An analog to digital converter by using an exponential-logarithmic model includes an exponential circuit which acquires an analog input voltage and generates an analog output voltage that is an exponential function of the input voltage. A positive feedback circuit that succeeds the exponential circuit exhibits a natural logarithmic characteristic. A comparator is connected to the positive feedback circuit to compare an output voltage of the positive feedback circuit with a reference voltage. Via the exponential-logarithmic conversion technique, the time interval or pulse produced by the positive feedback circuit is a linear function of the magnitude of the input voltage. Based on the comparator output, a counter is employed to translate the analog input signal to its digital representation.