The Instrum entation A mplifier Handbook Including Applications Neil P. Albaugh Burr- Brown Corporation Tucson, Arizona DRAFT COPY Contents: INSTRUMENTATION AMPLIFIERS 2-1 Overview 2-1 THE WHEATSTONE BRIDGE SENSOR 2-2 Error Sources 2-5 AMPLIFIER TOPOLOGIES 3-1 The Difference Amplifier 3-1 The “Classical” Three Op Amp Instrumentation Amplifier 3-3 Internal vs. External Gain Setting Resistors 3-7 The Two Op Amp Instrumentation Amplifier 3-8 AVOIDING INSTRUMENTATION AMPLIFIER PITFALLS 4-1 Input Bias Current Effects 4-1 Avoiding the “Reference Pin” Trap 4-5 A Common Mistake: Floating Inputs 4-8 Common- Mode Voltage Limitations 4-11 Calculating Common Mode Voltage Range Painlessly 4-12 Maintaining “Truth In Output” 4-18 Noise Filtering The IA Input The Wrong Way—Making A Bad Situation Worse 4-20 SOLVING THE GROUND LOOP PROBLEM 5-1 What Are “Ground Loops”? 5-1 Op Amps and Ground Loops 5-1 Instrumentation Amplifiers and Ground Loops 5-2 OFFSET VOLTAGE TRIM 5-3 Trimming Offset Voltage At The Input 5-3 Trimming Offset Voltage Using REF Pin 5-4 BOOSTING THE OUTPUT 5-4 Driving Heavy Loads : The Wrong Way 5-4 Driving Heavy Loads : The Right Way 5-5 APPLICATION CIRCUITS: GENERAL 6-1 A +/-20V Input Diff Amp With +/-200V CMV Applications Circuit 6-1 Two and Three Op Amp IA Applications 6-3 Difference Amplifier Applications In Single- Ended Circuits 6-3 APPLICATIONS CIRCUITS: AUDIO 6-10 Low Noise Applications 6-10 Low Distortion Applications 6-11 DIFFERENCE AMPLIFIER INPUT RESISTANCE 6-12 APPLICATIONS CIRCUITS: CURRENT MEASUREMENT 7-1 Current Shunts 7-1 Low-Side Current Sensing 7-2 High- Side Current Sensing 7-5 MICROPOWER & BATTERY - POWERED APPLICATIONS 8-1 Single Supply Considerations 8-1 Minimizing Supply Current 8-4 APPLICATIONS CIRCUITS: UNUSUAL 8-7 Extending Common Mode Range To 1kV 8-7 An Adjustable Gain Difference Amplifier 8-8 Advantages Of Assymmetrical Power Supplies 8-10 VLF & LF Loop Antenna Amplifiers 8-10 APPLICATIONS CIRCUITS: OPTOELECTRONICS 8-10 Differential Photodetectors (“Edge Detectors”) 8-10 X-Y Position (Quadrant) Detectors 8-10 CW Diode Laser Current Driver 8-10 SELECTING YOUR INSTRUMENTATION AMPLIFIER 9-1 Sensor Source Impedance Considerations 9-1 Very High Impedance Sensor 9-1 Very Low Impedance Sensor 9-3 Power Supply Constraints 9-3 Common Mode Voltage Range Requirements 9-4 Improving Common Mode Rejection 9-4 Bandwidth And Settling Time 9-9 Noise And Distortion 9-10 “Rail- To- Rail” Input & Output Swing 9-12 RFI PROBLEMS 10-1 Input Rectification- the Most Common Problem 10-1 Typical Instrumentation Amplifier Swept- Power RFI Tests 10-2 Input RFI Filtering 10-4 Other RFI Considerations 10-9 MISCELLANEOUS APPLICATIONS CIRCUITS 11-1 Absolute- Value Amplifier 11-1 AVOIDING DIFFERENCE AMPLIFIER PITFALLS 12-1 Adding External Resistors—Don’t!! 12-1 Adding External Resistors—Sometimes? 12-4 TABLE OF FIGURES FIGURE2 - 1. A “BLACK BOX” REPRESENTATION OF AN INSTRUMENTATION AMPLIFIER 2-1 FIGURE2 - 2. (A.) CONVENTIONAL BRIDGE CIRCUIT (B.) REDRAWN BRIDGE CIRCUIT 2-2 FIGURE2 - 3. BALANCED BRIDGE GENERATES CMV BUT NO DIFFERENTIAL OUTPUT VOLTAGE 2-2 FIGURE2 - 4. UNBALANCED BRIDGE GENERATES A DIFFERENTIAL OUTPUT VOLTAGE 2-4 FIGURE2 - 5. A MODEL OF INPUT- REFERRED AMPLIFIER ERRORS 2-7 FIGURE3 - 1. THE UNITY- GAIN DIFFERENCE AMPLIFIER 3-1 FIGURE3 - 2. THE“ CLASSICAL” THREE OP AMP INSTRUMENTATION AMPLIFIER 3-3 FIGURE3 - 3. THREE OP AMP INSTRUMENTATION AMPLIFIER GAIN ANALYSIS 3-5 FIGURE3 - 4. THREE OP AMP IA COMMON MODE VOLTAGE ANALYSIS 3-6 FIGURE3 - 5. THE TWO OP AMP INSTRUMENTATION AMPLIFIER 3-8 FIGURE3 - 6. TWO OP AMP IA GAIN ANALYSIS 3-9 FIGURE3 -7. TWO OP AMP IA COMMON MODE REJECTION ANALYSIS 3-11 FIGURE4 - 1. AN UNSUITABLEIA CHOICE FOR A HIGHI MPEDANCE TRANSDUCER 4-1 FIGURE4 - 2. HOW TO RUIN YOUR CMR BY DRIVING THE IA REFERENCE PIN INCORRECTLY 4-6 FIGURE4 - 3. OFFSETTING ANI NSTRUMENTATION AMPLIFIER BY DRIVINGI TS REFERENCE PIN CORRECTLY 4-7 FIGURE4 -4. WHAT'S WRONG WITH THIS CIRCUIT? HINT: WHERE DOES THE BIAS CURRENT COME FROM? 4-8 FIGURE4 - 5. ADDING BIAS CURRENT RETURN RESISTORS SOLVE THE PROBLEM 4-9 FIGURE4 - 6. INSTRUMENTATION AMPLIFIER INPUT BIAS CURRENT CAN ALSO BE RETURNED TO GROUND THROUGH ANI NDUCTIVES OURCE 4-10 FIGURE4 - 11. CMVP LOT REVEALS THE EFFECTS OF GAIN (A.) 100V/V (B.) 1V/V 4-14 FIGURE4 - 12. DISTINCTIVE CMV RANGE SHAPES (A.) THREE OP AMP IA, (B.) TWO OP AMP IA 4-15 FIGURE4 - 13. TEN VOLTS APPLIED TO THE REFERENCE PIN MODIFIES CMVR ANGE (A .) THREE OP AMP IA (B.) TWO OP AMP IA 4-16 FIGURE4 - 14. LOW SUPPLY VOLTAGES MUST BE USED WITH CAUTION! (A.) UNUSEBLE THREE OP AMP CMV RANGE (B.) A LARGER TWO OP AMP CMVR ANGE 4-18 FIGURE4 - 15. NOISE ANDR FI FILTERING-- THE WRONG WAY 4-21 FIGURE4 - 16. WORST-CASE MISMATCHED-POLE RFI FILTERS WITH 1% RESISTORS AND 5% CAPACITORS 4-22 FIGURE4 - 17. MISMATCHED NOISE FILTER COMPONENTS CREATE MISMATCHED CMV LOW PASS FILTER POLES 4-22 FIGURE4 - 18. DIFFERENTIAL VOLTAGE CREATED BY MISMATCHED COMMON MODE LOW PASS FILTER POLES 4-23 FIGURE4 - 19. CMV FREQUENCYR ESPONSE OF AN INA118 INSTRUMENTATION AMPLIFIER WITH MISMATCHED COMMON MODE LOW PASS FILTER POLES 4-23 FIGURE4 - 20. AN IMPROVED METHOD OF INSTRUMENTATION AMPLIFIERINPUT NOISE FILTERING 4-24 FIGURE4 - 21. FREQUENCYR ESPONSE OF AN INA118 INSTRUMENTATION AMPLIFIER WITH MISMATCHED" IMPROVED" NOISE REJECTION FILTER 4-25 FIGURE5 - 1. DRIVING A HEAVY LOAD—THE RIGHT WAY 5-6 FIGURE6 - 1. ANE XTERNAL OP AMP BOOSTS THEI NA117 DIFFERENCEI NPUT RANGE TO +/-20V BUT STILL HANDLES +/-200 V COMMON MODE VOLTAGES 6-1 FIGURE6 - 2. EVEN WITH THE0 PA27 OP AMP FEEDBACK CIRCUIT GAIN OF 0.5V/V AND 1000PF LOAD, THES TABILITY OF THE INA117 CIRCUIT IS EXCELLENT. 6-2 FIGURE6 - 3. (A.) A PRECISION GAIN OF- 1.000V/V (B.) PRECISION GAIN OF +2.000V/V 6-4 FIGURE6 -4. A DIFFERENCE AMPLIFIER CONNECTED AS (A.) ANA VERAGE VALUE AMPLIFIER (B.) A 2-INPUT SUMMING AMPLIFIER 6-5 FIGURE6 - 5. A PRECISION GAIN OF +0.500V/V. 6-6 FIGURE6 - 6. A DIFFERENTIAL INPUT/ DIFFERENTIAL OUTPUT AMPLIFIER. 6-7 FIGURE6 - 7. AN AMPLIFIER WITH A CONTINUOUSLY ADJUSTABLE GAIN RANGE OF -1.000V/V TO +1.000V/V 6-8 FIGURE6 - 8. ADDING AS WITCH TO A DIFFERENCE AMPLIFIER CREATES TURNS IT INTO A SYNCHRONOUS DETECTOR, A.K.A. PHASE SENSITIVE DETECTOR. 6-9 FIGURE6 - 9. SCHEMATIC CAPTURE DRAWING: SIMULATED UNITY-GAIN DIFFERENCE AMPLIFIER WITH TWOI NDEPENDENT SIGNAL SOURCES 6-12 FIGURE6 - 10. UNEXPECTEDB EHAVIOR? DIFFERENCE AMPLIFIER INPUTS EXHIBIT "DIFFERENT" LOADING OF THEIR RESPECTIVES IGNALS OURCES 6-13 FIGURE6 - 11. IS THEI NPUT LOADING OF THIS CIRCUIT BETTER THAN FIGURE6 - 10? 6-14 FIGURE7 - 1. (A.) HIGHS IDE CURRENT SHUNT (B .)LOW SIDE CURRENT SHUNT 7-1 FIGURE7 - 2. SHUNT RESISTOR KELVIN CONNECTION 7-2 FIGURE7 - 3. DIFFERENTIAL SENSING OF THE VOLTAGE DROP ACROSS A LOW- SIDE SHUNT RESISTOR MINIMIZES GROUND LOOP ERRORS 7-3 FIGURE7 - 4. ALTERNATIVE CONNECTION OF SHUNT RESISTOR. RE: FIGURE7 - 3. 7-4 FIGURE7 - 5. LOW- SIDE SHUNT AMPLIFIER WITH SINGLE SUPPLY 7-5 FIGURE7 -6. HIGH ACCURACYC URRENT MEASUREMENT WITH UP TO 200V COMMON MODE VOLTAGE 7-6 FIGURE7 - 7. ACCURATE LOW CURRENT MEASUREMENTS WITH UP TO +/-200V COMMON MODE VOLTAGE 7-8 FIGURE8 - 1. A FAST R-R SINGLE SUPPLYI NSTRUMENTATION AMPLIFIER WITH A GAIN OF 100V/V 8-2 FIGURE8 - 2. CMOS INSTRUMENTATION AMPLIFIER SWINGS TO WITHIN 10MV OF THE SUPPLY RAILS 8-3 FIGURE8 - 3. A 1KV CMV DIFFERENTIAL AMPLIFIER MADE WITH A PRECISION 100:1 VOLTAGE DIVIDER ADDED TO AN INSTRUMENTATION AMPLIFIER. 8-8 FIGURE8 - 4. A “DIFFERENT” RESISTOR NETWORK PLUS AN OP AMP YIELDS AN ADJUSTABLE- GAIN DIFFERENCE AMPLIFIER 8-8 FIGURE8 - 5. ADDING GAIN TO COMPENSATE FOR AN INPUT VOLTAGE DIVIDER YIELDS A UNITY GAIN DIFFERENCE AMPLIFIER WITH A 50VC OMMON MODE VOLTAGE RANGE. 8-10 FIGURE9 - 1. AMPLIFIER FOR PH MEASUREMENT 9-2 FIGURE9 - 2. (A.) DIFFERENCE AMPLIFIER CMR TRIM (B.) ALTERNATE CMR TRIM 9-5 FIGURE9 - 3. CMR TRIMMING AC LASSICAL INSTRUMENTATION AMPLIFIER WITH A NEGATIVE IMPEDANCE CONVERTER 9-6 FIGURE9 --4. “MIRROR IMAGE” AMPLIFIER EXTENDS HIGH FREQUENCY CMR OF TWO OP AMP INSTRUMENTATION AMPLIFIERS. 9-7 FIGURE9 - 5. MEASURED CMR OF A “MIRROR- IMAGE” INSTRUMENTATION AMPLIFIER VS . SINGLE INA126. 9-8 FIGURE9 - 6. UNITY GAIN DIFFERENCE AMPLIFIER COMMON MODE RANGE AUTOMATICALLY FOLLOWS HIGH-SIDE SHUNT VOLTAGE 9-13 FIGURE9 - 7. (A.) INA132 COMMON MODE RANGE WITH+ 12/-5VDC SUPPLIES (B.) INA132 COMMON MODE RANGE WITH+ 28/-5VDC SUPPLIES 9-13 FIGURE9 - 8. A RAIL-TO-RAIL INPUT AND RAIL-TO-RAIL OUTPUT INSTRUMENTATION AMPLIFIER WITH A GAIN OF 10V/V 9-14 FIGURE9 - 9. OPA340 INSTRUMENTATION AMPLIFIER RAIL-TO-RAIL OUTPUT SWINGI NTO A 10K LOAD WITH V = – 2.5VDC AND V =2 50 MVP -P. 9-15 S IN FIGURE1 0- 1. I-V CURVES FOR GERMANIUM AND SILICON DIODES. 10-2 FIGURE1 0- 2. UNFILTERED INA129 INPUT OFFSET SHIFT: NOTE+ 200, -700 MV VERTICAL SCALE. 10-3 FIGURE1 0- 3. UNFILTERED INPUT INA129 RFI TEST CIRCUIT. 10-4 FIGURE1 0- 4. ONE APPROACH TO INPUT RFI FILTERING—THE“ AD FILTER.” 10-5 FIGURE1 0- 5. INPUT OFFSET SHIFT WITH ANALOG DEVICES’ FILTER: NOTE – 5MV VERTICAL SCALE. 10-5 FIGURE1 0- 6. AN IMPROVED ALL PASSIVEI NPUT FILTER-- DUBBED THE “BB FILTER.”. 10-6 FIGURE1 0- 7. INPUT OFFSET SHIFT WITH IMPROVED “BB FILTER”: NOTE – 2MV VERTICAL SCALE. 10-7 FIGURE1 0- 8. RFI TEST CIRCUITS FOR MEASURING OFFSET SHIFT OF INA129 WITH “AD” LPF. 10-8 FIGURE1 0- 9. RFI TEST CIRCUITS FOR MEASURING OFFSET SHIFT OF INA129 WITH “BB” LPF. 10-8 FIGURE1 1- 1. UNITY GAIN ABSOLUTE VALUE CIRCUIT—POSITIVE OUTPUT 11-1 FIGURE1 1- 2. ABSOLUTE-VALUE CIRCUIT INPUT (BOTTOM CURVE) VS. OUTPUT (TOP CURVE) TRANSFER FUNCTION. THE AMPLIFIER’S OUTPUT IS ALWAYS POSITIVE. 11-2 FIGURE1 2- 1. HOW TO RUIN YOUR DIFFERENCE AMPLIFIER’S COMMON MODE REJECTION— ADD EXTERNAL RESISTORS 12-1 FIGURE1 2- 2. CIRCUIT DIAGRAM TO INVESTIGATE EFFECTS OF ADDING EXTERNAL RESISTORS TO A RATIO- TRIMMED DIFFERENCE AMPLIFIER 12-2 FIGURE1 2- 3. THEORETICAL CMR WITH AND WITHOUT EXTERNAL 40K RESISTORS—A BIG DIFFERENCE 12-3 TABLES TABLE4 - 1. RESISTOR % MATCH REQUIRED TO ACHIEVE CMR. 4-6 TABLE8 - 1 CELL DATA FOR BATTERIES COMMONLY USEDI N PORTABLE ELECTRONIC INSTRUMENTS 8-5 TABLE8 - 2. A FEW DEVICES THAT ARE RECOMMENDED FOR SINGLE SUPPLY OR BATTERY OPERATEDI NSTRUMENT APPLICATIONS 8-6 TABLE9 - 1. A SELECTION OF INSTRUMENTATION AMPLIFIERS AND DIFFERENCE AMPLIFIERS 9-4 TABLE9 - 2. INSTRUMENTATION AMPLIFIERS AND DIFFERENCE AMPLIFIERS RECOMMENDED FOR AUDIO APPLICATIONS 9-11 TABLE1 0- 1 RF INPUT POWER TO RF INPUT VOLTAGE CONVERSION TABLE 10-7 Instrumentation Amplifiers Overview The term “instrumentation amplifier” is properly used to describe a category of true differential- input amplifiers that emphasize high common mode rejection (CMR) and accuracy. Although both instrumentation amplifiers and difference amplifiers use op amps as basic architectural “building blocks”, they are distinctly different from their op amp cousins. Op amps are “single-ended” and they are usually intended to operate in a variety of applications-- with their feedback determining their functions. Instrumentation amplifiers and difference amplifiers are used primarily to provide differential gain and common mode rejection. Employing feedback from output to input is not intended. In some instances this term has been widely misused and this has created confusion as to the correct definition of an instrumentation amplifier (IA). In the early days of monolithic operational amplifiers, one well-known vendor referred to their new precision op amp as an instrumentation amplifier. What they meant to say was that it was an “instrumentation-grade” op amp. In addition, large laboratory bench-top amplifiers and even traveling- wave tube (microwave) amplifiers have been called instrumentation amplifiers. It is not surprising, then, that so much confusion exists about what an IA really is and what it does. Most common IAs are one of three types: the simple “Difference Amplifier”, the “Two Op Amp Instrumentation Amplifier”, and the “Classical Three Op Amp Instrumentation Amplifier” architecture. As we shall see, these three architectures are interrelated but their performance differs in certain important aspects. For now, let’s just think of the IA as a “black box” differential amplifier. +Vs - Inverting Input tuptuO + Noinn-v erting Input -Vs Figure 2- 1. A "Black Box" Representation of an Instrumentation Amplifier.