voltage divider 9 of fig1 the voltage divider 9 comprises a feedback resistor 14 having constant resistance r 1 and a programmable resistor 16 having variable resistance r 2 the programmable resistor 16 comprises a fixed resistor 31 0 having resistance r 0 and a plurality of additional resistors 31 1 31 2 31 n having equal resistance r e and disposed reciprocally in series between the output terminal 2 b and the fixed resistor 31 0 intermediate nodes 32 0 32 1 32 n u2212 1 connected to the output terminal 2 b by means of respective selection switches 36 0 36 1 36 n u2212 1 are present between pairs of adjacent resistors 31 0 31 1 31 2 31 n furthermore a selection switch 36 n is interposed between the additional resistor 31 n and the output terminal 2 b each selection switch 36 0 36 1 36 n is controlled by a respective closure signal s 0 s 1 s n where n is equal to the number of additional resistors 31 1 31 2 31 n which are present the closure signals s 0 s 1 s n are generated by a control unit 18 so as to control the closure of one only of the selection switches 36 0 36 1 36 n at a time keeping all the other switches open when for example the selection switch 36 j controlled by the closure signal s j j being a number between 0 and n inclusive is closed the corresponding intermediate node 32 j is connected directly to the output terminal 2 b consequently the fixed resistor 31 0 and j additional resistors 31 1 31 2 31 j are connected in series between the feedback node 6 and.