IC Applications Laboratory Experiment no. 27 VERIFICATION OF 4 BIT DAC USING OP AMP AIM: To design 4- bit Di al to Analog Converter for required step size APPARATUS: 1. Op-amp Ic741 INo, 2. Resistors(R) 10KQ 4No. 2R 20K 2 6No, 3. Potentiometer 1No, 4, Multimeter INo CIRCUIT DIAGRAM: xR R : , ote y, Fig. 7.1 A 4 — Bit DAC using R-2R Ladder network THEORY: A Digital-to-Analog Converter is used when a Binary output from a digital system must be converted to some equivalent Analog voltage or Current, The Binary output from a digital system is difficult to interpret. However a DAC makes. the interpretation easier. The function of DAC is exactly opposite to that of ADC. ‘Advantages: 1) tis simpler in construction when compared to ADC 2) It can be used to form the ADC Types of D/A Converters ‘There are two types of DACS available 1. D/A converter with Binary -weighted Resistors 2. D/A converter with R and 2R resistors. ‘MVGR College Of Engineering(A) WOPECE 48IC Ipplications Laboratory ites A D/A converter in its simplest form uses an op-anp and either binary weighted resistors or Rand 2R resistors, DIA converter with Binary -weighted Resistors The word D and hhematie of a DAC is shown in Fig.7.2, The input is an n-bit bit combined with a reference voltage Ve to give an analog output signal, The output ‘ofa DAC can be either a voltage or current, Theory o/p voltage Vo=R Vrs(bi2"'b *b,2") «I Where Vu output voltage Vis = fll sea'e output voltage K = scaling factor usually adjusted to unity SSieabs enti binary fractional word with the decimal point located at the left ‘st significant bit (MSB) with a weight of Ves/2 least significant bit (LSB) with a weight of Vis/2" + he by, (MSB) | Lo Bray J yom] pac | Word B ure = b (CSB) Fig.7.2 Schematic ofa DAC OPERATION: D/A converter with R and 2R Fig7.1 shows D/A converter with R and 2R resistors. As before, the binary inputs are simulated by switches b0 through b3 and the output is proportional to the binary inputs. Binary inputs can be in either the high (+5y) or low (Ov) state. Assume that the smost significent bit (MSB) switch b3 is connected to +SV and other switches are connected to ground, as in Fig.7.1 Thevenizing the circuit tothe left of switch b3 .Thevenin’s equivalent resistance Rn is Riv = [{{QRI] 2R +R) |] 2R}ER} |] AER 2R= 20K0 MVGR College Of Engineering(A) Deparin 49JC Applications Laborat The resu {[ 2. Fig.8.3 Equivalent Circuit when bs is high and bo,br In this Fig, the (-) input is at virtual ground (V2 20V) ; therefore . the current through ou (= 2R) is zero However , a h 2R connected to +5V is Rru(= 2R) However , the current through 2R connected t ome 0.25mA .The same current flows through Rr and in tum produces the output voltage Vo = -(20K) (0.25mA) = - 5V Using the same analysis, the output voltage corresponding to all possible combinations of binary inputs can be calculated. The maximum or full -scale output of 9.375 V is obtained when all the inputs are high The output voltage equation can be written as Where each of the inputs b3, b2, Bl and 60 may be either high(#5V) or low (OV) ‘The great advantage of the D/A Converter of Fig8.2 is that it requires only two sets of precision resistance values; nevertheless, it requires more resistors and is also ‘more difficult to analyze than the binary -weighted resistor type. [As the numberof binary inputs is increased beyond four, both DYA converter circuit get complex and their accuracy degenerates. Therefore, in critical applications an integrated circuit specially designed as D/A converter should be used PROCEDURE: 1 me Ut as Noon iv dv Conner tHe ¢ q (eorett the Ue “ba, 1, bs» 03 wean ‘MVGR College Of Engineering(A) Department Of ECE 501C Applications Laboratory hak mene (Oke Moor “ak qeond 46v Cornbinahe rs op he Me fiom cow ly mr 7 fot lh mr 2 obsee the Ap qo omp TE OBSERVATIONS: J written as OUTPUT EQUATION: The output voltage equation can bs =r (23 2 By HO) 7-100 oe Vo="Re opt aR BR TOR (a. =e Where each of the inputs b3, b2, bl and 'bO may be either high(+SV) or low (ov) 'S. | Digital | Analog Output | No | Inputs || bz | br [bo | Theoretical | Practical ofojo| 0 =0-0! aaa ofo0/1|-0 @s¢ [=p.43 o1 3 5 re O[1f0l-poy 0 T ofiji aw 1lolo|-28 10/1 [-3. bs TT fo lee t¢ Ti To. aa 0/0}0) -50 olo}1|-~ 56> oft fol = 49s “3275 opr] géas mma te tfofof. ai Output Versus Inputs TifcOr itfe=" geting 1[1]0| ¢-4as | ~6.99 Tyr [5-3 |= 44 5 3, b Lb, ) ; Voz Rs) P32 Bes Bey be CALCULATIONS: _ US ye sr ce gs Be % Be {} >» 0 MD O : i) o Oo Bg 8, 8, B an) 10 0 oso o Oo fl 10 0! Oo I 00 1 Oo 10 ve VGR College Of Engineering(A ee ieee PoP ng sr prova o2e% 110 poy “0 aril | eo | | | °