An analog to digital converter apparatus arranged to convert a plurality C (= 4) of different input signals to digital representations therof. The apparatus comprises a multiplexer arranged sequentially to couple each of the 4 input signals to a fast a-to-d converter (such as a successive approximations converter) at a rate such that each input signal is coupled N (= 8) times in a period M (= 20m secs) of an a.c. series mode interfacing signal (at 50 Hz) and at equal time intervals M/N (20/8 = 2.5m secs). The N (= 8) digital values of each input signal are averaged to provide at the end of a conversion period a digital value for each of the 4 input signals (C) which is the mean magnitude of the signal during the 20m sec period (M).