Canon SD940 IS has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon ELPH 140 IS has a 16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4+ processor.
Below you can see the SD940 IS and ELPH 140 IS sensor size comparison.
Canon SD940 IS and Canon ELPH 140 IS have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon SD940 IS has 32% larger pixel area compared to Canon ELPH 140 IS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.