Transcript electronic instrumentation & plc dkt314

ELECTRONIC
INSTRUMENTATION & PLC
DKT314
3. Signal Conditioning Circuits
Signal Conditioning Functions
• Amplification
– Increase the level of input signal to better suit the
DAQ.
– Improve the sensitivity and resolution of the
measurement.
• Filtering
– Reject useless noise within certain frequency range.
– Prevent signal aliasing and distortion.
• Attenuation
– Contrary to amplification.
Signal Conditioning Functions
…continued
• Isolation
– Solve improper grounding problem of the
system.
• Multiplexing
– Sequentially transmit a number of signals into
single digitiser.
• Simultaneous Sampling
– Issue of measuring more than one signals at
the same time.
• Digital Signal Conditioning
DC Signal Conditioning System
DC Signal Conditioning System…
continued
• 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.
AC Signal Conditioning System
AC Signal Conditioning System…
continued
• 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.
Amplifiers
• 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
Input Impedance
• 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
Input Impedance… continued
• 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.
Output Impedance
• Output impedance required for an amplifier depends on
the input impedance of the next sub-system.
• 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.
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 power or voltage.
• Amplifier gain also can be expressed in Decibel
(dB).
 Pout 
 Vout 




A( dB )  10 log
 20 log


 Pin 
 Vin 
Gain… continued
• 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.
Type of amplifier circuits
• Several amplifier circuits can be
constructed using the operational amplifier
(such as µA741). These are:
– Non-Inverting Amplifier
– Inverting Amplifier
– Differential Amplifier
– Instrumentation Amplifier
Filters
• 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
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
Types of Filters… continued
a) Low Pass
b) High Pass
c) Band Pass
d) Band Stop
e) All Pass
The responses of various filters
are shown in figure above:
• Fig. (a)
– The gain is constant over a frequency range starting from zero
to a cut off frequency fc
– The output of any signal having a frequency greater than fc will
be attenuated, i.e: there will be no output voltage for
frequencies greater than cutoff frequency fc. Hence output will
be available faithfully from 0 to fc with constant gain and is zero
from fc onward.
• Fig. (b)
– The gain is zero starting from zero to a frequency fc (the cutoff
frequency)
– Above the cutoff frequency, the gain is constant and equal to A.
Hence signal of any frequency beyond fc will be faithfully
reproduced with a constant gain, and frequencies from 0 to fc
will be attenuated.
The responses of various filters are
shown in figure above: Cont…
• Fig. (c)
– The band pass filter reproduces signals falling between fc1 and fc2,
while signals between 0 to fc1 and frequencies greater than fc2 are
attenuated.
– There is an output corresponding to signals having frequencies between
fc1 and fc2 but no output for signals having frequencies below fc1 and
above fc2. Hence this filter passes a band of frequencies.
• Fig. (d)
– The band stop filter attenuates a particular band of frequencies from fc1
to fc2, while passing all frequencies between 0 to fc1 and fc2 onwards.
– This filter also called a notch filter.
• Fig. (e)
– All pass filter is filter are passed all frequencies without attenuation. The
important feature of this filter is that provides predictable phase shift for
frequencies of different input signals. Mostly used in communications.
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 and voltage gain equal to unity.
– At very high frequencies, the capacitive reactance is very low
therefore Vo is very small compared to Vi. The gain fall and
drops off gradually as the frequency is increased.
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.
AHPF 
RC
1  RC 
1
fc 
2RC
2
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 clo wer
1
1

, f cu p p er 
2R2C2
2R1C1
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
Active Filters
• Generally the impedances are used in the inverting amplifiers using
operational amplifiers.
• Advantages of Active Filter over Passive Filter:
– Gain and frequency adjustment flexibility
• Since the OpAmp is capable of providing a gain, the input signal is not
attenuated, as in a passive filter. Active filter easy to tune.
– No loading problem
• Because of the high input resistance and low output resistant of the OpAmp,
the active filter does not cause loading of the source or load.
– Cost
• Active filter more economical than passive filter because of the variety of
cheaper OpAmps available and the absence of inductors.
• Basic relationship can be used to obtain the desired filter sections is
as follows (in the case of inverting amplifiers).
• The voltage can also be amplified.
Active Filters… continued
• Low Pass Filters
(LPF)
R fH
V0  
V1
R1H 1  jR fH C fH 
1
H 
R fH C fH
•
H refers to
characteristic high
frequency
Active Filters… continued
• High Pass Filters
(HPF)
V0  
jR fLC1L
1  jR1L C1L
L 
•
V1
1
R1L C1L
L refers to
characteristic low
frequency
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