Precision Converters Although semiconductor diodes available toda

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Precision Converters Although semiconductor diodes available toda

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Precision Converters
Precision Converters
Although semiconductor diodes available today close ``ideal'' devices they have severe limitations level applications Silicon diodes have threshold which must overcome before appreciable conduction occurs placing diode feedback loop operational amplifier threshold voltage divided open loop gain amplifier With threshold virtually eliminated possible rectify millivolt signals
National Semiconductor Linear Brief August 1969
Figure shows simplest configuration eliminating diode threshold potential voltage non-inverting input amplifier positive output LM101A
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FIGURE Precision Diode swings positive When amplifier output swings positive becomes forward biased negative feedback through forces inverting input follow non-inverting input Therefore circuit acts voltage follower positive signals When input swings negative output swings negative With current flows load except bias current LM101A conduction threshold very small since less than change input will cause output LM101A swing from negative positive
useful variation this circuit precision clamp shown Figure this circuit output precisely clamped from going more positive than reference voltage When more positive than EREF LM101A functions summing amplifier with feedback loop closed through Neglecting offsets negative feedback keeps summing node therefore output within voltage non-inverting input When about more negative than EREF output swings positive reverse biasing Since prevents negative feedback from controlling voltage inverting input clamping action obtained both circuits Figures output clamp diode added help speed response clamp prevents operational amplifier from saturating when reverse biased When reverse biased either circuit large differential voltage appear between inputs LM101A This necessary proper operation does damage since LM101A designed withstand large input voltages These circuits will work with amplifiers protected with back back diodes across inputs Diode protection conducts when differential input voltage exceeds would connect input output together Also unprotected devices such LM709 damaged large differential input signals circuits Figures relatively slow Since there 100% feedback positive input signals necessary unity gain frequency compensation Also when reverse biased feedback loop around amplifier opened input stage saturates Both these conditions cause errors appear when input frequency exceeds high performance precision half wave rectifier shown Figure This circuit will provide rectification with accuracy frequencies from Further easy extend operation full wave rectification precision converters
EREF must have source impedance less than 200X used
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FIGURE Precision Clamp
FIGURE Fast Half Wave Rectifier
LB-8
C1995 National Semiconductor Corporation
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RRD-B30M115 Printed
Precision Converters
This precision rectifier functions somewhat differently from circuit Figure input signal applied through summing node inverting operational amplifier When signal negative forward biased develops output signal across with inverting amplifier gain When signal goes positive non-conducting there output However negative feedback path provided path through reduces negative output swing prevents amplifier from saturating Since LM101A used inverting amplifier feedforward compensation used Feedforward compensation increases slew rate reduces gain error high frequencies This compensation allows half wave rectifier operate higher frequencies than previous circuits with loss accuracy addition second amplifier converts half wave rectifier full wave rectifier shown Figure half wave rectifier connected inverting amplifier sums half wave rectified signal input signal provide full wave output negative input signals output zero current flows through NeR7 glecting moment output positive input signals sums currents through EOUT
Hence output alR6 ways absolute value input output Filtering averaging obtain pure output very easy capacitor placed across rolls frequency response give output equal average value input filter time constant R7C2 must much greater than maximum period input signal values given Figure time constant about seconds This converter better than conversion accuracy above less than ripple output calibrated read value sine wave input with high frequency circuit some care must taken during construction Leads should kept short avoid stray capacitance power supplies bypassed with disc ceramic capacitors Capacitive loading fast rectifier circuits must less than decoupling becomes necessary diodes should reasonably fast film type resistors used Also amplifiers must have bias currents REFERENCES Dobkin ``Feedforward Compensation Speeds National Semiconductor Corporation LB-2 March 1969
Feedforward compensation used make fast full wave rectifier without filter
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FIGURE Precision Converter 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
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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 Converters Although semiconductor diodes available toda

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