An apparatus for extracorporeal blood treatment (1) is described, comprising a treatment unit (3) having at least a first chamber (4) and at least a second chamber (5) separated from one another by a semi-permeable membrane (6) at least a blood removal line (7) connected to an inlet port (4a) of the first chamber (4) and predisposed to remove blood from a patient (P) at least a blood return line (8) connected to an outlet port (4b) from the first chamber (4) and predisposed to return treated blood to the patient (P), wherein the blood removal line (7), the blood return line (8) and the first chamber (4) are part of an extracorporeal blood circuit (2) at least a peristaltic pump (9) operating at the extracorporeal blood circuit (2) for moving the blood in the circuit at least a pressure sensor (13, 14) associated to the extracorporeal blood circuit (2) and configured to enable determining pressure values in the extracorporeal blood circuit (2) at least a fluid drainage line (23) connected to an outlet port of the second chamber (5) a control unit (CU) connected to the at least a pressure sensor (13, 14) and to the peristaltic pump (9) and configured to: move the peristaltic pump (9) with a cyclic motion to generate a blood flow receive from the at least a pressure sensor (13, 14) a signal (Ps) indicating the pressure of the blood in the extracorporeal blood circuit (2). The signal (Ps) indicating the blood pressure is detected with a time domain frequency depending on the velocity (ω) of the peristaltic pump (9), when the peristaltic pump (9) is in a plurality of predefined positions (pp1-ppy). The pressure signal (Ps) is therefore sampled with a constant frequency in the spatial domain (constant at each pump cycle) and independent of the velocity of the pump (9).Linvention concerne un appareil pour un traitement de sang extracorporel (1), comprenant une unité de traitement (3) ayant au moins une première chambre (4) et au moins une seconde chambre (5) séparées lune
