Precision Comparator Runs from Logic Supply digital systems, some

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Precision Comparator Runs from Logic Supply digital systems, some

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Precision Comparator Runs from Logic Supply
Precision Comparator Runs from Logic Supply
digital systems, sometimes necessary convert level analog signals into digital information. example this might detector illumination level photo-diode. Another would zero crossing detector magnetic transducer such magnetometer shaft-position pickoff. These transducers have low-level outputs, with currents microamperes voltages millivolts. Therefore, level circuitry required condition these signals before they drive logic circuits. voltage comparator perform many these precision functions. comparator essentially high-gain designed open loop operation. function comparator produce logic "one" output with positive signal between inputs logic "zero" with negative signal between inputs. Threshold detection accomplished putting reference voltage input
National Semiconductor Application Note Robert Widlar October 1970
signal other. Clearly, used comparator, except that response time tens microseconds which often slow many applications. unique comparator design will described here along with some applications digital systems. Unlike older comparators amps, will operate from same supply logic circuits. will also operate with single negative supply used with logic. Hence, level functions performed without extra supply voltages previously required. versatility comparator along with minimal circuit loading considerable precision recommend many uses, digital systems, other than detection level signals. used oscillator multivibrator, digital interface circuitry even voltage analog circuitry. Some these applications will also discussed.
Circuit Description
00730301
FIGURE Simplified Schematic Comparator
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AN007303
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Circuit Description
(Continued)
order understand this comparator, necessary look briefly circuit configuration. Figure shows simplified schematic device. transistors buffer differential input stage input currents without sacrificing speed. PNP's drive standard differential stage, output this stage further amplified pair. This feeds which provides additonal gain drives output stage. Current sources used determine bias currents, that performance greatly affected supply voltages. output transistor Q11, protected which limit peak output current. output lead, since connected other point circuit, either returned positive supply through pull-up resistor switch loads that connected voltage higher than positive supply voltage. circuit will operate from single supply negative supply lead connected ground. However, negative supply available, used increase input common mode range. Table summarizes performance comparator when operating from supply. circuit will work with supply voltages with corresponding increase input voltage range. Other characteristics essentially unchanged higher voltages. TABLE Important electrical characteristics LM111 comparator when operating from single, supply 25°C) Parameter Input Offset Voltage Input Offset Current Input Bias Current Voltage Gain Response Time Common Mode Range Output Voltage Swing Output Current (DTL/TTL) Supply Current Limits V/mV Units
00730302
FIGURE Zero Crossing Detector Magnetic Transducer Figure shows connection operating with logic. This level detector photodiode that operates -10V supply. output changes state when diode current reaches Even this current, error contributed comparator less than
00730303
FIGURE Level Detector Photodiode Higher threshold currents obtained reducing proportionally. switching point, voltage across photodiode nearly zero, leakage current does cause error. output switches between ground -10V, driving data inputs logic directly. circuit Figure can, course, adapted work with supply. rate, accuracy circuit will depend supply-voltage regulation, since reference derived from supply. Figure shows method making performance independent supply voltage. temperature-compensated reference diode with 1.23V
Level Applications
circuit that will detect zero crossing output magnetic transducer within fraction millivolt shown Figure magnetic pickup connected between inputs comparator. resistive divider, biases inputs 0.5V above ground, within common mode range output will directly drive TTL. exact value pull resistor, determined speed required from circuit since must drive capacitive loading positive-going output signals. optional offset-balancing circuit using included schematic.
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Level Applications
(Continued)
breakdown voltage. acts shunt regulator delivers stable voltage comparator. When diode current large enough (about make voltage drop across equal breakdown voltage output will change state. been added make threshold error proportional offset current comparator, rather than bias current. eliminated bias current error considered significant.
creased common-mode slew obtained without balancing potentiometer shorting both balance terminals positive-supply terminal. Increased input bias current price that must paid faster operation.
00730305
FIGURE Zero Crossing Detector Driving Logic
Digital Interface Circuits
00730304
FIGURE Precision Level Detector Photodiode zero crossing detector that drives data input logic shown Figure Here, both positive supply -10V supply circuits used. Both supplies required circuit work with zero common-mode voltage. alternate balancing scheme also shown schematic. differs from circuit Figure that raises input-stage current factor three. This increases rate which input voltage follows rapidly-changing signals from 7V/µs 18V/µs. This
Figure shows interface between high-level logic TTL. input signal, with logic states attenuated 2.5V threshold level comparator that switches when input goes through 15V. response time circuit controlled with desired, make insensitive fast noise spikes. Because error currents LM111, possible input impedances even higher than obtained with indicated resistor values. comparator strobed, shown Figure addition With logic base approximately drawn strobe terminal LM111, making output high independent input signal.
00730306
FIGURE Circuit Transmitting Data Between High-Level Logic
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Digital Interface Circuits
(Continued)
Multivibrators Oscillators
free-running multivibrator Figure another example versatility comparator. inputs biased within common mode range stability, which insures starting, provided negative feedback through negative feedback reduced high frequencies some frequency, positive feedback through will greater than negative feedback; circuit will oscillate. component values shown, circuit delivers square wave output. frequency changed varying adjusting through while keeping their ratios constant.
Sometimes necessary transmit data between digital equipments, maintain high degree electrical isolation. Normally, this done with transformer. However, transformers have problems with low-duty-cycle pulses since they preseve level. circuit Figure more satisfactory method obtaining isolation. transmitting end, gate drives gallium-arsenide light-emitting diode. light output optically coupled silicon photodiode, comparator detects photodiode output. optical coupling makes possible electrical isolation thousands Megohms potentials thousands volts. maximum data rate this circuit MHz. lower rates (200 kHz) eliminated.
00730307
FIGURE Data Transmission System with Near-Infinite Ground Isolation Because input current comparator, large circuit impedances used. Therefore, frequencies obtained with relatively-small capacitor values: problem down using capacitor. speed comparator also permits operation frequencies above kHz.
00730308
Fanout two.
FIGURE Free-Running Multivibrator
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Multivibrators Oscillators
(Continued) frequency oscillation depends almost entirely resistance capacitor values because precision comparator. Further, frequency changes only change supply voltage. Waveform symmetry also good, symmetry varied changing ratio crystal-controlled oscillator that used generate clock slower digital systems shown Figure similar free running multivibrator, except that positive feedback obtained through quartz crystal. circuit oscillates when transmission through crystal maximum, crystal operates series-resonant mode. high input impedance comparator isolating capacitor, minimize loading crystal contribute frequency stability. shown, oscillator delivers square-wave output.
00730309
FIGURE Crystal-controlled Oscillator
Frequency Doubler
digital system, relatively simple matter divide integer. However, multiplying integer quite another story especially operation over wide frequency range waveform symmetry required.
00730310
Frequency Range Input Output
FIGURE Frequency Doubler frequency doubler that satisfies above requirements shown Figure comparator used shape input signal feed integrator. shaping required because input integrator must swing between supply voltage ground preserve symmetry output waveform. LM108 amp, that works from logic supply, serves integrator. This feeds triangular waveform second comparator that detects when waveform goes through voltage equal average value. Hence, shown Figure output second comparator delayed half duration input pulse. comparator outputs then combined through exclusive-OR gate produce double-frequency output.
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Frequency Doubler
(Continued)
limit peak current something less than This especially important when inputs outside piece equipment where they could accidentally connected high voltage sources. impedance sources cause problem unless their output voltage exceeds negative supply voltage. However, supplies zero when they turned off, isolation usually needed. Large capacitors input (greater than should treated source impedance isolated with resistor. charged capacitor hold inputs outside supply voltage supplies abruptly shut off. Precautions should taken insure that power supplies this other never become reversed even under transient conditions. With reverse voltages greater than conduct excessive current, fusing internal aluminum interconnects. This usually takes more than 0.5A. there possibility reversal, clamp diodes with adequate peak current rating should installed across supply bus. attempt should made operate circuit with ground terminal voltage exceeding either supply voltage. Further, output-voltage rating applies potential between output terminal. Therefore, comparator operated from negative supply, maximum output voltage must reduced amount equal voltage terminal. output circuitry protected shorts across load. will not, example, withstand short voltage more negative than ground terminal. Additionally, with sustained short, power dissipation become excessive voltage across output transistor exceeds about 10V. input terminals exceed positive supply voltage without causing damage. However, maximum rating between inputs terminal must observed. mentioned earlier, inputs should driven more negative than terminal.
00730311
FIGURE Waveforms Frequency Doubler With component values shown, circuit operates frequencies from kHz. Lower frequency operation secured increasing both
Application Hints
problems encountered using earlier comparators like LM710 LM106 that they were prone erratic operation caused oscillations. This direct result high speed devices, which made mandatory provide good input-output isolation low-inductance bypassing supplies. These oscillations could particularly puzzling when they occurred internally, showing external terminals only erratic characteristics. general, LM111 less susceptible spurious oscillations both because lower speed (200 response time because better power supply rejection. Feedback between output input lesser problem with given source resistance. However, LM111 operate with source resistance that orders magnitude higher than earlier devices, stray coupling between input output should minimized. With source resistances between impedance (both capacitive resistive) both inputs should made equal, this tends reject signal feedback. Even difficult completely eliminate oscillations linear region with source resistances above because open loop gain comparator about However, this does affect characteristics problem unless input signal dwells within transition level. oscillation does cause difficulties, eliminated with small amount positive feedback around comparator give hysteresis. Stray coupling between output balance terminals also cause oscillations, attempt should made keep these leads apart. usually advisable balance pins together minimize effect this feedback. balancing used, same result accomplished connecting capacitor between these pins. Normally, individual supply bypasses every device unnecessary, although long leads between comparator bypass capacitors definitely recommended. large current spikes injected into supplies switching output, bypass capacitors should included these points. When driving inputs from impedance source, limiting resistor should placed series with input lead
Conclusions
versatile voltage comparator that perform many precision functions required digital systems been produced. Unlike older comparators, operate from same supply voltage digital circuits. comparator particularly useful circuits requiring considerable sensitivity accuracy, such threshold detectors level sensors, data transmission circuits stable oscillators multivibrators. comparator also used many analog systems. operates from standard supplies, accuracy equals some best amps. also order magnitude faster than amps used comparators. comparator considerably more flexible than older devices. only will drive RTL, logic; also interface with logic deliver analog switches. output switch 50V, loads, making useful driver relays, lamps light-emitting diodes. Further, unique output stage enables drive loads referred either supply ground provide ground isolation between comparator inputs load. LM111 plug-in replacement comparators like LM710 LM106 applications where speed prime concern. Compared predecessors other respects, many improved electrical specifications, more design flexibility fewer application problems.
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Precision Comparator Runs from Logic Supply
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