Analog-to-digital converting device for increasing the number of analog

Abstract

An analog-to-digital (A/D) converting device which substantially increases the number of input channels without increasing the number of input terminals. When an external channel selecting circuit is asserted 'low', switches are selected to deliver a 3-bit control word to a decode circuit. The decode circuit delivers a corresponding analog signal to a multiplexer in accordance with the control word. An A/D converter converts the analog signal to a digital signal. When the external channel selecting circuit is asserted 'high', switches deliver a 3-bit channel control word to an output terminal. Simultaneously, the 'high' signal from the external channel selecting circuit is applied to an OR gate, which in turn is applied to a multiplexer. An externally inputted analog signal is also applied to the multiplexer. The inputted analog signal is applied to an A/D converter, via the multiplexer, and the analog signal is converted to a digital signal. Therefore, since the device can process an analog signal input through an external channel input terminal selected by the channel control word and then the predetermined analog input terminal, it makes it possible to substantially increase the number of analog input channels without increasing the number of the built-in analog input terminals.