An apparatus for premixing fluids to form dialysate for an extracorporeal blood treatment includes at least a first feed portion 12 for supplying at least a first fluid from at least a first fluid reservoir, at least a second feed portion 14 for supplying at least a second fluid from at least a second fluid reservoir, a premixing portion 18 configured to premix the fluids supplied via the feed portions (12, 14) in counter flow to form a fluid mixture and at least one drain portion 16 for draining the fluid mixture premixed in the premixing portion as dialysate. A method carries out appropriatesteps. In the conductivity measurement along the drain portion 16, the conductivity of the premixed fluid mixture is measured by means of a digital filter adapted to be parameterized based on at least two state variables of the premixing process at least in two phases with a first predictive phase and a second corrective phase.