Disclosed is a sensor apparatus comprising a plurality of pixels, a digital to analog converter for providing a ramp signal, A comparator for comparing the output level of each pixel to said ramp signal, and memory for storing the digital value that corresponds to said output level for each pixel, the sensor apparatus thereby converting the analog output level of each pixel to a digital value. The apparatus operates by providing an analog output that is sourced from the digital to analog converter used to provide said ramp signal.