Transcript EKT314/4 - ELECTRONIC INSTRUMENTATION

ELECTRONIC
INSTRUMENTATION
EKT314/4
4. Signal Conditioning Circuits
Contents



Introduction
Amplifiers
Filters
2
Introduction

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
Signal from detector stage has to be modified
(conditioned) in order to make it more
usable.
This signals is then to be use in later stage of
system that may consist of indicating,
recording and processing elements.
Proper selection of signal conditioning circuit
can improve the quality and system
performance.
Signal conditioning functions can be
amplification and filtering.
3
Example… Electronic-Aided
Measurement
Source: [Kalsi2005] pg. 461
4
Signal Conditioning Functions

Amplification



Filtering

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
Increase the level of input signal to better suit the
DAQ.
Improve the sensitivity and resolution of the
measurement.
Reject useless noise within certain frequency
range.
Prevent signal aliasing and distortion.
Attenuation

Contrary to amplification.
5
Signal Conditioning Functions
…continued

Isolation


Multiplexing
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Sequentially transmit a number of signals into
single digitiser.
Simultaneous Sampling

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Solve improper grounding problem of the system.
Issue of measuring more than one signals at the
same time.
Digital Signal Conditioning
6
DC Signal Conditioning System
Source: [Kalsi2005] pg. 462
7
DC Signal Conditioning
System… continued
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DC bridge can be Wheatstone’s Bridge which
can be balanced by a potentiometer or can
be calibrated for unbalanced conditions.
Amplifier should be thermally good and stable
for a long term.
Low pass filter for eliminating high
frequencies components or noise.
Main disadvantages – problem of drift.
8
AC Signal Conditioning System
Source: [Kalsi2005] pg. 462
9
AC Signal Conditioning
System… continued
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AC system is to overcome the problem of DC
system.
Transducer can be variable resistance or
variable inductance types.
Bridge circuit for modulating the amplitude of
the output from the transducer stage.
The signal is then amplified and demodulated
before pass through the low pass filter.
10
Contents



Introduction
Amplifiers
Filters
11
Amplifiers
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

Required in the system to improve the signal
strength which is typically in the low level
range of less than a few mV.
In some cases, amplifiers is necessary in
providing impedance matching and isolation.
Basic characteristics involved in designing
amplifiers are:




Input impedance
Output impedance
Gain and frequency response
Noise
12
Input Impedance
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Input impedance of an amplifier regularly
depends on the output impedance from the
transducer stage.
Source impedance may vary from few Ω up to
hundred MΩ.
Considering the loading effect of the input
impedance of the amplifier to the transducer,
the effective input, ei can formulated as
follows.
Rin
ei 
es
Rin  Rs
13
Input Impedance
14
Input Impedance… continued


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The error between effective input and source voltage
reflect the overall sensitivity of the system.
A very high input impedance (approaching infinity)
amplifier can be used to reduced an error.
Practically it is simpler to design an amplifier with
input impedance of 10 to 50 times source impedance
and calibrate the system sensitivity combining the
effect of the amplifier itself and the transducer.
In some cases, the amplifier with a very high input
impedance is needed to overcome the changes in the
sensitivity of the system.
15
Input Impedance… continued
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
High input impedance is not always a good thing
though, for example if you want to transfer as much
power as possible then the source and load
impedance should be equal.
For a current input a low input impedance (ideally
zero) is desired, for example in a
transimpedance (current to voltage) amplifier.
16
Output Impedance
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Output impedance required for an amplifier
depends on the input impedance of the next subsystem.
The value of output impedance can be explicit so
that the loading effect for next sub-system can
be calibrated.
Generally the output impedance on an amplifier
need to be sufficiently low enough (less than an
Ω).
Beside output impedance, the factor of output
drive capability of an amplifier also need to
considered.
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Gain


Gain of an amplifier is the result of an amplification
of the input signal.
Gain factor of an amplifier can be generally
expressed as:
aout
A
ain


aout and ain can be output and input power or voltage.
Amplifier gain also can be expressed in Decibel (dB).
A( dB )
 Pout 
 Vout 
  20 log

 10 log

V 
P
in


 in 
18
Gain… continued
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

Considering the relativity of the input
impedance of an amplifier and the output
impedance of the transducer, there are
attenuation occur before amplification.
The effective amplification is the product of
attenuation and the gain factor of the
amplifier.
Amplifier gain affect the system’s sensitivity
and calibration, therefore the high gain
stability become and important criteria.
19
Frequency Response
The frequency response refers
to the variation of the gain of
an amplifier with the
frequency.
•Usually specified in terms of
frequency when the -3dB take
place in the amplifier’s gain.
•Typically the frequency
response of an amplifier tends
to reduce the amplification at
high frequencies.
•The response often affected
by the output impedance of
the amplifier. Need to consider
the final load impedance when
calculating frequency response
of the system.
•
20
Noise



The performance of the amplifiers
that use an active devices are
influenced by noise that probably
internally generated.
Noise added usually frequency
dependent and its spectral density
can be expressed as noise
voltage, current and power.
At very low frequencies, the noise
spectral density increase due to
the flicker noise (pink noise, 1/f)
whereas at high frequencies the
spectral increase due to the
limitations of the frequency
response of the amplifier.
21
DC Amplifiers
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An amplifier that can produce a constant DC output signal.
Not necessary produce a zero DC output when input is zero.
This error known as offset voltage and it is depends on the input signal not
the amplifier gain. This offset voltage also varies with time and temperature.
The presence of finite DC current at the input of the amplifier is another
source of error in DC amplifier.
This DC bias current will create additional offset errors depending on the DC
output impedance.
22
Operational Amplifier

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DC amplifier with high gain in the
form of integrated circuit.
Can be used to performs an
important functions like isolation,
addition, inversion, multiplication,
subtraction and division.
Other mathematical operations can
be perform such as integration and
differentiation.
Variety of operational amplifiers
available commercially with different
specifications. For the purpose of
this course, the discussion is focus
on the 741 op-amps.
23
Operational Amplifier…
continued


Operational amplifier typically have more than one
cascaded differential amplifiers which led to achieving a
very high overall gain.
As in figure below, every stage give different value of gain
and in the end the total gain is obtained by a
multiplication of them all.
Source: [Kalsi2005] pg. 464
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Operational Amplifier…
continued
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From figure, the first stage is dual input balanced output
differential amplifier with constant current source that can
have gain up to 60. This stage contribute huge voltage
gain to the overall gain and also the input impedance is
resolve here.
Intermediate stage is where the overall gain is increased
furthermore with the dual input unbalanced output
differential amplifier. Intermediate stage can have gain up
to 30.
Level shifter is used to stabilize the high DC output
voltage from intermediate stage to the ground potential.
Last stage is push-pull amplifier with low output
impedance and minimum offset that have a gain up to 10.
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Ideal Operational Amplifier

Characteristics of ideal operational amplifier
can be listed as follows:
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Infinite input impedance
Zero output impedance
Infinite open loop gain
Infinite bandwidth, slew rate and CMRR
For ideal operational amplifier the output
voltage is zero whenever there is equal
voltage is applied to both of its inputs.
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Common Mode Rejection Ratio
27
Equivalent circuit of
operational amplifier

Output voltage can be
expressed as follows;
Vo = A(V1 – V2)
where A is large signal
voltage gain, V1 is voltage at
inverting terminal and V2 is
voltage at non-inverting
terminal, Vid in the figure is
differential input voltage,
take note that V1 and V2 are
both with respect to ground
Source: [Kalsi2005] pg. 465
28
741 Operational Amplifier
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For this actual operational amplifiers, the
characteristics is contrary to the ideal
operational amplifier as it does not have
infinite input impedance, non-zero output
impedance and input current.
Other important factor is offset voltage.
The µA741 operational amplifier is
manufactured by Fairchild Semiconductor
(LM741) and its specifications are discussed
in detail here.
29
741 Operational Amplifier…
continued

µA741 features
(adapted from LM741
datasheet by Fairchild
Semiconductor)
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Internal frequency
compensation
Excellent temperature
stability
Offset voltage null
capability
High input voltage range
Short circuit protection
30
Parameters of 741 Operational
Amplifier

There are typical parameters that associated
with µA741 (or any other) operational
amplifier that need to be taken into account
when using it the design.
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Absolute maximum ratings
Input parameters
Output parameters
Dynamic and other parameters
These parameters generally has been
presented in the operational amplifier
datasheet by the manufacturer.
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Absolute Maximum Ratings
#find example diff input/input

Supply Voltage
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Differential Input Voltage
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This value indicated the maximum voltage that can be applied across the + and – negative
inputs of the operational amplifier.
Input Voltage


This is the value of the maximum positive or negative voltage that can be supply to the
operational amplifier safely.
Maximum voltage that can be applied simultaneously between both input with reference to
ground.
Output Short Circuit Duration

The amount of time that the operational amplifier’s output can be shorted to supply voltage.
32
Absolute Maximum Ratings…
continued

Power Dissipation

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Operating Temperature Range


Maximum power that operational amplifier can
dissipate with respect to certain temperature.
Ambient temperature range that the operation of
operational amplifier meet the manufacturer's
specifications.
Storage Temperature Range

The range of temperature that the operational
amplifier can be stored into.
33
Absolute Maximum Ratings…
continued
Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001.
34
Input Parameters

Input Offset Voltage
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Input Offset Current
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Average of both current flowing into both of the inputs.
Input Resistance
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Differences of two input bias current when the output
voltage is zero.
Input Bias Current
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Voltage that must be applied to one of the input pins in
order to give zero output voltage.
Resistance of the operational amplifier at either input when
the other grounded.
Input Voltage Range

Voltage that common to both inputs and ground.
35
Input Parameters… continued
Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001.
36
Output Parameters

Output Short Circuit Current
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Output Voltage Swing
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Maximum output current that the operational
amplifier can deliver to the load.
Maximum output voltage (peak) that the
operational amplifier can give without distortion or
clipping. This depends on the load resistance.
Output Resistance

Resistance at the operational amplifier’s output.
37
Output Parameters…
continued
Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001.
38
Dynamic Parameters

Large Signal Voltage Gain
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
Slew Rate
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The ratio of the maximum voltage swing to the
change in the input voltage required to drive the
output from zero to a specified voltage.
Rate of change of the output voltage with the
operational amplifier having a unity gain.
Open-Loop Voltage Gain

Output voltage to input voltage ratio of the
operational amplifier without feedback.
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Other Parameters

Supply Current
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Common Mode Rejection Ratio
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
A quantify of the ability of the operational amplifier to reject
signals that are simultaneously present at both inputs
Bandwidth
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
Current that the operational amplifier will draw from the
supply.
Usually specified as unity gain bandwidth.
Power Consumption

The power consume by the operational amplifiers when
operated.
40
Dynamic and Other
Parameters
Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001.
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Comparison of Operational
Amplifiers
Source: [Rangan2004] pg. 243
42
Type of amplifier circuits

Several amplifier circuits can be
constructed using the operational
amplifier (such as µA741). These are:

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
Non-Inverting Amplifier
Inverting Amplifier
Differential Amplifier
Instrumentation Amplifier
43
Non-Inverting Amplifier

A close loop gain
(feedback is used) of
the non-inverting
amplifier can be
expressed as follow:
vo  R f 

AF 
 1 
vi 
R1 

Generally AF is made
very large.
44
Inverting Amplifier


The input signal for
inverting amplifier is
applied to its negative
input terminal.
The close loop gain of
this amplifier can be
computed as follows:
AF  
Rf
R1
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Inverting & Non-inverting
46
Differential Amplifier



The output voltage of the differential
amplifier is relative to the difference between
the two input voltages.
Output voltage, vo can be expressed as
follows:
vo = Ad (v+ - v-)
Where Ad is a differential gain that is
designed to be at very high value.
Ad also implies that both of the input voltages
is almost equal. i. e. v+≈ v-.
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Differential Amplifier…
continued




Input impedance for differential
amplifier is significantly large.
Ad also known as Difference Mode
Gain and (v+-v-) is Difference
Signal.
Equally applied input voltage is
common mode signals as the
differential amplifier will output
zero voltage theoretically.
Practically, the output of
differential amplifier is not equal to
zero when both inputs are equal.
48
Gain = R2/R1
Input impedance
= R1+R2
49
CMRR in Differential Amplifier


The distinction between the gain desired for difference signals
and the gain for the common mode signals can be made
available.
Common mode gain
Ac 

vo
vc
Where vc is common mode input signal.
The CMRR is defined as follows:
CMRR 

Ad
Ac
CMRR is the measure of the desired signal to the undesired
signal. The larger the CMRR the better the amplifier.
50
Common Mode Rejection Ratio
51
Advantages of Differential
Amplifier

Noise Immunity



Can be used in the situations where the operations of single
ended operational amplifiers is impractical due to the ground
potential differences and interferences from pick-up.
Differential amplifier only responds to the differences
between its input signals not the noise pick-up or ground
voltage that came in phase on both signals.
Drift Immunity


Changes in voltage gain and level due to ageing and
variations of temperature can be eliminate.
Two inputs and outputs of one differential amplifier can be
imagine as it is made up of two amplifier. Therefore any
changes in voltage and variations of temperature affect both
of its inputs.
52
Instrumentation Amplifier

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

Dedicated differential amplifier with very high
input impedance.
High common mode rejection features make
instrumentation amplifier useful in recovering
small signals hidden in large common mode
offsets and noise.
Instrumentation amplifier is a close loop
devices with certain value of gain.
Can be optimised as signal conditioner for low
level signals (DC) in high noise environments.
53
Instrumentation Amplifier…
continued


Instrumentation amplifier is divided
into two stages; first stage give a
very high input impedance to both of
input signals and with single resistor
gain setting, second stage is a
differential amplifier with the
negative feedback, ground
connections and output are all taken
out.
Input stage consists of two matched
operational amplifiers.
54
Instrumentation Amplifier…
continued


Both of input signals is
applied to the noninverting input terminal
of the operational
amplifiers.
The operational
amplifier are configured
as voltage follower
which give the
instrumentation
amplifier a very high
input impedance.
55
Instrumentation Amplifier…
continued

Rg is a gain setting resistor in the following
formulae for computing vo:
 2R 

vo  v 2 v1 1 
 Rg 



From the equation, the smaller the value of
Rg, the larger the output voltage vo. It is
clear that Rg can be used in setting the gain
of this first stage.
56
Instrumentation Amplifier…
continued

The second stage of the
instrumentation
amplifier is a differential
amplifier with unity
gain. The full
instrumentation
amplifier circuit uses
three operational
amplifiers hence it is
called three amplifier
configuration.
57
Instrumentation Amplifier…
continued

Important features of instrumentation
amplifier:





Differential input capability with high gain
common mode rejection.
Selectable gain with high gain accuracy and
linearity.
High stability of gain with low temperature
coefficient.
Low DC offset and drift errors referred to input.
Low output impedance.
58
Contents



Introduction
Amplifiers
Filters
59
Filters

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


Filter is the network used to attenuate certain
frequencies but allow others without attenuation.
Consist at least one pass band, which is a band of
frequencies that the output is approximately equal to
input and attenuation band that the output is equal
to zero.
Cut-off frequencies is the frequencies that separate
the various pass and attenuation bands.
Important characteristic of filter networks is its
construction make use of purely reactive elements.
Two types of filter:


Passive Filters
Active Filters
60
Types of Filters



Passive filters only use passive circuit component
such as resistors, capacitors and inductors.
Active filters use active elements like operational
amplifiers in addition to passive elements like
resistance, capacitance and inductance.
Both of passive and active filters can be classified as
follows:





Low Pass Filter
High Pass Filter
Band Pass Filter
Band Stop Filter
All Pass Filter
61
Types of Filters… continued
Source: [Kalsi2005] pg. 505
62
Passive Filters

Low Pass Filters (LPF)



A RC network.
At low frequencies, the
capacitive reactance is
very high, therefore the
capacitor circuit acts like
an open circuit. These
condition gives Vo = Vi.
At very high frequencies,
the capacitive reactance
is very low therefore Vo is
very small compared to
Vi.
1
fc 
2RC
63
Passive Filters… continued

High Pass Filters
(HPF)



A RC network.
At low frequencies,
the gain is small,
therefore Vo is small
compared to Vi.
As the frequencies
goes high the gain
approaches unity.
1
fc 
2RC
64
Passive Filters… continued

Band Pass Filters (BPF)



Can be constructed by
cascading LPF and HPF.
At frequencies below the
pass band, BPF behave
like HPF while above the
pass band frequencies
the BPF acts like LPF.
In pass band, the BPF
circuit is almost as a
resistive network.
ABPF
R2

R1  R2
f clower
1
1

, f cupper 
2R2C2
2R1C1
65
Passive Filters… continued

Band Stop Filters (BSF)



Simple RC filters.
Twin T BSF; At the very
low and high frequencies
the gain is almost unity,
but between the two
there is a frequency
where the gain become
zero.
The frequency is known
as Notch Frequency, f0.
1
f0 
2RC
66
67
Active Filters


Generally the impedances are used in the
inverting amplifiers using operational
amplifiers.
Basic relationship can be used to obtain the
desired filter sections is as follows (in the
case of inverting amplifiers).
V0 Z f

V1 Z1

The voltage can also be amplified.
68
Active Filters… continued

Low Pass Filters
(LPF)
V0  
R fH
R1H 1  jR fH C fH 
1
H 
 2f c
R fH C fH
H refers to
characteristic high
frequency
V1
1
fc 
2R f C
1
R 
2fC
69
Active Filters… continued

High Pass Filters
(HPF)
V0  
jR fLC1L
1  jR1LC1L
1
L 
 2f c
R1LC1L
L refers to
characteristic low
frequency
V1
1
fc 
2R1C
1
R 
2fC
70
Active Filters… continued

Low Pass Filters
(LPF)… non-inverting
configuration
V0 
R f  R1
R1 1  jRH CH 
V1
1
H 
RH C H
H refers to
characteristic high
frequency
71
Active Filters… continued

High Pass Filters
(HPF)… non-inverting
configuration.
V0

R

f
 R1  jRLCL
R1 1  jRLCL 
V1
1
L 
RLC L
L refers to
characteristic low
frequency
72
Active Filters… continued

Band Pass Filters (BPF)
V0 
R
f2
 R2 R f 1  R1  jRLCL
R2 R1 1  jRLCL 1  jRH CH 
V1
73
Band stop filter
74