ve, to wary Wher Mr 9 trag Atetnewe tro ener on a fm the snares ein aes terns tenet ty wd Pern ina Sete Se one foe voting EER tr ee arian SS Serabina 9 a4 That the tag REE Seth 8 wot ge ae rene ok Currents ‘ne enti eA sn paistried evwttunmenss ute He mente Ha to on mn fof AMA TIEMTOE a FED OAUTH. aed a eee ae satemna reorrarnis the fuRlacale vahar The tease ene 3 Mm enna 19 ofieet HOM PERO 1 8m than a coor 6 0 that a current of fe ef t9 represent a0 open circum ripe the Hint m eRtAtOn Oe steps eae recetving ve comment the Current om proportional eer Sract can be mppHed to the trpuit of the AD converter pA CONVERTER OPERATION, iNTERFACING, AND APPLICATIONS Se aera foe we tise eTISOTN fo get electrical signals a terres ot eta peri ores eng, rape tony ud ethene ec er Tima ie cy em be fo ah on ee BA cere 1 tthe nae to ii frm feta merce son werk enh, ower ee Ap converters have D/As as part of thetr ctreuitry, we oer DA Converter Operation and Specifications OPERATION ‘the purpose of a digital-to-analog converter Is to convert i bury word to a proportional current or voltage. To tr how this ts done, let’s look at the simple 4-Input adder circuit in Figure 10-14, ‘Since the noninverting Input of the ep amp is ounded, the op amp will work day and night to held fe inverting input also at 0 V. Remember that the ‘eerting input in this circuit ts feferred fo as the suinming point. When one of the switches ts closed, @ current will low from ~ 5 V (Vg through that resistor fo ‘he summing point. The op amp will pull the curren’ Of thtaagh the feedback resistor to produce a proportional feat voltage. If you close switch DO, for example, & Carte of 0165 mA wil flow snto the summands pout: [ Ry 10K. Woe ty fn 'GURE 10.14 Simple 4-bit D/A converter: sane aot ph et Sreite hs putt ehue erorwane etnseagh the ferettoae k & rag of OM HA ate ehoue warttelt ug pouest order ‘ise feeeftancl 4048 the eye an > tnmest pe O8 Venn 4 enyepun. ¥6 seoet anwenen fF mu rte te ait fe Pl the we of th strona theo afar. the og amp taut to output a voltae of Ae ORD TAY ro * Fhe point here that the hemuare wanghted reaietors: proetuce binary weighted carente wiiels are mumimedt by the ops amp to prortuce 4 proportinnal The binary word appiied to the awitches pent Proportional output voltage. Teetinbealty the ont pet vole Age te “digital” because tt can only have certaie: fared values pint as the display on a digital vote However, the output simutates an anim signal, 20 refer to it as analog. Switets DEY en Figure 10-14 repre sents the mont signifleant bit because closing I proxtuces the hargest current, Note that stnee Veer # negative, Ue output will go positive as switches are closed [As you ace here, the heart of a DYA converter is & set of Dinary-weighted current sources which can be reitehed on or aff according to a binary word applied to Its inputs Since these current sources are usually tsicle an IC we dont need to diseuss the different ways the Dinary-weughted currents ean be produced. The op, ap circuit in Figure 10-14 converts the sum of the currents toa proportional voltage. D/A CHARACTERISTICS AND SPECIFICATIONS Figure 10-15 shows the elreuit for an inexpensive IC DA converter with an op-amp circuit as a current:to- Vagy *20.V 06 Wilma son mreouit | jive — Veer el YT ae | us a4 sctsoous oS) ciao ate aso] ALY 4 AS, AO, AS cana See Mags wedhonm {Jodo bobed dete} cov {(B] mv rola MCH08 B-bit D/A with current FIGURE 1095 Motor jowollage converter, INTERFACING AND INDUSTRIAL CONTROL 301 ANALOGVoltage converter. We will use this circult for our discus- Sion of D/A characteristies ‘The first eharacteristte ofa D/A converter 10 4s resolution. This Is determined by the 4m the Input binary word, A converter with 8 binary Inputs, such as te one tn Figure 10-15. Is 2° or 256 Possibte output levels, so tts resolution fs 1 part in 256- As another example, a 12-bit converter has a resolution of 1 part in 24 or 4096. Resolution ts sancti expressed as a percentage. The resolution of an reent or about 0.39 percent yaverter in Figure 10-15, seale output voltage. For the co the current for all the switches is supplied by Veer through R14. The current output from pin 4 of the D/A is pulled through R, to produce the output voltage. The formula for the output voltage is shown under the circuit in Figure 10-15. In the equation the term Al, for example, represents the condition of the switch for that bit. Ifa switch Is closed, allowing a current to flow, put a1 in that bit, Ifa switch 1s open, put a 0 in that bit. ‘As we also show in Figure 10-15, if all the switches are closed, the output will be 10 V x. (255/256) or 9.961 V. Even though the output voltage can never actually get to 10 V, this is referred to as a 10-V output converter. The maximum output voltage of a converter will always have a value 1 least significant bit less than the named value. As another example of this, suppose that you have a 12-bit, 10-V converter. The value of 1 LSB will be (10 VV4096 or 2.44 mV. The highest voltage out of this converter when it is properly adjusted will then be (10.0000 ~ 0.0024) V or 9.9976 V. Several different binary codes, such as straight binary, BCD. and offset binary, are commonly used as inputs to D/A converters, We will show examples of these codes ina later discussion of A/D converters. The accuracy specification for a D/A converter is a comparison between the actual output and the expected output. It is specified as a percentage of the full-scale output voltage or current. If a converter has a full-scale output of 10 V and £0.2 percent accuracy, then the maximum error for any output will be 0.002 x 10.00 V or 20 mv. Ideally the maximum error for a D/A converter should be no more than + the value of the LSB, Another important specification for a D/A converter Is Uneartty. Linearity is a measure of how much the output. ramp deviates from a straight line as the converter is stepped from no switches on to all switches on. Ideally, the deviation of the output from a straight line as the converter is stepped from no switches on to all switches on. Ideally, the deviation of the output from a straight line should be no greater than 1 the value of the LSB to maintain overall accuracy. However, many D/A converters are marketed which have linearity errors greater than that, National Semiconductor, for example, markets the DAC1020, DACI021, DAC1022 series of 10-bit-resolution converters. The linearity specification for the DAC1020 is 0.05 percent, which is appropriate for a 10-bit converter. The DACIO2I has a linearity specification of 0.10 percent, and the DAC1022 hee 2 specification of 0.20 percent. The question that may 302 © cHarTeR TEN ‘occur to you at this point 1s, What good 15 It to have a To-bit converter if the linearity ts only equtvalent to that of an 8 or 9-bit converter? The answer to this question ts that for many applications, the resolution given by 4 10-bit converter 1s needed for small output signals, but It doesn’t matter if the output value 1s somewhe, nonlinear for large signals. The price you pay fora bya converter ts proportional not only to its resolution, but also to its linearity spectfication, Sull another D/A specification to look for ts sett, time. When you change the binary word applied to tng input of a converter, the output will change to the appropriate new value. The output, however, may over shoot the correct value and “ring” for a while before finally settling down to the correct value. The time the output takes to get within +1 LSB of the final value ig called settling time. As an example, the Nationa) DAC1020 10-bit converter has a typical setting time of 500 ns for a full-scale change on the output. This specification 1s important because if a converter ig operated at too high a frequency, it may not have time to settle to one value before it 1s switched to the next D/A Applications and Interfacing to Microcomputers D/A converters have many applications besides those where they are used with a microcomputer. Ina compact. disk audio player, for example, a 14- or 16-bit DiA converter is used to convert the binary data read off the disk by a laser to an analog audio signal. Most speech. synthesizer ICs contain a D/A converter to convert stored binary data for words into analog audio signals. Here, however, we are primarily interested in the use of a D/A converter with a microcomputer. The inputs of the D/A circuit (Al through A8) in Figure 10-15 can be connected directly to a microcom. puter output port. As part of a program, you can produce any desired voltage on the output of the D/A. Here are some Ideas as to what you might use this circult for. As a first example, suppose that you want to bulld a microcomputer-controlled tester which determines the effect of power supply voltage on the output voltage of Some integrated-circuit amplifiers. If you connect the Output of the D/A converter to the reference input of @ Programmable power supply or simply add the high- Current buffer circuit shown in Figure 10-16 to the Cutput of the D/A, you have a power supply which you can vary under program control. To determine the output voltage of the IC under test as. you vary its supply voltage, connect the input of an A/D converter to the IC Sutput, and connect the output of the A/D converter {0 an input port of your microcomputer. You can then read in the value of the output voltage on the IC. ‘other applicatioy buffer for (PPication you might use a D/A and a pow s to vary the voltage supplied to a si Teslative heater under program conte Ao, the Cuamall de motors is proportional to the amount ©! smal S84 through them, so you could connec! ® controle 9 the output of the power bullet Ca to the Din. Peed of the motor with the value you ou'P™ SDV. Note that without feedback control. thes