On-chip design-for-testing structure for CMOS APS (active pixel sensor) image sensor

A method for verifying operation of a group of at least one active pixel sensor within an array of active pixel sensors and of a chain of circuitry connected to each active pixel sensor for capturing an output signal from said active pixel sensor, whereby said chain of circuitry includes a column bus circuit and a signal conditioning and readout circuit, and whereby said method comprises the step of:

• testing functionality of said group of active pixel sensors and the chain of circuitry connected to each active pixel sensor of the group of active pixel sensors by the steps of:
• activating said group of active pixel sensors,
• applying one of a group of voltage levels that vary incrementally from a first voltage level to charge a capacitance of a photo-conversion device of each active pixel sensor of the group of active pixel sensors to a first charging voltage level and sampling and holding said first charging voltage level from the capacitance of the photo-conversion device of each active pixel sensor of the group of active pixel sensors,
• applying the first voltage level to charge the capacitance of the photo conversion device of each active pixel sensor of the group of active pixel sensors to a second charging voltage level,
• sampling and holding a second charging voltage level from said capacitance of the photo-conversion device of each active pixel sensor of the group of active pixel sensors,
• differentially comparing the first charging voltage level with the second charging voltage level to create a first difference voltage, whereby said first difference voltage indicates the functionality of each active pixel sensor of the group of active pixel sensors and the chain of circuitry connected to said active pixel sensor.