A device whose output data are a representation of the sum of the numbers represented by its input data. Note: An adder may be serial or parallel, digital or analog. 2. A device whose output data are a representation of the sum of the quantities represented by its input data. Note: An adder can add things other than representations of numbers. It can add voltages, etc. analogue adders are not limited to summing representations of numbers. An adder may operate on digital or analogue data.