An image sensor having an array of pixel cells, each including a photo-conversion device. The array has first, second, and third groups of pixel cells. The first group of pixel cells receives light and the second and third groups are shielded from light. Each pixel cell of the second group is configured to output a black reference signal for determining a black level of the array. Each pixel cell of the third group has at least one first transistor coupled to the photo-conversion device, and each transistor coupled to the photo-conversion device has a gate coupled to a power supply voltage.