CHAPTER 8. DATA ACQUISITION AND PROCESS CONTROL1618.4Digital-to-Analog ConversionThe process of converting a number held in a digital register to an analog voltage or currentis accomplished with a digital-to-analog converter (DAC). The DAC is a useful interfacebetween a computer and an output transducer.8.4.1Current Summing and IC DevicesDACs are normally switched current devices designed to drive the current-summing junctionof an operational ampliﬁer. Figure 8.2 shows a simple 4-bit DAC. VrefIm = ---- 2RIm/8Im/4Im/2Im16R8R4R2RMSBDCBAIRIrefVrefVoutFigure 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 im-prove the circuit by replacing it with a circuit that requires fewer distinct resistor values.8.4.2DAC LimitationsThe output of a DAC can only assume discrete values. The relationship between the inputbinary number and the analog output of a perfect DAC is shown in ﬁgure 8.3.Common DAC limitations are an anomalous step size between adjacent binary numbers,non-monotonic behaviour, or a zero output.8.5Analog-to-Digital ConversionThe analog-to-digital converter (ADC) is used to convert an analog voltage to a digitalnumber (ﬁgure 8.4).