COMPUTING

Chapter 5 Operational Amplifiersrequires a large number of precision resistors. The DAC of Figure 5.83, known as R−2R laddernetwork, requires more resistors, but only two sets of precision resistance values, R and 2R.2RR R R2R 2R 2R 2RV analogV A V B V C V DFigure 5.83. Digital−to−analog converter using the R−2R ladder networkWe can prove that the equivalent analog voltagethe relationV analogVV A + 2V B + 4V C + 8V D + …analog= -----------------------------------------------------------------------------shown in Figure 5.83 is obtained from(5.72)where n is the number of digital inputs. The proof is left as an exercise at the end of this chapter.Figure 5.84 shows a four−bit R−2R ladder network and an op−amp connected to form a DAC.The op amp shown is an inverting amplifier and in this case the reference voltage should benegative so that the amplifier output will be positive. Alternately, a non−inverting op amp couldbe used with a positive value of .V ref2 nV ref2RRRRR f2R2R2R 2R V analogV refFigure 5.84. Typical R−2R DAC circuitExample 5.22Figure 5.85 shows a four−bit DAC where all four switches are set at the ground level. Find theanalog voltage value at the output of the unity gain amplifier for each of the sets of the switchpositions shown in Table 5.2. Fill−in the right−most column with your answers.5−54Electronic Devices and Amplifier Circuits with MATLAB Computing, Second EditionCopyright © Orchard Publications