D/A conversion is the process of converting a value represented in digital code such as binary or BCD into a voltage or current which is proportional to the digital value. Fig represents the symbol of a typical 4 bit D/a converter. Each of the digital inputs A,B,C and D can assume a value 0 or 1, therefore there are 24 = 16 possible combination of inputs. For each input number, D/A converter outputs a unique value of voltage. The analog output voltage Vout is proportional to the input binary number. So, Analog output = ( K *digital input) where K is proportionality factor and is constant for a given DAC.
Figure above represents a DAC using op-amp. The summing amplifier has four input resistances corresponding to four binary inputs D0,D1,D2 and D3.Switches are provided with each input. If a switch is open ,it indicates ‘0’And for closed switch it indicates ‘1’.
The output of the 4 bit DAC is : Iin = (Vref/R)(D3+2-1D2+2-2D1+2-3D0.
So the total voltage available at the output of the op-amp is the total of input voltage levels which represents the equivalent analog signals of the 4-bit digital input.