DACs are normally switched current devices designed to drive the current-summing junction of an operational amplifier. Figure 8.2 shows a simple 4-bit DAC.
Figure 8.2: The current DAC uses the summing input to an
op-amp to yield a voltage output. LSB and MSB refer to the least and
most significant bits of a binary number.
Each current is proportional to the value of a bit position in a binary number.
The design requires a number of different precisely defined resistor values. We can improve the circuit by replacing it with a circuit that requires fewer distinct resistor values.