A device for detecting electric potentials includes a plurality of measuring inputs (9) for connecting to measuring electrodes (11), which can be placed on the body of a patient (3), a plurality of measuring amplifiers (Op1, . . . , OpN), and a potential output (27) for connecting to an additional electrode (31), which can be placed on the body of the patient (3), to which a preset voltage can be applied. A summing unit (17) sends a signal, which is an indicator of the mean value of the signals sent by the measuring amplifiers (Op1, . . . , OpN). A current-measuring device (29) sends a current signal, which is proportional to the current flowing through the potential output. An analyzing unit (35) is connected to receive a potential output voltage signal, the summing unit output (19) signal and the current-measuring device signal. The analyzing unit is configured to generate an impedance signal from the fed signals.