A signal processing circuit and processing method are provided for measuring an analog signal from a photo-detector. Generally, the method includes steps of: (i) sampling and storing a characteristic of the signal at a first predetermined time following a reset of the circuit; (ii) sampling the characteristic of the signal at a second predetermined time following a reset or initialization of the circuit; (iii) determining a difference between the stored characteristic of the signal sampled at the first predetermined time and the characteristic of the signal sampled at the second predetermined time; and (iv) converting the determined difference to a digital value and determining a slope of the signal from the digital value and the difference between the first and second predetermined times. Thus, the measurement of the slope is independent of and substantially unaffected by absolute values of the characteristics measured at the first and second predetermined times.