An ECG signal acquisition system comprising: an analogue readout unit configured to receive an analogue ECG signal (BS1) and to extract, from that received analogue ECG signal, an analogue measured ECG signal (BS2) and an analogue electrode-tissue impedance signal (REF1); an ADC unit configured to provide a digital version of the analogue measured ECG signal (BS2) and the analogue electrode-tissue impedance signal (REF1); a reference signal processing unit configured to convert the electrode-tissue impedance signal (REF1) into a new reference signal (REF2); wherein the reference signal processing unit is configured to perform a polarity calculation based on the correlation between the measured ECG signal (BS2) and the electrode-tissue impedance signal (REF1); and wherein the reference signal processing unit generates a new reference signal (REF2) that is the same as the electrode-tissue impedance signal (REF1) if the polarity information has a first value indicating that the electrode-tissue impedance signal (REF1) has the same polarity as the measured ECG signal (BS2), or generates a new reference signal (REF2) that has a polarity opposite to the polarity of the electrode-tissue impedance signal (REF1) if the polarity information has a second value indicating that the electrode-tissue impedance signal (REF1) has an opposite polarity than the measured ECG signal (BS2); a digital filter unit configured to calculate a digital motion artifact estimate (MA) based on the digital version of the measured ECG signal (BS2) and the new reference signal (REF2) provided by the reference signal processing unit . an analogue readout unit configured to receive an analogue ECG signal (BS1) and to extract, from that received analogue ECG signal, an analogue measured ECG signal (BS2) and an analogue electrode-tissue impedance signal (REF1); an ADC unit configured to provide a digital version of the analogue measured ECG signal (BS2) and the analogue electrode-tissue impedance signal (REF1); a reference signal processing unit configured to convert the electrode-tissue impedance signal (REF1) into a new reference signal (REF2); wherein the reference signal processing unit is configured to perform a polarity calculation based on the correlation between the measured ECG signal (BS2) and the electrode-tissue impedance signal (REF1); and wherein the reference signal processing unit generates a new reference signal (REF2) that is the same as the electrode-tissue impedance signal (REF1) if the polarity information has a first value indicating that the electrode-tissue impedance signal (REF1) has the same polarity as the measured ECG signal (BS2), or generates a new reference signal (REF2) that has a polarity opposite to the polarity of the electrode-tissue impedance signal (REF1) if the polarity information has a second value indicating that the electrode-tissue impedance signal (REF1) has an opposite polarity than the measured ECG signal (BS2); a digital filter unit configured to calculate a digital motion artifact estimate (MA) based on the digital version of the measured ECG signal (BS2) and the new reference signal (REF2) provided by the reference signal processing unit . <EOS>