A method and device is disclosed for continuously and simultaneously measuring an impedance signal and a biopotential signal on a biological subject's skin. In one aspect, the method includes attaching input and output electrodes to the biological subject's skin and applying a predetermined alternating current having a first frequency to the output electrodes for creating an alternating voltage signal over the input electrodes. The first frequency is above a predetermined minimum frequency. The method also includes measuring an input signal from the input electrodes which includes a biopotential signal and the alternating voltage signal. The method also includes extracting from the input signal the biopotential signal and the alternating voltage signal, and determining the impedance signal from the alternating voltage signal. The alternating voltage signal is extracted by amplifying and demodulating the input signal using a control signal having a frequency equal to the first frequency.