Where N is the binary number from 0 to 15 formed by the digital input. If the analog input is a fixed de reference, the circuit is a conventional 4-bit D to ac signal, in which case the output is the product of the analog signal and the digital signal. The circuit on the left is a programmable attenuator with weights of 0, 1/4, 1/2, or 3/4. The circuit on the right is a noninverting adder, which adds weights to the first output of 0, 1/16, 1/8, or 3/16.
If four quadrant multiplication is required, place a phase selector circuit in series with either the analog input or output. The D0 input of that stage becomes the + or -sign bit of the digital input.