AN-275 CMOS Converters Match Most Microprocessors With doubl

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AN-275 CMOS Converters Match Most Microprocessors With doubl

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CMOS Converters Match Most Microprocessors
AN-275
CMOS Converters Match Most Microprocessors
With double buffering, 10-bit multiplying units useful microprocessor control gain attenuation. family complementary multiplying digital-to-analog converters arrived scene promises make microprocessor interfacing truly universal. double-buffered MICRO-DACunits eliminate many common problems, bridging host applications that include microprocessor-controlled gain, attenuation, multiplication. proliferation microprocessor electronic circuits brought with equal proliferation microprocessor-compatible converters. Many these converters, however, have shortcomings that they often require additional external components truly microprocessor-compatible. Furthermore, depending converter's resolution data format, designer sometimes forced adopt additional interfacing circuitry total microprocessor compatibility. Transient output voltage errors occur during updating 10-bit converter from 8-bit microprocessor bus, when words transferred converter. Left-justified (fractional binary) right-justified (positionally weighted binary) converter data formats require different interfacing schemes. these problems must considered interfacing microprocessor unit.
LEVELS BUFFERING MICRO-DAC family multiplying converters consists 10-bit accurate units designed interface directly with 8080, 8048, 8085, Z-80, other popular
National Semiconductor Application Note James Cecil July 1981
microprocessors. converters appear microprocessor memory location input/output port require interfacing logic. Each levels input buffers input latch register (Figure converter's register holds digital data undergoing conversion, while input latch kept busy acquiring input data. digital input data used update converter. double buffering feature allows bits microprocessor data assembled from data bytes. also prevents analog output from changing while digital input word updated. Even when used with 16-bit microprocessors, double buffering feature necessary simultaneous updating many converters. Double buffering establishes proper conditions next test lets system parameters same time. groups MICRO-DAC converters available. DAC1000, DAC1001, DAC1002 24-pin units with 8-bit accuracy levels, respectively. Each contains necessary logic functions interfacing with right-justified left-justified microprocessor data. DAC1006, DAC1007, DAC1008 20-pin units designed left-justified data accuracy levels bits, respectively. members this family multiplying converters feature standard chip select (CS) write (WR) microprocessor control signals. Data microprocessor written into converter standard write cycle.
AN-275
MICRO-DACand BI-FETare trademarks National Semiconductor Corp.
1998 National Semiconductor Corporation
AN008715
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PrintDate=1998/05/01 PrintTime=13:33:04 40115 an008715 Rev.
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AN008715-1
FIGURE Double buffered. MICRO-DAC family 10-bit digital-to-analog converters levels input buffers input latch register. They designed interface with 8080-, 8048, 8085, Z-80, other popular microprocessors, with interfacing logic. HANDLING DIFFERENT DATA FORMATS Different data formats exist many converter products, which must readily handled when interfacing with microprocessor. Left-justified (fractional number VREF) right-justified (positional number VREF/1,024) main ones. Initially, converter manufacturers favored left-justified approach which most significant labeled Newer converters have changed right-justified approach match data format microprocessor data buses. Nevertheless, left-justified approach still widely used. previously mentioned, MICRO-DAC family readily handle left- right-justified data formats with additional interfacing circuitry. When MICRO-DAC converter uses either 8-bit (two write cycles) 16-bit (one write cycle) data bus, locations converter's input latch enabled first write cycle from microprocessor. Depending data format, next write cycle, used, overwrites locations proper data rate. Digital data transferred from input latch register internally three ways: automatically when second write byte occurs; through microprocessor control, which allows updating several converters this necessary; through external strobe. converter's CMOS logic levels made compatible with those special biasing circuit that uses parasitic bipolar transistor available CMOS chip. bipolar transistor supplies base-emitter voltage (VBE) that acts reference converter's digital inputs. supplies input threshold voltage that same amplitude that devices. Details biasing circuit shown Figure Note that reference N-channel field-effect transistor, tied feedback loop have gate voltage biased level VTHN, causing conduct shown drain circuit. three transistors loop voltage VTHN. output emitter-follower, causes loss VBE, thus producing voltage reference VTHN logic input circuits. Each input stages FETs like whose source digital input applied whose geometry same that Like also current feeding drain. When logic input voltage equals VBE, conducts, thereby pulling input standard CMOS inverter level. This threshold continues independent converter's supply voltage. logic threshold voltage standard gates. ACHIEVING HIGH ACCURACY design MICRO-DAC's resistor network simple, even though provides high levels converter accuracy. Figure shows current switching inverted R-2R ladder used, which consists passive components. operation ladder network requires that legs connect ground, level. This means that external operational amplifier shown must have minimal offset voltage. Only offset voltage introduce 0.01% linearity error into converter's operation. Operational amplifiers like National's LM308A series available with offset voltages, they require zero adjustments. When zero adjustment operational amplifier's offset voltage required, resistor temporarily switched between converter's IOUT terminal (which tied amplifier's negative input terminal) ground. balancing resistance should used operational amplifier's grounded positive input terminal, since create errors. operational amplifier, BI-FETLF356 (made with bipolar field-effect transistors), input bias current which makes ideal choice
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current-to-voltage converter. amplifier's offset voltage should adjusted with digital input zeros force IOUT converter zero current level. manually switched-in resistor provides gain about
offset voltage makes zeroing easier sense. converter chip provides feedback resistor good initial matching well tracking over temperature.
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VTHRESHOLD
FIGURE Threshold. This basic logic threshold loop provides biasing MICRO-DAC family converters interface with voltage levels. This circuit uses parasitic bipolar structure, which delivers input threshold biasing circuit. LOOKING INSIDE examination internal details single-pole, double-throw current-mode switches employed converters shows that N-channel FETs' gates driven from converter's supply voltage. contrast supply, level reduces FETs' on-resistances thereby improves converter's performance. MICRO-DAC converters relatively stable gain linearity during variations supply voltage. example, drop supply voltage down results gain error only -0.1%. Even smaller change linearity error same supply voltage swing just -0.005%. usefulness converter determined magnitude linearity errors resulting from changes reference voltage. applications, like multiplication, that require small values reference voltage, small linearity errors essential. case MICRO-DAC converters, reducing reference voltage from results worst-case linearity error change approximately 0.005%.
Figure shows typical application MICRO-DAC unipolar voltage output device. This circuit inverts negative reference voltage positive output, with maximum value 1,023/1,024 reference voltage multiplied VREF. BI-FET operational amplifier used LF356 that slews settles within
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FIGURE Ladder. current-switching, current-mode R-2R resistor ladder MICRO-DAC family converters simple, provides high levels converter accuracy. external operational amplifier chosen minimal offset voltage least converter linearity error.
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AN008715-4
VOUT 9.990
FIGURE Unipolar. typical unipolar application, MICRO-DAC converter inverts negative reference voltage positive one. positive output 1,023/1,024 negative reference voltage multiplied 9.990 VDC. output amplifier slews within Operating MICRO-DAC's R-2R resistor ladder voltage switching mode shown Figure gives faster slewing settling time ladder being used backwards. reference voltage that derived from LM336 reference diode applied IOUT pin. output voltage produced converter's where reference voltage previously located Figure This output voltage ranges from (1,023/1,024) (2.49 VDC). LF356 operational amplifier used supplies gain little more than overall output voltage ranging from less than 9.990 VDC). compensating diodes ends full-scale adjustment potentiometer LM336 reference improve temperature stability reference voltage. bipolar output voltage, circuit Figure used. bipolar output voltage results from adding subtracting reference voltage from converter's output voltage. output operational amplifier ranges from -1,023/1,024 VREF -9.990 VDC). This voltage then applied operational amplifier where gain doubles voltage range. offset voltage output operational amplifier provided adding converter's reference voltage amplifier's input. Resistors circuit operational amplifier must stay matched even during temperature changes circuit Figure order work properly. bipolar converter Figure adjusted first entering digital code composed zeros into converter. Next, offset potentiometer operational amplifier adjusted zero amplifier output voltage then offset potentiometer operational amplifier adjusted amplifier output voltage -10,000 VDC. Finally, digital code applied, potentiometer, series with RfBof converter, adjusted output voltage 9.98 VDC. This voltage VREF LSB, where VREF/512.
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AN008715-5
VOUT 9.990
FIGURE Voltage mode. Operating MICRO-DAC converter's resistor ladder voltage-switching mode provides faster slewing settling time (1.8 than that Figure Note that converter's R-2R ladder being used backwards.
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VOUT 9.98
FIGURE Bipolar. adding subtracting MICRO-DAC converter's reference voltage from output voltage, bipolar output results. this circuit work properly, however, resistors circuit must stay matched during temperature changes.
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AN008715-7
VOUT -VIN/M Where digital input (expressed fractional binary number)
FIGURE Control. MICRO-DAC converter used microprocessor control amplifier circuit. Since converter 4-quadrant multiplication capability, signals handled. feedback resistors referred shown converter. USING MICROPROCESSOR CONTROL MICRO-DAC multiplying converter used microprocessor-controlled amplifier circuit feedback element amplifier (Figure Since converter 4-quadrant multiplication capability, both signals handled. feedback resistor (not shown) internal converter's chip. converter Figure automatically provides output voltage that causes current from converter's IOUT terminal VREF terminal equal input current, VINRfB. Note that when microprocessor provides data converter with relatively large value reference voltage needed balance input current. This value corresponds maximum gain -1,024. minimum gain -1,024/1,023 obtained converter digital input all, 1,023 gain steps provided. addition another amplifier converter's IOUT produces microprocessor-controlled amplifier attenuator. Compared with gain circuit that appears Figure gain here noninverted ranges from 1,022. POINT BEST-STRAIGHT-LINE maximize their product yields, manufacturers digital-to-analog converters like best-straight-line linearity guarantee. Unfortunately, this method based iteration zero full-scale converter adjustments, that errors optimally split equidistant from straight line. converter user, best-straight-line specification means that converter must undergo sophisticated adjustment procedure linearity proven. Furthermore, each converter different best-straight-line fit, making necessary adjust every them individually. Another specify converter linearity end-point method. current output converter, offset voltage current-to-voltage output amplifier first adjusted output. Then converter adjusted with full-scale input digital code produce full-scale output voltage. This simple technique ensures that each 10-bit unit's 1,024 steps within stated linearity specification. Further, pretrimmed output amplifier used eliminate zero offset adjustment, leaving only full-scale adjustment. differences between best-straight-line end-point specification techniques shown illustration (below), where converter with error least significant shown failing end-point linearity test. Note that readjusting converter's full-scale output, converter's error optimally split either side ideal line best-straight-line fit, which time-consuming procedure, particularly when done large number individual converters. many application where converter already mounted printed circuit board, end-point adjustment zero full-scale much less time-consuming. Furthermore, this end-point procedure more stringent guarantee converter linearity than best-straight-line approach. end-point method used converters MICRODAC family.
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AN008715-8
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PrintDate=1998/05/01 PrintTime=13:33:05 40115 an008715 Rev.
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CMOS Converters Match Most Microprocessors
LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT NATIONAL SEMICONDUCTOR CORPORATION. used herein: critical component component life support Life support devices systems devices sysdevice system whose failure perform reatems which, intended surgical implant into sonably expected cause failure life support body, support sustain life, whose faildevice system, affect safety effectiveness. perform when properly used accordance with instructions provided labeling, reasonably expected result significant injury user.
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AN-275
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National does assume responsibility circuitry described, circuit patent licenses implied National reserves right time without notice change said circuitry specifications.
PrintDate=1998/05/01 PrintTime=13:33:05 40115 an008715 Rev.
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AN-275 CMOS Converters Match Most Microprocessors With doubl

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