Embodiments of an image sensor including a pixel array with a plurality of pixels arranged into rows and columns. Control circuitry coupled to the pixels in each row, and an analog-to-digital converter is coupled to the pixels in each column of the pixel array. Each analog-to-digital converter includes a ramp comparator, and a variable current source coupled to the ramp comparator to provide a variable bias current to the ramp comparator. The bias current can adjusted during reading of a row of pixels according to a dynamic bias current profile that maintains at least a specified margin between the random noise of the pixels and an acceptable noise level. Other embodiments are disclosed and claimed.