Transcript EKT314/4 Electronic Instrumentation

EKT314/4
Electronic Instrumentation
Week 4-5
Chapter 3: Analog Signal
Conditioning
REFERENCE BOOK FOR ANALOG SIGNAL CONDITIONING
EKT314/4 - Electronic Instrumentation
2
Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
3
Introduction
Objectives
1.Explain the purpose of analog signal conditioning.
2.Design a Wheatstone bridge circuit to convert resistance change to voltage
change.
3.Design RC low-pass and high pass filter circuits to eliminate unwanted
signals.
4.Draw the schematics of four common op amp circuits and provide the
transfer functions.
5.Explain the operation of an instrumentation amplifier and draw its
schematic.
6.Design an analog signal-conditioning system to convert an input range of
voltages to some desired output range of voltage.
7.Design analog signal conditioning so that some range of resistance variation
is converted into a desired range of voltage variation.
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Signal Conditioning
-Operations performed on signals to
convert them to a form suitable for
interfacing with other elements in the
process-control loop. This chapter only
concern about analog conversions.
-Effect of the SC is described by the term
transfer function.
EKT314/4 - Electronic Instrumentation
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Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
7
Signal Level
and Bias
Changes
Concept of
Loading
Analog
Signal
Conditioning
Principles
Linearization
Conversion
Filtering &
Impedance
matching
8
Signal Level
and Bias
Changes
Adjusting the level
(magnitude) and bias (zero
values) of some voltage
representing a process
variable.
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Linearization
The purpose of linearization
is to provide an output that
varies linearly with some
variable even if the sensor
output does not.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
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)
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Filtering &
Impedance
matching
-Filtering-Eliminate
unwanted signals in the
process-control loop
-Impedance matchingtransducer internal
impedance or line
impedance can cause error in
measurement of a dynamic
variable.
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Concept of
Loading
Concern -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 V and a single
series resistor R.
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FIGURE 2.2
The Thévenin equivalent circuit of a sensor allows easy visualization of how loading occurs.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
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EKT314/4 - Electronic Instrumentation
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FIGURE 2.3
If loading is ignored, serious errors can occur in expected outputs of circuits and gains of amplifiers.
Example: 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.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
17
Divider
circuits
Analog
Signal
Conditioning
Bridge
Circuits
Passive
Circuits
RC Filters
18
FIGURE 2.4
The simple voltage divider can often be used to convert resistance variation into voltage variation.
Divider
circuits
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKT314/4 - Electronic Instrumentation
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Example
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Bridge
Circuits
Bridge circuits are used to convert
impedance variations into voltage
variations
EKT314/4 - Electronic Instrumentation
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FIGURE 2.5
The basic dc Wheatstone bridge.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
if
Zero difference and zero voltage
across the detector
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Example
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Example
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FIGURE 2.6
When a galvanometer is used for a null detector, it is convenient to use the Thévenin equivalent circuit of the bridge.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Example
EKT314/4 - Electronic Instrumentation
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Bridge resolution
-A function of the resolution of the
detector used to determine the bridge
offset.
EKT314/4 - Electronic Instrumentation
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Example
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FIGURE 2.7
For remote sensor applications, this compensation system is used to avoid errors from lead resistance.
LEAD COMPENSATION
-When 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
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.8
The current balance bridge.
CURRENT BALANCE BRIDGE
-this method uses a current to null
the bridge
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Example
EKT314/4 - Electronic Instrumentation
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Potential measurement using bridges
-A bridge circuit is useful for measuring small
potentials at very high impedance using either a
Wheatstone bridge or current balance bridge.
-performs by placing the potential to be
measured in series with the detector.
EKT314/4 - Electronic Instrumentation
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FIGURE 2.9
Using the basic Wheatstone bridge for potential measurement.
Using Wheatstone bridge
Using Current balance bridge
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
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Example
EKT314/4 - Electronic Instrumentation
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Example
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ac bridges
EKT314/4 - Electronic Instrumentation
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FIGURE 2.10
A general ac bridge circuit.
ac bridges
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
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FIGURE 2.11
The ac bridge circuit and components for Example 2.10.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Example
EKT314/4 - Electronic Instrumentation
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FIGURE 2.12 (a) Bridge off-null voltage is clearly nonlinear for large-scale changes in resistance. (b) However, for small ranges of
resistance change, the off-null voltage is nearly linear.
Primary application of bridge circuits
-To Convert variations of resistance into
Variations of voltage
If the range of resistance variation is
Small and centered about the null value Then
then nonlinearity of voltage Resistance is
small.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
RC Filters
To eliminate unwanted noise signals
from measurements, it is often
necessary to use circuits that block
certain frequencies or bands of
frequencies.
EKT314/4 - Electronic Instrumentation
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FIGURE 2.13
Circuit for the low-pass RC filter.
Low-pass RC Filter
-It blocks high
frequencies and
passes low
frequencies.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.14
Response of the low-pass RC filter as a function of the frequency ratio.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Critical Frequency=fc
-ratio of the output to the input voltage is approximately 0.707
EKT314/4 - Electronic Instrumentation
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Design Guideline
Find the critical frequency that will satisfy the design criteria.
EKT314/4 - Electronic Instrumentation
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Example
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EKT314/4 - Electronic Instrumentation
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FIGURE 2.15
Circuit for the high-pass RC filter.
High-Pass Filter
-Passes High frequencies
-Blocks low frequencies
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.16
Response of the high-pass RC filter as a function of frequency ratio.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Example
EKT314/4 - Electronic Instrumentation
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Band-Pass Filter
Filter that blocks frequencies below a
low limit and above a high limit while
passing frequencies between the
limits.
EKT314/4 - Electronic Instrumentation
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FIGURE 2.19
The response of a band-pass filter shows that high and low frequencies are rejected.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.20
A band-pass RC filter can be made from cascaded high-pass and low-pass RC filters.
Good passband filter.
-critical frequencies be as far
as possible
-resistor ratio below 0.01
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.21
Band-pass response for the filter in Example 2.15.
Example
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKT314/4 - Electronic Instrumentation
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Band-Reject Filter
-Filter that blocks specific range of
frequencies.
-Difficult to design using RC
combinations, possible using inductor
and capacitors
-Most success using active circuits.
EKT314/4 - Electronic Instrumentation
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FIGURE 2.22
Response of a band-reject, or notch, filter shows that a middle band of frequencies are rejected.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.23
One form of a band-reject RC filter is the twin-T.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.24
The twin-T rejection notch is very sharp for one set of components.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Example
EKT314/4 - Electronic Instrumentation
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EKT314/4 - Electronic Instrumentation
63
Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
64
Characteristics
Analog
Signal
Conditioning
Operational
Amplifier
Specification
65
FIGURE 2.25
The schematic symbol and response of an op amp.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.25 (continued)
The schematic symbol and response of an op amp.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.26
The op amp inverting amplifier.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.27
Nonideal characteristics of an op amp include finite gain, finite impedance, and offsets.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.27 (continued)
Nonideal characteristics of an op amp include finite gain, finite impedance, and offsets.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.28
Some op amps provide connections for an input offset compensation trimmer resistor.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.29
Input offset can also be compensated using external connections and trimmer resistors.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
73
Current to
Voltage
Converter
Linearization
Voltage to
Current
Converter
Differentiator
Integrator
Voltage Follower
OpAmp Circuit
in
Instrumentation
Differential
Instrumentation
Amplifier
Inverting
Amplifier
Non
Inverting
Amplifier
74
FIGURE 2.30
The op amp voltage follower. This circuit has unity gain but very high input impedance.
High impedance
Low impedance
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.31
The op amp summing amplifier.
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Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.33
A noninverting amplifier.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.34
The basic differential amplifier configuration.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.35
An instrumentation amplifier includes voltage followers for input isolation.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKT314/4 - Electronic Instrumentation
80
FIGURE 2.39
A voltage-to-current converter using an op amp.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKT314/4 - Electronic Instrumentation
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EKT314/4 - Electronic Instrumentation
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FIGURE 2.40 A current-to-voltage converter using an op amp. Care must be taken that the current output capability of the op amp is
not exceeded.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.41
An integrator circuit using an op amp.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.42
This circuit takes the time derivative of the input voltage.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 2.43
A nonlinear amplifier uses a nonlinear feedback element.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
88
Range
Parameter
Analog
Signal
Conditioning
Design
Guideline
Input
Impedance
Output
Impedance
89
FIGURE 2.45
Model for measurement and signal-conditioning objectives.
Curtis Johnson
Process Control Instrumentation Technology, 8e]
Copyright ©2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKT314/4 - Electronic Instrumentation
91
EKT314/4 - Electronic Instrumentation
92
Principles
Introduction
Analog
Signal
Conditioning
Design
Guidelines
Passive
Circuit
Operational
Amplifier
(OP AMP)
OPAMP circuits
in
Instrumentation
93
EKT314/4
Electronic Instrumentation
Week 3
End