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

OPAMP circuits in Instrumentation
Passive Circuit Principles Analog Signal Conditioning Operational Amplifier (OP AMP) Introduction The purpose of this chapter is to introduce the reader to a variety of analog signal-conditioning methods used in process-control systems. Both passive methods and active methods, based upon the use of op amps, are defined. After reading this chapter and working through the examples and problems you will be able to. 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 variations is converted into a desired range of voltage variation. OPAMP circuits in Instrumentation Design Guidelines

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 variations is converted into a desired range of voltage variation. EKT314/4 - Electronic Instrumentation

Effect of the SC is described by the term transfer function.
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

Passive Circuit Principles Analog Signal Conditioning Operational Amplifier (OP AMP) Introduction OPAMP circuits in Instrumentation Design Guidelines

Signal Level and Bias Changes Filtering & Impedance matching
Concept of Loading Linearization Analog Signal Conditioning Principles Conversion Filtering & Impedance matching

Signal Level and Bias Changes
Adjusting the level (magnitude) and bias (zero values) of some voltage representing a process variable. -Example- sensor output (0.2V-0.6V). Further processing equipment requires voltage varies from 0V-5V. -SC- changing the zero to occur when the sensor output is 0.2V. Subtracting 0.2 from the sensor output (Zero Shift), or bias adjustment. -Sensor output-(0.2V-0.4V). Multiply the voltage by 12.5, the new ouput (0V-5V) This is called amplification, 12.5 (Gain) Some cases we need the sensor output to be smaller(attenuation). Both used circuit amplifier only gain is greater than or less than unity. EKT314/4 - Electronic Instrumentation

Linearization The purpose of linearization is to provide an output that varies linearly with some variable even if the sensor output does not. For example suppose a sensor output varied nonlinearly with a process variable, a linearization circuit would ideally conditioned the sensor output so that a voltage was produced which was linear with the process variable. Curtis Johnson Process Control Instrumentation Technology, 8e]

Convert one type of electrical variation into another.
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) -transmitting signal in wire 4-20mA (process control standard) current level in wire. -convert resistance and voltage level to an appropriate current level at transmitting end. -Convert the current back to voltage at the receiving end. Actually, you can use any range you choose! But if you do, you will have to design and manufacture both the transmitter and receiving controller to match. The 4-20 ma range is a "standard" adopted by tradition and also ANSI 50.1, and most process instruments and controllers use it. It was developed during a time when teletypewriters used a 20 ma current loop for communications, so parts for that type of circuit were readily available. There once was a ma current loop, but it has pretty much faded out. It does have several advantages: The 4 ma "bottom of span" signal allows the receiver to detect a broken wire or failed instrument. Since the loop current never falls to zero, the instrument can be loop powered, allowing for two-wire devices. 20 ma is low enough to be made intrinsically safe, making it easier to apply in hazardous locations. The constant-current feature of a current loop cancels out voltage drop errors due to long wiring runs (of course this would also be true if you selected different current values for zero and span). The 4-20 ma signal dropped across a 250 or 500 ohm resistor creates very convenient 1-5V and 2-10V voltage ranges, respectively, also fairly standard in the industry. So, while technically a manufacturer could use ma or V as their range, who would buy their pressure transmitter? No one! That's both the beauty and trap of a "standard". There is a huge investment out there in process controls that use 4-20 ma. That's the real reason, Im afraid. You will not change that until you find a different range that has some huge benefit over the current (pun intended) standard. EKT314/4 - Electronic Instrumentation

Filtering & Impedance matching
-Filtering-Eliminate unwanted signals in the process-control loop -Impedance matching-transducer internal impedance or line impedance can cause error in measurement of a dynamic variable. EKT314/4 - Electronic Instrumentation

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. 1. Calculate the output voltage, VAB, when in open circuit condition (no load resistor—meaning infinite resistance). This is VTh. 2. Calculate the output current, IAB, when the output terminals are short circuited (load resistance is 0). RTh equals VTh divided by this IAB. EKT314/4 - Electronic Instrumentation

FIGURE 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 All rights reserved.

EKT314/4 - Electronic Instrumentation

FIGURE 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. Vt=20mV/ ° Cx50V=1000mV=1V Vout=10Vx1V=10V Vin=Vt(1-5/(5+10k)=0.67 Vout=10(0.67)=6.7V Curtis Johnson Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

Divider circuits Bridge Circuits Analog Passive Circuits
Signal Conditioning Passive Circuits Bridge and divider circuits are two passive techniques that have been extensively used for signal conditioning for many years. Voltage divider- convert resistance variation into voltage variation. Bridge-accurate means of measuring changes in impedance especially when fractional changes in impedance are very small. RC filters-common in the industry environment to find signals that posses high and low frequency noise as well as the desired signal measurement data. RC Filters

FIGURE 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 All rights reserved.

Bridge Circuits Bridge circuits are used to convert impedance variations into voltage variations EKT314/4 - Electronic Instrumentation

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 All rights reserved.

Zero difference and zero voltage across the detector

*-ve result means that Vb larger Va.

FIGURE 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 All rights reserved.

Bridge resolution -A function of the resolution of the detector used to determine the bridge offset.

-Problem many effect that change the resistance.
FIGURE 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 All rights reserved.

FIGURE 2.8 The 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 All rights reserved.

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

Using Wheatstone bridge
FIGURE 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 All rights reserved.

ac bridges EKT314/4 - Electronic Instrumentation

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 All rights reserved.

FIGURE 2.11 The ac bridge circuit and components for Example 2.10.

Primary application of bridge circuits
FIGURE (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 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

FIGURE 2.13 Circuit for the 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 All rights reserved.

FIGURE 2.14 Response of the low-pass RC filter as a function of the frequency ratio.

Critical Frequency=fc
-ratio of the output to the input voltage is approximately 0.707 EKT314/4 - Electronic Instrumentation

Design Guideline Find the critical frequency that will satisfy the design criteria. EKT314/4 - Electronic Instrumentation

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 All rights reserved.

FIGURE 2.16 Response of the high-pass RC filter as a function of frequency ratio.

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

FIGURE 2.19 The response of a band-pass filter shows that high and low frequencies are rejected.

-critical frequencies be as far as possible -resistor ratio below 0.01
FIGURE 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 All rights reserved.

FIGURE 2.21 Band-pass response for the filter in Example 2.15.

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

FIGURE 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 All rights reserved.

FIGURE 2.23 One form of a band-reject RC filter is the twin-T.

FIGURE 2.24 The twin-T rejection notch is very sharp for one set of components.

Characteristics Analog Signal Conditioning Operational Amplifier Specification

FIGURE 2.25 The schematic symbol and response of an op amp.

FIGURE 2.25 (continued) The schematic symbol and response of an op amp.

FIGURE 2.26 The op amp inverting amplifier.

FIGURE 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 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 All rights reserved.

FIGURE 2.28 Some op amps provide connections for an input offset compensation trimmer resistor.

FIGURE 2.29 Input offset can also be compensated using external connections and trimmer resistors.

Current to Voltage Converter
Integrator Differentiator Current to Voltage Converter Voltage Follower OpAmp Circuit in Instrumentation Linearization Inverting Amplifier Voltage to Current Converter Non Inverting Amplifier Differential Instrumentation Amplifier

High impedance Low impedance
FIGURE 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 All rights reserved.

FIGURE 2.31 The op amp summing amplifier.

FIGURE 2.33 A noninverting amplifier.

FIGURE 2.34 The basic differential amplifier configuration.

FIGURE 2.35 An instrumentation amplifier includes voltage followers for input isolation.

FIGURE 2.39 A voltage-to-current converter using an op amp.

FIGURE 2. 40 A current-to-voltage converter using an op amp
FIGURE 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 All rights reserved.

FIGURE 2.41 An integrator circuit using an op amp.

FIGURE 2.42 This circuit takes the time derivative of the input voltage.

FIGURE 2.43 A nonlinear amplifier uses a nonlinear feedback element.

Range Parameter Input Impedance Analog Signal Conditioning Design Guideline Output Impedance

FIGURE 2.45 Model for measurement and signal-conditioning objectives.

EKT314/4 Electronic Instrumentation
Week 3 End

FIGURE 2.17 Cascaded high-pass RC filter for Example 2.13.

FIGURE 2.18 Analysis of loading for a high-pass RC filter in Example 2.14.

EKT314/4 Electronic Instrumentation

Similar presentations

Presentation on theme: "EKT314/4 Electronic Instrumentation"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

EKT314/4 Electronic Instrumentation

Similar presentations

Presentation on theme: "EKT314/4 Electronic Instrumentation"— Presentation transcript:

Similar presentations

About project

Feedback