1. A signal processing apparatus for converting an inputted analog signal to a digital signal and deriving sum of products, comprising: a circuit for comparing said inputted analog signal with each voltage of a group of analog reference voltages and generating a thermometer code depending upon said inputted analog signal; a plurality of memory circuits having, as word selection lines, signal lines indicating a change point of said thermometer code; and means, including a plurality of arithmetic elements, for inputting output signals from the plurality of memory circuits into the plurality of arithmetic elements, respectively, to execute filter arithmetic processing.