A memory read-out approach for a 0.5 μm CMOS image sensor

In image sensors with passive pixels the column capacitance is large compared to the capacitance of the pixel. The charge-to-voltage conversion occurs in the column amplifier relatively far from the pixel. This may result in a high sensitivity to interference, especially in case other electronic circuitry is located on the same chip. Two types of CIF CMOS imagers are presented that use different read-out options to counter this effect. Both designs use differential read-out as DRAM's do. This means that the pixel is compared to a reference cell. The first type uses a reference cell on the same row; the second type utilizes a fully symmetrical way of read-out, similar to digital memories by having its reference on the same column. Furthermore, two other means of image quality improvement are applied. A boost circuit is used to generate a negative voltage for driving the selecting transistor to insure that it is completely switched on during pixel reset. By this, threshold differences between pixels do not affect the reset voltage. The second is a well thought-out column amplifier that calibrates its offset before reading the pixel information.