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EKT 314ELECTRONIC INSTRUMENTATION Elektronik Instrumentasi

Semester 2 2012/2013 Chapter 3

Analog Signal ConditioningSession 1 & 2

school of computer and communication engineering. universiti malaysia perlis

Mr. Fazrul Faiz Zakaria

Wednesday, March 27, 13

Signal Conditioning

Sensors produce different kind of signalsVoltage output or Current outputCan’t necessarily take sensor output and put right into Microprocessor ADC or logic inputSignal may need:

High to low impedance buffer, current to voltage conversion, gain, detection, & filtering.

The connection between a sensor and an interface circuit. In the image on the left, the sensor has voltage output. In the image on the right, the sensor has current output.

Wednesday, March 27, 13

Wednesday, March 27, 13

The ObjectivesExplain the purpose of analog signal conditioning.Design a Wheatstone bridge circuit to convert resistance change to voltage change.Design RC low-pass and high pass filter circuits to eliminate unwanted signals.Understanding the schematics of four common op amp circuits and provide the transfer functions.Explain the operation of an instrumentation amplifier and draw its schematic.Design an analog signal-conditioning system to convert an input range of voltages to some desired output range of voltage.Design analog signal conditioning so that some range of resistance variations is converted into a desired range of voltage variation.

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The Principles

Signal level and bias changesLinearization ConversionFiltering and impedance matchingConcept of loading

Convert signal to a form suitable for interface with other elements

Signal Conditioning

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Signal Level and bias changes

adjusting the level (magnitude) and bias (zero value) of some voltage representing a process variable amplification vs attenuation --> uses amplifier

Linearization

The purpose of linearization is to provide an output that varies linearly with some variable even if the sensor output does not.

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Conversion

Convert one type of electrical variation into another.Signal Transmission (Voltage to current, Current to Voltage converter)Digital Interface (ADC requires 0-5V input)

Filtering & impedance matching

Filtering-Eliminate unwanted signals in the process-control loopImpedance matching-transducer internal impedance or line impedance can cause error in measurement of a dynamic variable.

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Concept of loadingConcern -loading of one circuit by another.Thévenin's theorem for linear electrical networks states that any combination of voltage sources, current sources, and resistors with two terminals is electrically equivalent to a single voltage source Vth and a single series resistor Rth.

this equivalent voltage Vth is voltage obtained at the terminals A-B of the network with terminal A-B open circuitsthis equivalent resistance Rth is the resistance obtained at terminal A-B of the network with all its current source open circuited and all voltage sources short circuited

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An amplifier outputs a voltage that is 10 times the voltage on its input terminal. It has an input resistance

of 10 kΩ. A sensor outputs a voltage proportional to temperature with a transfer function of 20mV/ °C. The

sensor has an output resistance of 5.0 kΩ. If the temperature is 50 °C, find the amplifier output.

Example 1.1

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Solution for Signal Conditioning

Passive CircuitsDivider CircuitsBridge CircuitsRC Filters

Operational amplifier (OP-AMP)

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Divider CircuitsThe simple voltage divider can often be used to convert resistance variation into voltage variation.

R R RI R

VR RV

I RV V R RR

total 1 2

totaltotals

1 2s

total 2D s1 2

2

= += = +

= = +

D e m o

either R1 or R2 can be your sensorWednesday, March 27, 13

the sensor system shown above has R1=10Kohm and Vs=5V. suppose that R2 is our sensor whose resistance

varies from 4Kohm to 12Kohm as some dynamic variable varies over a range . Find (a) the minimum and maximum of VD (b) the range of output impedance and

(c) the range of power dissipated by R2

Example 3.2

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Bridge CircuitsBridge circuits are used to convert impedance variations into voltage variationsin the instrumentation situation this bridge is operate in the unbalance operationvoltage across the a & b point depend on the resistance ratio, when the ratio are equal where R1R4=R2R3, in that situation voltage across a & b will be zeroany change on the resistance value which cause the bridge unbalance, will produce a voltage across the a & b point.

Wednesday, March 27, 13

Bridge Circuits

R RR V and R R

R V

R RR V R R

R V

R R R RR R R R V

V potential of point a with respect to cV potential of point b with respect to c

V V V

V VVV

1 3

3s

2 4

4s

1 3

3s

2 4

4s

1 3 2 4

3 2 1 4s

ab

a b

a b: :

: :

:

T

T

T

= + = +

+ +

+ +

==

=

=

= -

-

-Q QT V V Y

D e m o

Wednesday, March 27, 13

Bridge CircuitsWhen a galvanometer is used for a null detector, it is convenient to use the Thévenin equivalent circuit of the bridge.

R R R RR R R R VV

1 3 2 4

3 2 1 4sth :+ += -Q QT V V YD e m o

R RR R

R RR RR

1 3

1 3

2 4

2 4th + += +D e m o

I R RV

Gth G

th= +

Wednesday, March 27, 13

A bridge circuit has resistance of R1=R2=R3=2Kohm and R4=2.05 Kohm and a 5V

of supply. If a galvanometer with a 50 ohm internal resistance is used for a detector, find

the offset current.

Example 3.3

Wednesday, March 27, 13

A bridge circuit has resistance of R1=R2=R3=R4=2Kohm and a 10V of supply.

clearly, the bridge is nulled. suppose a 31/2 digit DVM on a 200mV scale will be used for null

detector. find the resistance resolution for the measurements of R4

Example 3.4

Wednesday, March 27, 13

Bridge CircuitsLEAD COMPENSATIONWhen bridge circuit may be located at considerable distance from the sensor whose resistance changes are to be measured.Problem many effect that change the resistance.any changes in lead resistance are introduced equally into both arms of the bridge circuit, thus causing no effective change in bridge offset For remote sensor applications, this compensation system is used to avoid errors from lead resistance

Wire 3, is the power lead and has no influence on the bridge balanceIf wire 2 changes in resistance--> change R4. Wire 1 changes by thesame amount (& exposed to the same environment) --> change R3

Wednesday, March 27, 13

A bridge circuit is used with sensor located 100 m away. The bridge is not lead compensated and the cable to the sensor has a resistance of 0.45

ohm/ft. The bridge nulls with R1=3400 ohm, R2=3445 ohm and R3=1560 ohm.What is the

sensor resistance ?

Example 3.5

Wednesday, March 27, 13

Bridge CircuitsCurrent Balance BridgeThis method uses a current to null the bridge

R 4 >>R 5

R 2 +R 4( ) >>R 5

Now ;

Vb =V R 4 +R 5( )R 2 +R 4 +R 5

+ IR 5

so ;

ΔV =V R 3

R1+R 3

−R 4 +R 5

R 2 +R 4 +R 5

#

$%%

&

'((− IR 5

Wednesday, March 27, 13

A current balance bridge circuit has resistors R1=R2=10 Kohm, R3=1 Kohm R4=950 ohm and

R5=50 ohm and high-impedance null detector.find the current required to null the

bridge if R3 change by 1 ohm. the supply voltage is 10 V

Example 3.6

Wednesday, March 27, 13

Bridge CircuitsPrimary application of bridge circuits

To Convert variations of resistance into Variations of voltageIf the range of resistance variation is Small and centered about the null value Then then nonlinearity of voltage Resistance is small.

Bridge off-null voltage is clearly nonlinear for large-scale changes in resistance.

However, for small ranges of resistance change, the off-null voltage is nearly linear.

Wednesday, March 27, 13

RC Filters

To eliminate unwanted noise signals from measurements, it is often necessary to use circuits that block certain frequencies or bands of frequencies.

Wednesday, March 27, 13

RC FiltersLow Pass RC Filter

It blocks high frequencies and passes low frequencies.

fc = 12πRC

VoutVin

=1

1+ ffc

!

"#

$

%&

2

Wednesday, March 27, 13

RC FiltersDesign Guideline

find the critical frequency that will satisfy the design criteria

Wednesday, March 27, 13

A measurement signal has a frequency < 1 kHz, but there is unwanted noise at about 1MHz. Design a low pass filter that attenuates the

noise to 1%. what is the effect on the mesurement signal at its maximum of 1 kHz

Example 3.7

Wednesday, March 27, 13

RC FiltersHigh-Pass Filter

-Passes High frequencies-Blocks low frequencies

fc = 12πRC

VoutVin

=

ffc

!

"#

$

%&

1+ ffc

!

"#

$

%&

2

Wednesday, March 27, 13

Pulse for stepping motor are being transmitted at 2000 Hz. Design a filter to reduce 60 Hz

noise but reduce the pulses by no more than 3dB

Example 3.8

Wednesday, March 27, 13

Wednesday, March 27, 13