Method for determining or estimating an impedance of at least one electrode, the method being characterized by utilization of: a reference amplifier apparatus for processing physiological measuring signals, in particular EEG-signals (electroencephalogram signals), wherein the apparatus is provided with a plurality of channels (1) with signal inputs for receiving input signals, which input signals each comprise a specific signal component and a signal component common to all input signals, wherein each channel is provided with an impedance transforming input amplifier (3), wherein the apparatus is configured for supplying to a first input of each input amplifier a respective signal and to a second input an analogue reference signal which is equal for all channels, wherein the apparatus is provided with a digital signal processor (210) and one or more analogue digital converters (5) for supplying the signals provided by the input amplifiers (3) to the digital signal processor (210), wherein the signal processor (210) is designed for converting signals received from the one or more analogue digital converters (5) at least into one or more output signals, wherein the signal processor (210) is designed for processing the signals received from the one or more analogue-digital converters (5), for providing a digital reference signal, wherein the apparatus is provided with a digital-analogue converter (111) for converting the digital reference signal into said analogue reference signal. The invention also provides a system including a number of electrodes and a reference amplifier apparatus.
