Digital Electronics: Principles, Devices and Applications

482 Digital Electronics(Angle O/P)R R R2R2R2R 2R 2RV refMSB LSBOut-1-Out-2+(Digital Ground)Figure 12.9Voltage switching mode of operation of a D/A converter.voltage switching mode of operation. The output voltage is the product of the fractional binary valueof the digital input word and the reference voltage applied at the Out-1 terminal, i.e. DV ref As thepositive reference voltage produces a positive analogue output voltage, the voltage switching mode ofoperation is possible with a single supply. As the circuit produces analogue output voltage, it obviatesthe need for an opamp and the feedback resistor. However, the reference voltage applied to the Out-1terminal in this case will see different input impedances for different digital inputs. For this reason,the source of the input is buffered.12.5 BCD-Input D/A ConverterA BCD-input D/A converter accepts the BCD equivalent of decimal digits at its input. A two-digit BCDD/A converter for instance is an eight-bit D/A converter. Figure 12.10 shows the circuit representationof an eight-bit BCD-type D/A converter. Such a converter has 99 steps and accepts decimal digits00 to 99 at its input. A 12-bit converter will have 999 steps. The weight of the different bits in theleast significant digit (LSD) will be 1 (for A 0 , 2 (for B 0 , 4 (for C 0 and 8 (for D 0 The weights ofthe corresponding bits in the next higher digit will be 10 times the weights of corresponding bits inthe lower adjacent digit. For the D/A converter shown in Fig. 12.10 the weight of the different bitsin the most significant digit (MSD) will be 10 (for A 1 , 20 (for B 1 , 40 (for C 1 and 80 (for D 1 Ingeneral, an n-bit D/A converter of the BCD input type will have (10 n/4 − 1) steps. The percentageresolution of such a converter is given by [1/(10 n/4 − 1)] × 100.