CMOS Converter Interfaces Easily with Many Microprocessors With s

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CMOS Converter Interfaces Easily with Many Microprocessors With s

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CMOS Converter Interfaces Easily with Many Microprocessors
CMOS Converter Interfaces Easily with Many Microprocessors
With span accommodation down this 8-bit unit also replace 12-bit analog-to-digital device some applications help meet rising demand easier interfacing between analog-to-digital converters microprocessors complementary 8-bit ADC0801-05 been designed accommodate almost today's popular microprocessors requires only single supply power boot Housed 20-pin dual-in-line package successive approximation device includes Schmitt trigger circuit that allows driven from system clock well external network clock frequency conversion time What's more guaranteed linearity error least significant (typically LSB) encode analog signal span small performance that allows replace even 12-bit converters many applications Constantly decreasing converter prices raise comparative cost interface electronics increase demand simplicity interfacing growing emphasis simpler systems higher levels reliability also pushed this demand trend toward lower levels power dissipation with success power supply standard logic circuits linear circuits have been pressed operation Supporting ADC0801-05 converter such special operational amplifiers LM358 dual LM324 LM3900 quad amps that supplies also useful voltage comparators such LM393 dual LM339 quad devices Perhaps most versatile such linear devices LM392 comprising comparator
MORE COMPLICATIONS Complicating interfacing ever higher levels resolution monolithic converters with 10-bit types readily available 12-bit devices ready emerge soon despite their greater resolution 12-bit monolithic converters only more expensive than 8-bit designs also require more careful attention system noise problems management grounding simple interfacing converter must operate directly with signals available microprocessor control converter generally given address that mapped into memory input output space depending type microprocessor employed 6800 microprocessors their derivatives special input output addressing strobes available converter must appear memory location these processors microprocessors other hand only provide special interfacing also automatically insert wait state dur-
National Semiconductor Application Note July 1981
selection increase width read write strobe signals This eases interface requirements considerably since slower devices operate with much faster microprocessor units automatic wait state devices will loom larger importance next generation higher speed microprocessors evolves COMPATIBILITY CRITERIA DIFFER Microprocessor compatibility wide range meanings least according various converter data sheets True compatibility however involves meeting electrical specifications like proper logic voltage levels with adequate loading capability example true compatibility means ability maintain potential less) converter logic outputs while sinking current high state must maintained minimum while supplying least Furthermore interface protocols must This only means operating with proper signals also meeting necessary timing requirements converter must have valid data microprocessor within access time memory system with which happens working protocols interfacing standardized Some converters make standard chip select signal (CS) start conversion decoding voltage glitches cause converter begin conversion when desirable Both standard signal write strobe signal (WR) must therefore used that former signal qualifies latter prevents unwanted conversions address decoding glitches Care must also taken when using some converters that designed controllers problems arise when central processor control DIFFERENT STANDARDS 8080 6800 microprocessors (and their derivatives) different control standards Microprocessors based 8080 example make read write strobe signals specify operation (read write) requested Working with these microprocessors converters start conversion cycle upon microprocessor's issuance chip select signal (decoded from address bus) write strobe signal conversion (EOC) converter issues signal When dealing with older converters where signal typically during conversion process high microprocessors have difficulty because signal available data Furthermore signal does reset when converter serviced central processing unit (that when data been read)
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registered trademark Zilog Corp
8721 RRD-B30M115 Printed
C1995 National Semiconductor Corporation
Complications also occur when microprocessors interface with older devices during read operations proper interfacing such converters must have valid data within memory access time Interfacing requirements differ 6800-type microprocessors like 6502 68000 which read write control lines instead read write strobe signals obtain timing information from system clock signal addition they include valid memory address signal qualify address that placed Such features make interfacing these microprocessors different from that earlier 8080 types converter most useful microprocessor-based system must have such desirable analog features differential inputs should adjust accommodate various analog input signal ranges ADC0801-05 offers differential analog inputs converter's span accommodation that allows many unusual useful analog applications availability differential analog voltage inputs eliminates problem poor analog grounds since both inputs connected directly across analog signal source negative (normally grounded) analog input lead referenced desired offset voltage accommodate input signal range that does swing down ground offset thus used this input cause digital output desired input voltage FLEXIBLE SPAN Finally ability accommodate arbitrary span input dynamic voltage range desirable converter This easily achieved ADC0801-05 selecting magnitude converter's reference input example might permit analog input voltage range This accomplished tying
converter's negative input lead offset voltage supplying reference voltage that equal half span This application provides output code output code many applications (such weighing cans production line) even 16-bit converters often called upon needed high levels resolution those reduced-span applications 8-bit converter used instead considerble savings SAMPLED-DATA INPUT ADC0801-05 makes sampled-data comparator Sampled-data circuits cancel offset voltage provide essentially temperature-independent performance cancel frequency noise They however provide some differences application since there input stray capacitance ground shown Figure When switch closed stray input capacitance charged input analog potential VANALOG Note that with stray capacitance approximately switch resistance time constant only Thus becomes charged necessary accuracy level (within LSB) about Since input switches operating eighth input clock frequency there ample time settle comparisons made only clock period Note that switch analog input discharges stray capacitance this event causes input displacement currents flow Input bypass capacitors when placed directly analog inputs cause full-scale errors since they average current which will flow through source resistance analog input signal generator Input capacitors required they used full-scale adjustment will eliminate system errors
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FIGURE Equivalent Because sampled-data comparator input 8-bit ADC0801-05 monolithic analog-to-digital converter looks capacitive input signal source sampling switches operate eighth rate clock frequency
ADC0801-05 monolithic 8-bit CMOS converter operated with wide range VREF voltages that facilitates many different circuit applications Inexpensive ratiometric transducers such potentiometers tied across converter's supply voltage with wiper directly converter's input VREF which will bias tied second potentiometer that also hooked across supply voltage provide full-scale adjustment When VREF grounded converter then functions comparator yielding digital output when greater than VINb when less than VINb VREF feature also useful level analog voltage systems where operational amplifier normally used boost input signal prior digitization circuit with analog input voltage maximum example signal directly device saving cost amplifier VREF would thus biased CAREFUL GROUNDING minor drawback that this extra analog resolution leaves circuit more susceptible noise VREF voltage requires initial tolerance must stable
over temperature changes Grounding problems become more critical careful grounding must ADC0801-05 also used logarithmic converter extend input voltage dynamic range cover three decades Three input logging circuits (Figure provided transistors feedback loops operational amplifiers With these same temperature (all three common chip) there thermal problems with this circuit keep costs their lowest three transistors LM389 audio amplifier used fourth operational amplifier Figure used supply proper VREF voltage converter output voltage half that logarithmically compressed analog input voltage span OFFSET ADJUSTING another application ADC0801-05 automatically adjusting offset voltage under microprocessor control This useful transducer bridge networks where pair amplifiers normally used amplify differential signal Such output signal directly converter's inputs without requiring more costly instrumentation amplifier bridge network's arms will thus biased approximately
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FIGURE Logarithmic ADC0801-05 monolithic converter's VREF allows three-decade logarithmic circuit three transistors feedback loops operational amplifiers give better accuracy with changing temperature than diodes normally used
CMOS Converter Interfaces Easily with Many Microprocessors
Figure shows such circuit where microprocessor takes digital output device automatically adjusts output voltage operational amplifier This amplifier used isolate bridge network from offset adjustment circuit INS8255 programmable peripheral interface controls offset voltage adjustment analog switches CMOS buffer provides ideal analog level swings either binary resistor network binary resistor network extracts injects current from into causing small voltage drop across This corrects offset voltage that introduced anywhere system
AUTO ADJUSTMENT Electrically actuated switches allow automatic adjustment offset voltage should noted that
referenced side bridge network order cancel common-mode offset voltage effects converter acts high gain comparator because VREF provided voltage follower (amplifier switch circuits This allows microprocessor perform successive approximation routine null offset voltage system Resolution thus considerably better than normal 1LSB obtainable with conventional converter ADC0801-05 combines linear digital features converter that flexible easy microprocessor-based systems benefits sampled-data comparator unusual ladder make converter actually easier fabricate than digital-to-analog converter
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FIGURE Automatic Adjusting offset voltage differential amplifier pair transcript bridge network done automatically microprocessor provides this adjustment through programmable peripheral interface buffer integrated circuit LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT NATIONAL SEMICONDUCTOR CORPORATION used herein Life support devices systems devices systems which intended surgical implant into body support sustain life whose failure perform when properly used accordance with instructions provided labeling reasonably expected result significant injury user critical component component life support device system whose failure perform reasonably expected cause failure life support device system affect safety effectiveness
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CMOS Converter Interfaces Easily with Many Microprocessors With s

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