$1.$ Design an OpAmp interface circuit whose input connects to the output of a $4-$input R$-2$R ladder network, so that each increment in the binary number produces $-1$ V increase $($or decrease$)$in$$output$$voltage$$VDAC$,$e$.$g$.$a$(0001)2$ gives$$a$-1\mathrm{.}0$V$$output$,$a$(0010)2$ gives$$a$-2\mathrm{.}0$ V output. Assume that the input voltages can be either $0$V $($digital $0)$ or $4$ V $($digital $1).$ Give the circuit and the calculations to find the resistor values.