2, 21) comprising an oscillator, to generate a timing signal; a channel for each respective body surface sensor, each channel comprising: (i) a current modulator to receive said timing signal and supply a current drive signal timed by said timing signal to the transmitter of the respective body surface sensor; (ii) a synchronous receiver to receive said timing signal from the timing signal generator (2, 21) and an output signal timed by said timing signal from the charge-to-voltage converter of the respective body surface sensor and supply a broadband output signal; (iii) an analogue high-pass filter to receive the broadband output signal from the synchronous receiver and supply a filtered output signal; and (iv) an anti-aliasing analogue low-pass filter to receive the filtered output signal from the high-pass filter and supply an anti-aliased output signal; the apparatus further comprising: an analogue-to-digital converter to receive the anti-aliased output signal from the respective channels of each respective body surface sensor and supply a digital output signal for downstream signal processing; wherein the timing signal generator (2, 21) also generates a digital representation of a sine wave and comprises a digital-to-analogue converter to receive the digital sine wave representation at an update rate higher than the frequency of the timing signal and convert it into an analogue sine wave having a frequency greater than a highest frequency of the physiological signals and less than a cut-off frequency of the anti-aliasing low pass filter; a low-pass filter to receive the analogue sine wave from the digital-to-analogue converter and remove update rate components therefrom to provide a clean sine wave; and an amplitude modulator to receive the clean sine wave from the low-pass filter and add it to the timing signal. a timing signal generator (2, 21) comprising an oscillator, to generate a timing signal; a channel for each respective body surface sensor, each channel comprising: (i) a current modulator to receive said timing signal and supply a current drive signal timed by said timing signal to the transmitter of the respective body surface sensor; (ii) a synchronous receiver to receive said timing signal from the timing signal generator (2, 21) and an output signal timed by said timing signal from the charge-to-voltage converter of the respective body surface sensor and supply a broadband output signal; (iii) an analogue high-pass filter to receive the broadband output signal from the synchronous receiver and supply a filtered output signal; and (iv) an anti-aliasing analogue low-pass filter to receive the filtered output signal from the high-pass filter and supply an anti-aliased output signal; (i) a current modulator to receive said timing signal and supply a current drive signal timed by said timing signal to the transmitter of the respective body surface sensor; (ii) a synchronous receiver to receive said timing signal from the timing signal generator (2, 21) and an output signal timed by said timing signal from the charge-to-voltage converter of the respective body surface sensor and supply a broadband output signal; (iii) an analogue high-pass filter to receive the broadband output signal from the synchronous receiver and supply a filtered output signal; and (iv) an anti-aliasing analogue low-pass filter to receive the filtered output signal from the high-pass filter and supply an anti-aliased output signal; an analogue-to-digital converter to receive the anti-aliased output signal from the respective channels of each respective body surface sensor and supply a digital output signal for downstream signal processing; wherein the timing signal generator (2, 21) also generates a digital representation of a sine wave and comprises a digital-to-analogue converter to receive the digital sine wave representation at an update rate higher than the frequency of the timing signal and convert it into an analogue sine wave having a frequency greater than a highest frequency of the physiological signals and less than a cut-off frequency of the anti-aliasing low pass filter; a low-pass filter to receive the analogue sine wave from the digital-to-analogue converter and remove update rate components therefrom to provide a clean sine wave; and an amplitude modulator to receive the clean sine wave from the low-pass filter and add it to the timing signal.