EKT 314/4 WEEK 5 : CHAPTER 3 SIGNAL CONDITIONING ELECTRONIC INSTRUMENTATION

Chapter 3 Problem Statement Don’t know why signal conditioning needed. Don’t know where should the signal conditioning part to be located. Don’t know what actually the content of signal conditioning part. Don’t know what the function of signal conditioning parts.

Chapter 3 Objectives To understand why signal conditioning is important To know where exactly signal conditioning circuit exist To implement amplifier, modulator and filter circuit in system To suit each circuit to system implementation

Chapter 3 Content Introduction Signal Conditioning Circuit (SCC) Function Preliminary Requirement Signal Conditioning (SC) System Type Amplifier Modulator Filter

Chapter 3 Content Introduction Signal Conditioning Circuit (SCC) Function Preliminary Requirement Signal Conditioning (SC) System Type Amplifier Modulator Filter

Introduction

SCC Function 1. Perform Linear Task/Process 2. Perform Non-Linear Task/Process

Chapter 3 Content Introduction Signal Conditioning Circuit (SCC) Function Preliminary Requirement Signal Conditioning (SC) System Type Amplifier Modulator Filter

SCC Function 1. Perform Linear Task/Process › Amplification › Attenuation › Integration › Differentiation › Addition › Substraction 2. Perform Non-Linear Task/Process

SCC Function 1. Perform Linear Task/Process 2. Perform Non-Linear Task/Process › Modulation › Demodulation › Sampling › Filtering › Clipping & Clamping › Squaring & Linearizing › Multiplication

Chapter 3 Content Introduction Signal Conditioning Circuit (SCC) Function Preliminary Requirement Signal Conditioning (SC) System Type Amplifier Modulator Filter

Preliminary Requirement Passive Transducer Require excitation Require amplification Active Transducer Require amplification

Signal Excitation Only needed by passive transducer since they cannot generate their own voltage or current Excitation come from external source External source can be ac or dc

Chapter 3 Content Introduction Signal Conditioning Circuit (SCC) Function Preliminary Requirement Signal Conditioning (SC) System Type Amplifier Modulator Filter

SC System Type 1. DC Signal Conditioning System 2. AC Signal Conditioning System

SC System Type 1. DC Signal Conditioning System Generally used for common resistance transducers (e.g. potentiometer and straing gauges) 2. AC Signal Conditioning System Used for variable reactance transducers For systems where signal have to be transmitted via long cables to connect the transducers to signal conditioning equipment.

DC Signal Conditioning System

AC Signal Conditioning System

Signal Conditioning (SC) System Type DC SC SystemAC SC System

Chapter 3 Content Introduction Signal Conditioning Circuit (SCC) Function Preliminary Requirement Signal Conditioning (SC) System Type Amplifier Modulator Filter

Amplifier Two types Operational Amplifier (OpAmp) Instrumentation Amplifier

Amplifier Normal Amplifier Block Diagram OPAMPs Non-Inverting / Inverting Integrator / Differentiator Sum/Scale/Average Substractor Comparator Instrumentation Amplifier

Amplifier: Block Diagram Input Stage Dual input balanced output differential amplifer Double-ended high gain amplifier Gain = 60 Intermediate Stage Single-ended differential amplifier Dual input unbalanced output differential amplifier Gain = 30 Level Shifter Level translator circuit To bring dc output voltage to ground potential Output Stage Push-pull complementary amplifier Gain = 5 to 10

OPAMPs V o = AV id = A(V + - V - ) A – Large signal voltage gain V id – differential input voltage V + - noninverting terminal voltage V - - inverting terminal voltage

OpAmp Operation Non-Inverting Inverting Integrator Differentiator Summing Amplifier Scaling Amplifer Averaging Amplifier Substractor Comparator

Non-Inverting Amplifier Key: Input signal is applied to the non-inverting input terminal.

Inverting Amplifier Key: The input signal is applied to the inverting terminal.

OpAmp as Integrator

Key: Use C F instead of R F

Practical Integrator Circuit At low freq and dc signal, C F acts like an open circuit Close loop gain = open loop gain Produce too much output offset voltage.

Practical Integrator Circuit At zero freq (DC) and without negative feedback, the circuit treats the input offsets as a valid input signal charges the capacitor. This drive the output into +ve or –ve saturation.

Practical Integrator Circuit Insert R F in parallel with C F Resistor, R F used must be at least 10 times the input resistance, R 1

Integrator Application To use constant input voltage to produce a ramp voltage Analog Computer A/D Conversion Signal Wave Shaping

OpAmp as Differentiator

Key: Input resistor replaced by capacitor

Practical Differentiator Circuit This differentiator has tendency to undesirably oscillate The gain R F /X C1 increases at a rate of 20dB/decade with increases in frequency  become unstable

Practical Differentiator Circuit As the freq increases, input impedance X C decreases, making the circuit more susceptible to high frequency noise. This noise is superimposed after amplification, on the differential output signal.

Practical Differentiator Circuit Introduce R 1 and C F 0.01R F < R 1 < 0.1R F so close loop gain reduced to between -10 to -100.

Practical Differentiator Circuit Consideration: 1. f a is the highest freq range to be differentiated f a = 1/(2  R F C 1 )

Practical Differentiator Circuit Consideration: 2. f b is the gain limiting frequency at which the gain begins to decrease at a rate of 20db/decade. f b = 1/(2  R 1 C 1 ) R 1 C 1 =R F C F f b =20f a

Differentiator Application Detection of the leading and trailing edges of rectangular pulse Wave shaping circuit to detect high frequency components in an input signal. Rate of change detector in FM modulators. Triggering the time base generator in an oscilloscope.

OpAmp as Summing Key: Depends on relation of R a, R b, R c, R F, circuit can be summing, scaling or average amplifier.

Sum, Scale, Average Amplifier FunctionCondition 1SumR a =R b =R c =R F 2ScaleR a ≠R b ≠ R c 3AverageR a =R b =R c

OpAmp as Substractor Key: All R value are same.

OpAmp as Comparator Key: Opamp used without feedback. Amplifier goes either to saturation limit +V cc or – V EE One terminal considered as reference terminal. When V + > V -, V o  When V + < V -, V o 

OpAmp as Comparator If V - = GND, slight V + ++ result in V o = +V sat = +V CC If V - = GND, V + goes slightly below 0, V o = -V sat = -V EE Diodes used to protect opamp from damage due to excessive V in.

OpAmp as Comparator If V in feed to V + then it is called non-inverting comparator. If V in fed to V - then it is called inverting comparator.

Comparator Application Discriminator Voltage level detector Oscillator Digital interfacing Schmitt trigger

Amplifier Normal Amplifier Instrumentation Amplifier Important Features Difference with Normal Opamp

EKT 314/4 WEEK 5 : CHAPTER 3 END ELECTRONIC INSTRUMENTATION