An analog-to-digital converter is a device which converts continuous signals to discrete digital numbers. Typically, an ADC is an electronic device that converts an input analog voltage (or current) to a digital number proportional to the magnitude of the voltage or current. This circuit demonstrates the interfacing of ADC0808 with 8051 microcontroller (AT89C51). The output is taken on the LEDs. This is an intermediate circuit, which finds lot of applications. ADC0808 needs an external clock to run. The circuit describes how a D-flip flop can be used to provide the external clock.