Analog-to-digital converter with sigma-delta duty cycle encoded output

Abstract

An analog-to-digital converter employs a sigma-delta modulator that produces a pulsed output with an average duty cycle that varies with an analog input to the modulator. A counter averages the modulator's output duty cycle, and feeds the averaged information onto a one-line transmission circuit for delivery to a remote location. Both the sigma-delta modulator and the counter are synchronized to the same local clock, so that the counter's duty cycle remains substantially constant even if the clock frequency varies. This allows for a simple clock configuration that can be integrated along with the sigma-delta modulator and counter on a single IC chip of reasonable size. When used for temperature sensing, the counter's output duty cycle provides an indication of the locally sensed temperature, independent of temperature-induced variations in the clock frequency.