Schmitt trigger belongs to a class of bistable multivibrator circuits. In a bistable, there exist two D.C. couplings from each output to input of the other. But in Schmitt trigger circuit, there exists only one coupling. It can be recalled that if in the emitter coupled bistable the feedback network from the collector of transistor Q2 to the base of transistor Q1 is removed , it becomes a Schmitt trigger circuit.
Digital to analog converter is used to convert digital quantity into analog quantity. DAC converter produces an output current of voltage proportional to digital quantity (binary word) applied to its input.