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Image processing applications like tracking of moving objects typically involve the subtraction of two successive images. Doing this without the need for a large digital memory requires a sensor chip that does the subtraction in the analog domain. This paper provides insight in the critical issues of the design of such a differential image sensor. The presented sensor has two operation modes: a very low noise correlated double sampling (CDS) mode and a differential image (DI) mode. This sensor is optimized to have a high common mode rejection ratio (CMRR) which results in a small influence from the content of the image onto the differential image.