ee 4900: fundamentals of sensor design 1...2 ece 5900/6900 fundamentals of sensor design dr. suketu...

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ECE 5900/6900 Fundamentals of Sensor Design Dr. Suketu Naik

EE 4900: Fundamentals of Sensor Design

Lecture 14

Interface Electronics (Part 2)

2



Linearizing Bridge Circuits (Sensor Tech Hand

book)

Precision Op amps, Auto Zero Op amps,

Instrumentation Amplifiers (Art of Electronics)

Miniaturizing Sensor Systems

3


Linearizing Bridge Circuits

Why Linearize?-The change in resistance and hence voltage is very small

-The output of a Wheatstone Bridge with only a single

active resistive sensor is inherently nonlinear

- For all Bridge configurations: Nonlinearity comes from

variation of the resistors in the bridge, wiring resistance

and the sensor itself

R

R

R

R+∆R

Sensor

(Strain Gauge,

RTD, Thermistor)

4



R

R

R

R+∆R

Sensor-Temp

R

R

R-∆R

Sensors-Pressure

-FlowR+∆R

R-∆R

Sensors-load cells

-strain gaugesR+∆RR-∆R

R+∆R

Good Better

Best

Sensors-load cells

-strain gauges

Bridge Configuration: 1,2 or 4 element circuits

5



Current or Voltage Drive?Linearity Error for Current Drive Bridge

Constant current source: free of wiring resistance

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Amplification

Use small tolerance resistors, low noise opamp, and

decouple the power supply

Advantage:

-Quick (& Dirty)

Amplifier

-Single Power Supply

(use matched RF resistors

to bring up DC level to

Vs/2)

-Single Opamp

Disadvantage:

-Noise

-Nonlinear

-Low CMRR

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Precision, Low Noise Amplification

Use small tolerance resistor, low noise opamp, and

decouple the power supply

Advantage:

-Gain accuracy

-Balanced operation

(differential)

-High CMRR

-Small Nonlinearity

can be corrected in software

Disadvantage:

-Cost

-Gain: may need more

amplification

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Active Bridge with Single Sensor

1) Output voltage is equal in

magnitude but opposite in

polarity to the change in

sensing voltage due to ∆R

2) Linear output voltage

given small change ∆R

3) Second amplifier is

required

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Active Bridge with Two Sensors

1) Output voltage is equal in

magnitude but opposite in

polarity to the change in

sensing voltage due to ∆R

2) Linear output voltage

given small change ∆R:

twice the sensitivity as single

element

3) Second amplifier is

required

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Active Bridge with Single Sensor and Noniverting Amplifier

1) Bridge opamp maintains

the current level through

the sensor

2) Non-inverting opamp

requires precise matching

for accurate gain

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Problem:Wiring Resistance and Noise Pickup

1) Temperature rise will result in rise in the output voltage

2) Use RCOMP to offset the error, or adjust the value in software

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Solution1 :Use 3-Wire Connection

1) Increase in temperature causes the lead resistor to change equally in

each half of the divider

2) Sense lead is connected to high output impedance (no vol change)

3) The imbalanced offset error is reduced in half

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Solution2 : Kelvin (4-wire Connection) Sensing

1) Two sense-leads connect to high impedance opamp inputs: no current

draw

2) Op amp ensures that any voltage variation due to wiring resistance will

be negated and bias voltage VB (and 0 V at the bottom) is maintained

Must have

highly accurate

bias voltage VB

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Solution3 : Ratiometric Technique with Kelvin (4-wire

Connection) Sensing

ADC minimizes offset and gain errors

Eliminate

dependency on

VB

24-bit Σ∆ ADC

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Solution3 : Ratiometric Technique with Kelvin (4-wire

Connection) Sensing

ADC minimizes offset and gain errors

Eliminate

dependency on

VB

24-bit Σ∆ ADC

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book)




17


Precision Op Amps

Why use Precision Op-amps?

1)Control circuits: must be accurate,

stable with time and temperature,

and predictable

2) Measurement/Sensing circuits: must be accurate, must have high

CMRR and precise gain selection, and must be stable

Examples:

Strain gauge (350 ohm): typical response= +/-10 mV

at some DC level

Light detector/Photo diode/Photo transistor:

Stability is important

Thermocouple: Accuracy is important

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Precision Op Amps

Important Characteristics of a Precision Op-amp Circuit

1) Input Impedance

2) Input Bias Current

- Variation with temperature

- Variation with common-mode input voltage

3) Input Voltage Offset

4) Common-mode Rejection(Ratio of differential vol. output vs. common mode vol. output)

5) Power Supply Rejection(Change in power-supply causes change in output voltage)

Radiation Hardened Precision

Op-amps in Space Applications

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Precision Op Amps

Choosing a Precision Op-Amp: What to look for

1) Supply Voltage and Signal Range

2) Single-supply vs. Dual-supply

3) Offset voltage

4) Noise (voltage noise, current noise, and 1/f noise)

5) Bias Current

6) CMRR and PSRR

7) Gain Bandwidth Product (GBW), Transition

Frequency (fT), and Slew Rate

8) Harmonic Distortion

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Auto Zero or Zero Drift Op Amps

Auto Zero Op-amp Characteristics1) Ultra-high precision

2) Very small input offset voltage (e.g. 2 mV)

3) Large Gain Bandwidth Product (e.g. 2 MHz)

4) Very Low Noise (e.g. 50 nV/√Hz)

5) Low Voltage Supply (e.g. 3.3 V, 5 V)

When to use them?

Load cells, Thermocouples, slow but accurate

measurements

Example: Microchip MCP6V26

Input Offset Voltage Drift

MCP6V26 Specifications

CMRR and PSRR vs Temperature

21


Auto Zero or Zero Drift Op Amps

Example CircuitUltra-high precision measurement

22


Instrumentation Amplifiers

Instrumentation Op-amp Characteristics1) Differential In, Single-ended out

2) Very high input impedance (10 MΩ-10 GΩ)

3) Wide gain (G=1-1000)

4) Very high CMRR at high gains (110-140 dB

at G=100

When to use them?

Strain gauge, Audio, Weigh scales/ Load cells, ECG and medical

electronis, process control

Example: Analog Device AD620

CMRR vs Frequency Voltage Noise vs Frequency

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Three Op-amp Design

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AD620 Instrumentation Amplifier

1) The input transistors Q1 and Q2

provide lower input bias current

through Superϐeta processing

2) Feedback through the Q1-A1-R1

loop and the Q2-A2-R2 loop maintains

constant collector current of the input

devices Q1 and Q2: this creates the

input voltage across RG

3) This creates a differential gain from

the inputs to the A1/A2 outputs given

by G = (R1 + R2)/RG + 1

4) The unity-gain subtractor, A3,

removes any common-mode signal,

yielding a single-ended output referred

to the REF pin potential

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AD620 Instrumentation Amplifier CircuitsPressure Monitor at 5 V Single Supply

ECG Monitor Circuit Voltage-to-current Converter

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Programmable Gain (Instrumentation) Amplifiers

Gain is programmable by microprocessor

Robotic arm with thermistor and torque sensor:

programmable gain amplifier with output directly interfaced with ADC (3.3 V)

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book)




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Data acquisition, process control and measurement can be

miniaturized with Smart Sensors

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What is a Smart Sensor?

Microcontroller Unit (MCU) must consume low power

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Analog Devices Microconverter Series

ADUC7124

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Analog Devices Microconverter Series

ADUC842

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Texas Instruments Wireless Sensor Node with low power

MCU eZ430-RF2500 kit

MSP430F2274

microcontroller CC2500 2.4-GHz

wireless transceiver

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Silicon Labs Wireless MCU Development Kit 1064-434-DK

1064-434-DK

(434 MHz)Si106x/8x Wireless MCU

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Xbee Wireless Sensor Node with Solar Power and Arduino-

Xbee Shield (Seedstudio)

Xbee is based on IEEE 802.15.4 networking protocol

for point-to-multipoint or peer-to-peer networkingRef:

https://www.sonoma.edu/users/f/farahman/sonoma/courses/cet543/resources/802_intro_01655947.pdf

Solar powered Xbee module Xbee-Arduino Shield

ee 4900: fundamentals of sensor design 1...2 ece 5900/6900 fundamentals of sensor design dr. suketu...

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