Instrumentational Amplifiers INTRODUCTION most useful analog subs

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Instrumentational Amplifiers INTRODUCTION most useful analog subs

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Instrumentational Amplifiers
Instrumentational Amplifiers
INTRODUCTION most useful analog subsystems true instrumentation amplifier faithfully amplify level signals presence high common mode noise This aspect performance makes especially useful input amplifier signal processing system Other features instrumentation amplifier high input impedance input current good linearity never been easy design high performance instrumentation amplifier however availability high performance IC's considerably simplifies problem amps available today that give very drifts well bias currents however most circuits have some drawbacks most commonly used instrumentation amplifier designs utilize either amps several precision resistors These capable excellent performance however high performance they require very precisely matched resistors common mode rejection these designs depends resistor matching overall gain Since amps available with exceedingly high CMRR this longer problem CMRR instrumentation amplifier approximately equal half resistor mismatch plus gain resistor mismatch CMRR limited plus gain referred input
National Semiconductor Linear Brief June 1973
Referred output common mode error independent gain fixed resistor mismatch match error match error These errors trivial high precision systems instrumentation amplifier shown here that compares favorably with multiple designs does require precisely matched resistors Further design allows single resistor adjust gain comparing this instrumentation multiple types there course some drawbacks gain linearity accuracy good multiple circuits errors appearing multiple circuits independent output signal level example common mode error output full scale error desired output signal only full scale With circuit maximum errors full scale output percentage output error decreases lower output levels
Figure shows general purpose instrumentation amplifier optimized wide bandwidth provide gains from under over 1000 with single resistor adjustment Gain linearity worst unity-gain gain stability better than from Typically over range gain stability Common mode rejection ratio about independent gain
Note Since LM114 obsolete part substitution LM194 recommended along with removal LM194 diodes This circuit been tested with LM194 included Also LM185-1 could substituted LM113
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8727
FIGURE Instrumentation Amplifier
C1995 National Semiconductor Corporation
8727
RRD-B30M115 Printed
Instrumentational Amplifiers
Transistor pair operated open-loop input stage give floating fully differential input Current sources operating current input pair obtain good linearity output current about twice current full differential voltage temperature sensitivity transconductance compensated making their operating current directly proportional absolute temperature been shown that biasing base transistor current sources output current varies absolute temperature LM113 diode provides constant current sources Both compensated large degeneration from give amplifier stable gain over wide temperature range
operation transistors convert differential input voltage differential output current their collectors This into standard differential amplifier obtain single ended output voltage Since diff does common mode input voltage resistors adequate Gain ratio (plus mentioned previously this circuit optimized wide bandwidth however easily modified other applications bias current needed resistors increased factor LM108 substituted LM318 Other possible improvements cascaded current sources modified Darlington input stage
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Instrumentational Amplifiers INTRODUCTION most useful analog subs

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