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Signal Conditioning

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Chapter Objective

• To understand the important of signal conditioning circuits

• To design the signal conditioner circuits and

make the signals from sensors/transducers

suitable for the next stage, i.e. Control

System

• To understand the standard signals for industrial

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Standard Signal for Industrial Process

• Electrical:

current 4-20 mA,

voltage 1-5 Volt

• Mechanical: 3-15 psi

Normally, sensors/transducers didn’t provide direct

these standard signals. Therefore, an interfacing

circuit is required between sensors/transducers

and standard industrial instrument.

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• Signal conditioning circuits are dealing with the output signals from sensors/transducers in such a way to make these signals appropriate for the next stage such as a control system and data acquisition system.

• Signal conditioning circuits can include amplification, attenuation, filtering , signal converting , isolation , etc.

Sensors/

Transducers

Control

SystemsSignal

Conditioner

Transmission Transmission

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Basic idea

1. Convert the sensor output to voltage. (In case of a long distance transmission or bad environment, recommend to use current or frequency(digital) transmission )).

2. Remove unwanted signals(Filtering)

3. Modify the signal range (either increase or decrease) to obtain the maximum accuracy for the further processing i.e. try to match with the input range of A/D converter to maximize the resolution.

4. Additional Requirement: Signal Isolation(Sensor-Control): Magnetic isolation(Transformer) or Optical isolation(LED and photodectector)

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Common Signal Conditioning

• Voltage to Voltage:

-Inverting Amp

-Non-inverting Amp

-Summing and Subtracting Amp

-Difference Amp

-Instrumentation Amplifier(IA)

-Zero and Span adjustment

• Voltage to Current and Current to Voltage

• Voltage to Frequency and Frequency to Voltage

• Optoisolation Circuit

• Filtering: Passive and Active

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Instrumentation Amplifier(IA)

• IA is dedicated differential amplifier.

• It is optimized for a signal conditioning for low-level signals in large

amounts of noises.

• Gain is adjustable by a single resistor.

• Finite, accurate and stable gain, usually between 1 and 1000.

• Extremely high input impedance.

• Extremely low output impedance

• Extremely high CMRR(Common Mode Rejection Ratio).

• CMRR is the ratio of the gain of the amplifier for differential-mode

signals (signals that are different between the two inputs) to the gain

of the amplifier for common-mode signals (signals that are the same

at both inputs).

vd

vcm

ACMRR

A

outvd

VA

V V

( ) / 2

outvcm

VA

V V

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• 2 Stages configuration:input stage and unity-gain difference amplifier stage

- Input stage(voltage follower configuration): allows high input impedance to both input and the gain is set by only one resistor.+ eliminate common-mode signal

- Unity-gain difference amplifier stage:

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• Voltage at point 1 is equal to V1 and voltage at point 2 is equal to

V2(no potential difference between op-amp input ).

• V1-2=V1-V2 and IRgain=(V1-V2)/Rgain

• V3-4= IRgain*(Rgain+2R) , substitute IRgain

• Since second stage is unity gain difference(resistors are the same,

all R) , the output voltage is the same as V3-4..

)2

1)(( 12

gain

outR

RVVV

2( ) 1V

gain

RA overall gain

R

Note. Difference amplifier with adjustable gain(without R2 and R1, difference amp will be normal

high gain op-amp or comparator(output is saturated at either negative or positive level ) )

22 1

1

( )( )o

RV V V

R

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IA applications

• IA is normally used to amplify the small signals obtained form the transducers

such as strain-gauge where there are common-mode noises picking up to the

transmission line.

• Applications for an instrumentation amplifier are small differential voltage

measurements, i.e. bridge outputs, strain gauge outputs, or any low level

transducer.

• The common mode noise can be eliminated by IA.( high CMRR).

Strain gauge amplifier circuit:

IA

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IA applications

Thermocouple output to A/D converter:

Reference: http://www.intersil.com/

IA

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IA applications

RTD output to A/D converter:

Reference: http://www.intersil.com/

IA

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Zero and Span adjustment

Ein

VOUT

m=slope=Rf/Ri

b=offset=(Rf/Ros)Vos

Zero adjust

Ein

VOUTm=slope=Rf/Ri

Span adjust

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Zero and Span adjustment

• Rf and Ri adjust the span(gain-slope)

• ±Vos adjust the zero(offset)

+

_

Rf

VOUT1= -(mx+b)

Ri

Ros

R

+

_

R±Vos

Ein

VOUT2= (mx+b)

Inverting Summing Amp

Inverting Amp

os

os

f

in

i

f

OUT VR

RE

R

RV 2

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Example 1

• Design the zero and span converter using OP-AMP 741

and supply ±5V to convert Ein (-0.25 V to 0.25 V) to Vout

(0 V to 5 V)

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Solution

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Solution

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Example 2

• Design a signal conditioning circuit to measure the speed of motor using

RE.0444 NV B C1X20 CA IP44 11X30 (DC Tachogenerator) as sensor

(Datasheet is given below). The motor speed to be measured is between 0 to 200 revolution per minute( rpm ). The desired output voltage is 0 to 5 V

for connecting to an analog input port of microcontroller.

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Homework

• Design the zero and span converter using OP-AMP 741 working with power supplies ±5 V to convert Ein (2 V to 4 V) to

Vout (0 V to 5 V). Students must use standard resistor values

in the design.

Standard Variable Resistors

Standard Fixed Value Resistors