PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction I 2 Bridge Circuits 3 Amplifiers for Signal Conditioning 4 Strain, Force, Pressure, and Flow Measurements 5 High Impedance Sensors 6 Position and Motion Sensors 7 Temperature Sensors 8 ADCs for Signal Conditioning 9 Smart Sensors 10 Hardware Design Techniques 2.0 a
RESISTANCE OF POPULAR SENSORS 120 Ω Ω , 350 Ω Ω , 3500 Ω Ω I Strain Gages 350 Ω Ω - 3500 Ω Ω I Weigh-Scale Load Cells 350 Ω Ω - 3500 Ω Ω I Pressure Sensors 100k Ω Ω - 10M Ω Ω I Relative Humidity 100 Ω Ω , 1000 Ω Ω I Resistance Temperature Devices (RTDs) 100 Ω Ω - 10M Ω Ω I Thermistors 2.1 a
MEASURING RESISTANCE INDIRECTLY USING A CONSTANT CURRENT SOURCE = = + + ∆ ∆ VOUT I R ( R ) I R + ∆ ∆ R 2.2 a
THE WHEATSTONE BRIDGE V B R 1 R 2 = = − − VO VB VB + + + + R 1 R 4 R 2 R 3 R4 R3 R 1 R 2 − − R 4 R 3 = = - + VB R 1 R 2 + + + + 1 1 V O R 4 R 3 AT BALANCE, R1 R2 R 1 R 2 = = = = VO 0 IF R 4 R 3 2.3 a
OUTPUT VOLTAGE AND LINEARITY ERROR FOR CONSTANT VOLTAGE DRIVE BRIDGE CONFIGURATIONS V B V B V B V B R+ ∆ ∆ R R −∆ −∆ R R+ ∆ ∆ R R −∆ −∆ R R R R R V O V O V O V O R+ ∆ ∆ R R+ ∆ ∆ R R+ ∆ ∆ R R −∆ −∆ R R+ ∆ ∆ R R R R ∆ R ∆ ∆ R ∆ ∆ R ∆ ∆ R ∆ V B V B V B V O : V B ∆ R ∆ ∆ R ∆ R R 4 2 2 R + R + 2 2 Linearity 0.5%/% 0.5%/% 0 0 Error: (D) All-Element (A) Single-Element (B) Two-Element (C) Two-Element Varying Varying Varying (1) Varying (2) 2.4 a
OUTPUT VOLTAGE AND LINEARITY ERROR FOR CONSTANT CURRENT DRIVE BRIDGE CONFIGURATIONS I B I B I B I B R+ ∆ ∆ R R −∆ −∆ R R+ ∆ ∆ R R −∆ −∆ R R R R R V O V O V O V O R+ ∆ ∆ R R+ ∆ ∆ R R+ ∆ ∆ R R −∆ −∆ R R+ ∆ ∆ R R R R ∆ R ∆ I B R I B I B ∆ R ∆ ∆ R ∆ ∆ R ∆ V O : I B ∆ ∆ R 4 2 2 R + 4 Linearity 0.25%/% 0 0 0 Error: (D) All-Element (A) Single-Element (B) Two-Element (C) Two-Element Varying Varying Varying (1) Varying (2) 2.5 a
BRIDGE CONSIDERATIONS I Selecting Configuration (1, 2, 4 - Element Varying) I Selection of Voltage or Current Excitation I Stability of Excitation Voltage or Current I Bridge Sensitivity: FS Output / Excitation Voltage 1mV / V to 10mV / V Typical I Fullscale Bridge Outputs: 10mV - 100mV Typical I Precision, Low Noise Amplification / Conditioning Techniques Required I Linearization Techniques May Be Required I Remote Sensors Present Challenges 2.6 a
USING A SINGLE OP AMP AS A BRIDGE AMPLIFIER FOR A SINGLE-ELEMENT VARYING BRIDGE V B R F R R +V S − − + R R F R+ ∆ ∆ R V S 2 2.7 a
USING AN INSTRUMENTATION AMPLIFIER WITH A SINGLE-ELEMENT VARYING BRIDGE V B +V S ∆ R ∆ R R V B ∆ R ∆ V OUT = GAIN 4 R + 2 − − R G IN AMP V OUT REF + R R+ ∆ ∆ R -V S * * SEE TEXT REGARDING SINGLE-SUPPLY OPERATION 2.8 a
LINEARIZING A SINGLE-ELEMENT VARYING BRIDGE METHOD 1 V B R R − − + +V S -V S R R+ ∆ ∆ R ∆ ∆ R = − = − VOUT VB 2 R 2.9 a
LINEARIZING A SINGLE-ELEMENT VARYING BRIDGE METHOD 2 V B ∆ ∆ VB R R 2 = = + + VOUT 1 2 R R 1 R R +V S + V OUT − − + +V S -V S − − R2 -V S R+ ∆ ∆ R R R1 2.10 a
LINEARIZING A TWO-ELEMENT VARYING BRIDGE METHOD 1 (CONSTANT VOLTAGE DRIVE) V B R R+ ∆ ∆ R − − + +V S -V S R R+ ∆ ∆ R ∆ ∆ R = = − − VOUT VB R 2.11 a
LINEARIZING A TWO-ELEMENT VARYING BRIDGE METHOD 2 (CONSTANT CURRENT DRIVE) +V S ∆ ∆ R R R+ ∆ ∆ R V OUT = I B GAIN 2 − − R G I B IN AMP V OUT REF + R -V S * R+ ∆ ∆ R +V S − − I B R SENSE * SEE TEXT REGARDING SINGLE-SUPPLY OPERATION + -V S * V REF 2.12 a
ERRORS PRODUCED BY WIRING RESISTANCE FOR REMOTE RESISTIVE BRIDGE SENSOR 100 FEET, 30 GAGE COPPER WIRE = 10.5 Ω Ω @ 25 ° ° C +10V TC = 0.385%/ ° ° C ASSUME +10 ° ° C TEMPERATURE CHANGE NUMBERS IN ( ) ARE @ +35 ° ° C 350 Ω Ω 350 Ω Ω R LEAD 10.5 Ω ( Ω ( 10.904 Ω) Ω) - + V O 0 → → 23.45mV STRAIN GAGE (5.44mV → → 28.83mV) 350 Ω → Ω → 353.5 Ω Ω FS 350 Ω Ω R LEAD 10.5 Ω ( Ω ( 10.904 Ω) Ω) R COMP 21 Ω Ω OFFSET ERROR OVER TEMPERATURE = +23%FS GAIN ERROR OVER TEMPERATURE = –0.26%FS 2.13 a
3-WIRE CONNECTION TO REMOTE BRIDGE ELEMENT (SINGLE-ELEMENT VARYING) 100 FEET, 30 GAGE COPPER WIRE = 10.5 Ω Ω @ 25 ° ° C +10V TC = 0.385%/ ° ° C ASSUME +10 ° ° C TEMPERATURE CHANGE NUMBERS IN ( ) ARE @ +35 ° ° C 350 Ω Ω 350 Ω Ω R LEAD 10.5 Ω ( Ω ( 10.904 Ω) Ω) - + V O STRAIN GAGE 0 → → 24.15mV I = 0 (0 → → 24.13mV) 350 Ω → Ω → 353.5 Ω Ω FS 350 Ω Ω R LEAD 10.5 Ω ( Ω ( 10.904 Ω) Ω) OFFSET ERROR OVER TEMPERATURE = 0%FS GAIN ERROR OVER TEMPERATURE = –0.08%FS 2.14 a
KELVIN (4-WIRE) SENSING MINIMIZES ERRORS DUE TO LEAD RESISTANCE +V B + +FORCE R LEAD – 6-LEAD +SENSE BRIDGE V O – SENSE – R LEAD – FORCE + 2.15 a
CONSTANT CURRENT EXCITATION MINIMIZES WIRING RESISTANCE ERRORS V REF + I R LEAD – 4-LEAD BRIDGE V O R LEAD I V REF I R SENSE I = R SENSE 2.16 a
DRIVING REMOTE BRIDGE USING KELVIN (4-WIRE) SENSING AND RATIOMETRIC CONNECTION TO ADC +5V +5V/+3V +FORCE R LEAD AV DD DV DD 6-LEAD +SENSE + V REF BRIDGE AD7730 ADC + A IN V O – A IN 24 BITS – SENSE – V REF GND R LEAD – FORCE 2.17 a
TYPICAL SOURCES OF OFFSET VOLTAGE THERMOCOUPLE VOLTAGE ≈ ≈ 35µV/ °C × × (T1 – T2) + V B I B + T1 V OS + + T2 V O AMP – – I B – KOVAR COPPER PINS TRACES 2.18 a
AC EXCITATION MINIMIZES OFFSET ERRORS E OS = SUM OF ALL OFFSET ERRORS + V B NORMAL DRIVE E OS VOLTAGES – + + + V O V A = V O + E OS – - V A – V B = (V O + E OS ) – (– V O + E OS ) = 2 V O REVERSE E OS DRIVE + – – + VOLTAGES V O V B = – V O + E OS + – + V B 2.19 a
SIMPLIFIED AC BRIDGE DRIVE CIRCUIT + V B Q1 Q3 V 3,4 + SENSE V O – SENSE V 1,2 Q2 Q4 + V B V 1,2 Q1,Q2 ON Q1,Q2 ON V 3,4 Q3,Q4 ON Q3,Q4 ON 2.20 a
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