Current steering ramp compensation scheme and digital circuit implementation

An imaging system, comprising:

• a pixel array including a plurality of pixel circuits configured to generate image signals in response to incident light;
• a control circuit coupled to the pixel array to control operation of the pixel array;
• and
• a readout circuit coupled to the pixel array to read out the image signals from the pixel array through a plurality of bitlines, wherein the readout circuit includes an analog to digital converter (ADC) including a comparator circuit coupled to the plurality of bitlines to receive the image signals, wherein the comparator circuit is further coupled to receive a ramp signal from a ramp generator, wherein the ramp generator comprises:
• a plurality of switched current sources coupled in parallel between a resistor and ground, wherein the ramp signal is configured to be generated at an output coupled to a node between the resistor and the plurality of switched current sources;
• and
• a digital ramp control signal generator comprising a counter configured to generate the ramp control signal in response to a clock signal, wherein the ramp control signal comprises a plurality of bits, wherein each bit of the ramp control signal is coupled to control switching of a respective one of the plurality of switched current sources to generate the ramp signal,
• wherein the digital ramp control signal generator is coupled to receive a reset signal to zero the ramp control signal, wherein the digital ramp control signal generator is further coupled to receive a set bits signal to initialize the ramp signal to a preset value after every reset of the ramp control signal to add a DC offset compensation current determined by the preset value to the ramp signal.