a RESISTANCE OF POPULAR SENSORS 120 , 350 , 3500 I Strain - - PowerPoint PPT Presentation

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Feb 12, 2024 11 likes •227 views

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

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2.0 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

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2.1 RESISTANCE OF POPULAR SENSORS

I Strain Gages 120Ω Ω, 350Ω Ω, 3500Ω Ω I Weigh-Scale Load Cells 350Ω Ω - 3500Ω Ω I Pressure Sensors 350Ω Ω - 3500Ω Ω I Relative Humidity 100kΩ Ω - 10MΩ Ω I Resistance Temperature Devices (RTDs) 100Ω Ω , 1000Ω Ω I Thermistors 100Ω Ω - 10MΩ Ω

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2.2 MEASURING RESISTANCE INDIRECTLY USING A CONSTANT CURRENT SOURCE

VOUT I R R = = + + ( ) ∆ ∆

R + ∆ ∆R I

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2.3 THE WHEATSTONE BRIDGE

VO R4 R1 R3 R2 VB VO R R R VB R R R VB = = + + − − + + 1 1 4 2 2 3 = = − − + +             + +             R R R R R R R R VB 1 4 2 3 1 1 4 1 2 3 AT BALANCE, VO IF R R R R = = = = 1 4 2 3 +

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2.4 OUTPUT VOLTAGE AND LINEARITY ERROR FOR CONSTANT VOLTAGE DRIVE BRIDGE CONFIGURATIONS

R R R R+∆ ∆R R+∆ ∆R R+∆ ∆R R+∆ ∆R R+∆ ∆R R−∆ −∆R R+∆ ∆R R−∆ −∆R R R R R−∆ −∆R VB VB VB VB

VO VO VO VO

(A) Single-Element Varying (B) Two-Element Varying (1) (C) Two-Element Varying (2) (D) All-Element Varying Linearity Error: VO: 0.5%/% 0.5%/% VB 4 ∆ ∆R ∆ ∆R 2 R + VB 2 ∆ ∆R ∆ ∆R 2 R + VB 2 ∆ ∆R R VB ∆ ∆R R R

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2.5 OUTPUT VOLTAGE AND LINEARITY ERROR FOR CONSTANT CURRENT DRIVE BRIDGE CONFIGURATIONS

R R R R+∆ ∆R R+∆ ∆R R+∆ ∆R R+∆ ∆R R+∆ ∆R R−∆ −∆R R+∆ ∆R R−∆ −∆R R R R R−∆ −∆R

VO VO VO VO

IB IB IB IB VO: Linearity Error: 0.25%/% IBR 4 ∆ ∆R ∆ ∆R 4 R + IB 2 ∆ ∆R IB ∆ ∆R IB 2 ∆ ∆R (A) Single-Element Varying (B) Two-Element Varying (1) (C) Two-Element Varying (2) (D) All-Element Varying R

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2.6 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

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2.7 USING A SINGLE OP AMP AS A BRIDGE AMPLIFIER FOR A SINGLE-ELEMENT VARYING BRIDGE

VB +VS R R R R+∆ ∆R RF RF + − − VS 2

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2.8 USING AN INSTRUMENTATION AMPLIFIER WITH A SINGLE-ELEMENT VARYING BRIDGE

VB R R R + − − IN AMP REF VOUT RG +VS

R+∆ ∆R

* SEE TEXT REGARDING

SINGLE-SUPPLY OPERATION VB 4 ∆ ∆R ∆ ∆R 2 R + VOUT = GAIN

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2.9 LINEARIZING A SINGLE-ELEMENT VARYING BRIDGE METHOD 1

VB R R R R+∆ ∆R + − − +VS

VOUT VB R R = = − −             ∆ ∆ 2

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2.10 LINEARIZING A SINGLE-ELEMENT VARYING BRIDGE METHOD 2

+ − − +VS

R R+∆ ∆R R R + − − +VS

VB R2 R1 VOUT

VOUT VB R R R R = =             + +             2 1 2 1 ∆ ∆

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2.11 LINEARIZING A TWO-ELEMENT VARYING BRIDGE METHOD 1 (CONSTANT VOLTAGE DRIVE)

VB R R R+∆ ∆R + − − +VS

VOUT VB R R = = − −             ∆ ∆ R+∆ ∆R

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2.12 LINEARIZING A TWO-ELEMENT VARYING BRIDGE METHOD 2 (CONSTANT CURRENT DRIVE)

R R + − − IN AMP REF VOUT RG +VS

R+∆ ∆R

* SEE TEXT REGARDING

SINGLE-SUPPLY OPERATION + − − R+∆ ∆R +VS

VREF RSENSE IB IB VOUT = IB ∆ ∆R 2 GAIN

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2.13 ERRORS PRODUCED BY WIRING RESISTANCE FOR REMOTE RESISTIVE BRIDGE SENSOR

+

0 →

→ 23.45mV (5.44mV → → 28.83mV) VO 350Ω Ω 350Ω Ω 350Ω Ω RCOMP 21Ω Ω 350Ω → Ω → 353.5Ω Ω FS +10V RLEAD 10.5Ω ( Ω (10.904Ω) Ω) RLEAD 10.5Ω ( Ω (10.904Ω) Ω) STRAIN GAGE 100 FEET, 30 GAGE COPPER WIRE = 10.5Ω Ω @ 25° °C TC = 0.385%/° °C ASSUME +10° °C TEMPERATURE CHANGE NUMBERS IN ( ) ARE @ +35° °C OFFSET ERROR OVER TEMPERATURE = +23%FS GAIN ERROR OVER TEMPERATURE = –0.26%FS

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2.14 3-WIRE CONNECTION TO REMOTE BRIDGE ELEMENT (SINGLE-ELEMENT VARYING)

+

0 →

→ 24.15mV (0 → → 24.13mV) VO 350Ω Ω 350Ω Ω 350Ω Ω 350Ω → Ω → 353.5Ω Ω FS +10V RLEAD 10.5Ω ( Ω (10.904Ω) Ω) RLEAD 10.5Ω ( Ω (10.904Ω) Ω) STRAIN GAGE 100 FEET, 30 GAGE COPPER WIRE = 10.5Ω Ω @ 25° °C TC = 0.385%/° °C ASSUME +10° °C TEMPERATURE CHANGE NUMBERS IN ( ) ARE @ +35° °C OFFSET ERROR OVER TEMPERATURE = 0%FS GAIN ERROR OVER TEMPERATURE = –0.08%FS I = 0

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2.15 KELVIN (4-WIRE) SENSING MINIMIZES ERRORS DUE TO LEAD RESISTANCE

6-LEAD BRIDGE RLEAD RLEAD +SENSE – SENSE +FORCE – FORCE + + +VB – – VO

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2.16 CONSTANT CURRENT EXCITATION MINIMIZES WIRING RESISTANCE ERRORS

4-LEAD BRIDGE RLEAD + – RLEAD RSENSE VREF VO I I I I = VREF RSENSE

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2.17 DRIVING REMOTE BRIDGE USING KELVIN (4-WIRE) SENSING AND RATIOMETRIC CONNECTION TO ADC

+5V AVDD GND + AIN – AIN + VREF – VREF RLEAD RLEAD 6-LEAD BRIDGE

AD7730 ADC 24 BITS

+SENSE – SENSE

VO

+FORCE – FORCE DVDD +5V/+3V

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2.18 TYPICAL SOURCES OF OFFSET VOLTAGE

+ VB VOS VO + – T1 T2 COPPER TRACES KOVAR PINS IB+ IB– + – THERMOCOUPLE VOLTAGE ≈

≈ 35µV/ °C × × (T1 – T2)

AMP

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2.19 AC EXCITATION MINIMIZES OFFSET ERRORS

+ VB + VB EOS EOS + – + – VO VO + – + – VA = VO + EOS +

– VB = – VO + EOS VA – VB = (VO + EOS) – (– VO + EOS) = 2 VO EOS = SUM OF ALL OFFSET ERRORS REVERSE DRIVE VOLTAGES NORMAL DRIVE VOLTAGES

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