Successive two-trip traversals of charges between gates G.sub.0 and G.sub.2 make it possible to obtain beneath gates G.sub.1 and G.sub.2 quantities of charges equal to Q.sub.R, Q.sub.R /2, Q.sub.R /2.sup.2 . . . Q.sub.R /2.sup.i. A readout device for reading charges and connected to gates G.sub.2 and G.sub.4 generates voltages V.sub.R and V.sub.Ri =a.sub.0 .multidot.V.sub.R +a.sub.1 .multidot.V.sub.R /2+ . . . +a.sub.i-1 .multidot.V.sub.R /2.sup.i-1 +V.sub.R /2.sup.i which are compared with a voltage sample V.sub.x to be coded in order to determine by successive approximations the coefficients a.sub.0 . . . a.sub.n which are equal to 0 or to 1 such that V.sub.x =a.sub.0 .multidot.V.sub.R +a.sub.1 .multidot.V.sub.R /2+ . . . +a.sub.n .multidot.V.sub.R /2.sup.n. Depending on the value of a.sub.i, each quantity of charges Q.sub.R /2.sup.i stored beneath gate G.sub.1 is removed beneath diode D.sub.e or stored beneath gate G.sub.3 and then transferred beneath gate G.sub.4.