Precision Comparator Runs from Logic Supply Robert Widlar Apartad

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

Precision Comparator Runs from Logic Supply Robert Widlar Apartad

Datasheet Thumbnail

Top Searches for this datasheet

Precision Comparator Runs from Logic Supply
Precision Comparator Runs from Logic Supply
Robert Widlar Apartado Postal Puerto Vallarta Jalisco Mexico
introduction digital systems sometimes necessary convert level analog signals into digital information example this might detector illumination level photodiode 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 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 order understand this comparator necessary look briefly circuit configuration Figure shows simplified schematic device transistors
National Semiconductor Application Note October 1970
7303
Figure Simplified schematic comparator 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
AN-41
C1995 National Semiconductor Corporation
7303
RRD-B30M115 Printed
output transistor 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 Table Important electrical characteristics LM111 comparator when operating from single supply 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 Units
mode range output will directly drive 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
Figure shows connection operating with logic This level detector photodiode that operates supply output changes state when diode current reaches Even this current error contributed comparator less than
7303
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 b10V driving data inputs logic directly circuit Figure course adapted work with supply rate accuracy circuit will depend supply-voltage regulation since reference derived from supply Figure shows method
supply voltages with corresponding increase input voltage range Other characteristics essentially unchanged higher voltages 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 above ground within com-
7303
Figure Precision level detector photodiode making performance independent supply voltage temperature-compensated reference diode with breakdown voltage acts shunt regulator delivers stable voltage comparator When diode current large enough (about make voltage drop
7303-2
Figure Zero crossing detector magnetic transducer
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 zero crossing detector that drives data input logic shown Figure Here both positive supply
comparator strobed shown Figure addition With logic base approximately drawn strobe terminal LM111 making output high independent input signal
7303
7303
Figure Zero crossing detector driving logic b10V 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 rapidlychanging signals from This increased common-mode slew obtained without balancing potentiometer shorting both balance terminals positive-supply terminal Increased input bias current price that must paid faster operation digital interface circuits Figure shows interface between high-level logic input signal with logic states attenuated threshold level comparator that switches when input goes through 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
Figure Circuit transmitting data between high-level logic 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 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 lower rates kHz) eliminated 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
7303
Figure Data transmission system with near-infinite ground isolation
shown circuit delivers square wave output frequency changed varying adjusting through while keeping their ratios constant 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
tive feedback obtained through quartz crystal circuit oscillates when transmission through crystal maximum crystal operates series-resonant
7303
Figure Crystal-controlled oscillator mode high input impedance comparator isolating capacitor minimize loading crystal contribute frequency stability shown oscillator delivers square-wave output frequency doubler digital system relatively simple matter divide integer However multiplying integer quite another story especially operation over wide frequency range waveform symmetry required frequency doubler that satisfies above requirements shown Figure comparator used shape
7303
Fanout
Figure Free-running multivibrator 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 posi-
Frequency Range Input Output
7303
Figure Frequency doubler
signal feed integrator shaping required because input integrator must swing between supply voltage ground preserve symmetry output waveform LM108 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
7303
Figure Waveforms frequency doubler second comparator delayed half duration input pulse comparator outputs then combined through exclusive-OR gate produce doublefrequency output With component values shown circuit operates frequencies from 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 back 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 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 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 Precautions should taken insure that power supplies this other never become reversed even under transient conditions With reverse voltages greater than conduct excessive current fuzing internal aluminum interconnects This usually takes more than there possibility reversal clamp diodes with adequate peak current rating should installed across supply 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
Precision Comparator Runs from Logic Supply
output circuitry protected shorts across load will example withstand short voltage more negative than ground terminal Additionally with sustained short power dissipation become excessive voltage across output transistor exceeds about 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 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 logic also interface with logic deliver analog switches output switch 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
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
National Semiconductor Corporation 1111 West Bardin Road Arlington 76017 1(800) 272-9959 1(800) 737-7018
critical component component life support device system whose failure perform reasonably expected cause failure life support device system affect safety effectiveness
AN-41
National Semiconductor Europe (a49) 0-180-530 Email cnjwge tevm2 Deutsch (a49) 0-180-530 English (a49) 0-180-532 Fran (a49) 0-180-532 Italiano (a49) 0-180-534
National Semiconductor Hong Kong 13th Floor Straight Block Ocean Centre Canton Tsimshatsui Kowloon Hong Kong (852) 2737-1600 (852) 2736-9960
National Semiconductor Japan 81-043-299-2309 81-043-299-2408
National does assume responsibility circuitry described circuit patent licenses implied National reserves right time without notice change said circuitry specifications

Precision Comparator Runs from Logic Supply Robert Widlar Apartad

Top Searches for this datasheet

Other recent searches