A ramp ADC consists of a comparator, a ramp input and a code counter. A limitation in ramp ADCs is conversion time (The time it takes to ramp/count all codes).
/rampADC.png)
Basic digital ramp ADC concept: A ramp voltage is compared with an analog input and a counter measures the time until the two signals cross. That counter value is then the digital equivalent of the analog input voltage.
Pixel noise follows a root curve, i.e. is more significant for darker pixels. Ideally, an ADC needs finer resolution for low codes. Here this can be achieved by introducing an accelerated (non-linear) ramp, resulting in finer resolution for low codes and a higher dynamic range.
Project assignment: Design and develop a non-linear ramp generator for per-column ramp ADCs in image sensors.