Signal Conditioning

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CHAPTER 6

• Signal Conditioning

Norazila Jaalam Fkee
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CHAPTER OUTLINE

• Introduction to signal conditioning
• Bridge circuits
• Amplifiers
• Protection
• Filters

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AMPLIFIER

• Introduction
• Operational Amplifier in Instrumentation.
• Inverting Amplifier
• Non-inverting Amplifier
• Unity Follower
• Summing Amplifier

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AMPLIFIER intro..

• the basis of signal conditioning
• is an electronic circuit which makes a signal
  bigger
• is often needed in systems using analogue signals

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AMPLIFIER intro..

• Has two inputs inverting i/p (-) and
  non-inverting i/p ()
• The output depends on the connections made to
  this i/p
• Operational amplifier (Op-amp) is a semiconductor
  device that consist dozen of transistors and
  resistors sealed in a package
• Simple amplifier for analog circuit can be
  developed by using this op-amp.

Normally VCC is set to 15V
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Properties for Ideal Op Amp
AMPLIFIER intro..
(Infinite gain)
(Infinite input resistance)
(Zero output resistance)
For idealized model
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AMPLIFIER intro..
Properties for practical Op Amp

• Very high voltage gain
• Very high input impedance
• Very low output impedance
• Wide bandwidth

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Inverting Amplifier

• Figure shows the connections made to amplifier
  when used as an inverting amplifier
• The input is taken to the inverting i/p through
  R1 with non-inverting i/p being connected to
  ground
• A feedback path is provided from the output via
  the resistor Rf to the inverting i/p

I1 If
The negative feedback stabilizes the voltage
gain indicates inversion
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EXERCISE 1 Find Vout for both circuits shown
below
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Non-Inverting Amplifier

• Figure below shows the connection to the op-amp
  when used as non-inverting amplifier with
  feedback
• The output can be considered to be taken from
  across a potential divider circuit consist of R1
  in series with Rf

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EXERCISE 2

Determine the gain of the amplifier in figure
below. Given Rf 100 k? and R1 4.7k?.
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UNITY FOLLOWER (or voltage follower)

• special case of non-inverting amplifier where
  all output voltage is fed back straight away to
  inverting (-) input
• has very high input impedance and very low
  output impedance
• ideal buffer amplifier for interfacing high
  impedance source and low impedance loads

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Summing Amplifier (with unity gain)

• Figure below shows the circuit of summing
  amplifier
• It is an inverting type of op-amp
• has two or more inputs (any number of input can
  be used)

For n input,
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EXERCISE 3Determine the output voltage of
figure below. Given R1 10k?, R2 15k?, R3
20k?, Rf 10k? and VIN1 3V, VIN2 1V, VIN3
8V.
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Gain of cascade circuit
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EXERCISE 4 Determine the total gain produced by
the network below. Then, determine the output
voltage, Vout
AV1
AV2
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EXAMPLE 5 Determine the total gain produced by
the network below. Then, determine the output
voltage, Vout
AV2
AV1
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PROTECTION
A unit after a sensor (eg. microprocessor) has a
possibility to damage as a result of high current
or high voltage
How to protect?
High Current
High Voltage

• a series resistor to limit the current to an
  acceptable level
• a fuse to break if the current exceed a safe
  level

• use a Zener diode circuit

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PROTECTION

• Zener diodes behave like ordinary diodes up to
  some breakdown voltage when they become
  conducting.
• Thus to allow a maximum voltage of 5 V, a Zener
  diode with a voltage rating of 5.1 V might be
  chosen.
• When the voltage rises to 5.1 V the Zener diode
  breakdown and its resistance drops to a very low
  value.
• Since the zener diode is a diode with low
  resistance for current in one direction and high
  resistance in opposite direction, therefore it
  also provides protection against wrong polarity

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FILTERS

• A filter is a device that shapes or modifies the
  frequency content (spectrum) of a signal or
  waveform.
• often used in electronic systems to emphasize
  signals in certain frequency range and reject
  signals in other frequency range/decrease the
  amplitude .
• 4 types of filter Low pass filter (LPF), high
  pass filter (HPF), Band pass filter (BPF) and
  band stop filter (BSF)

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Effect of filtering on a noisy signal
Signal with noise
FILTER
Signal Noise
Signal
Signal with noise removed
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TYPE OF FILTERS
Band pass filter
Low pass filter
High pass filter
fc cut-off frequency
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Low pass filter
0.707Am

• Passes frequency from dc to fc and significantly
  attenuates all other frequencies.
• fc (critical frequency or cut off frequency) is
  the end of the pass band and normally specified
  at the point where the response drops -3 dB of
  attenuation
• The output at the critical frequency is 70.7 of
  the input -3dB
• Approximate low frequency with ? ? 0 and high
  frequency with ?? ?
• at low frequency, gain 1,
• at high frequency, gain 0

20 log (0.707) -3dB
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Low pass filter
Noise is a high-frequency waveform
Low-pass filter will only allow low-frequency
signal to pass through the network
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Low pass filter design
Vi(t)
Vo(t)
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Low pass filter
Cut off frequency is where the gain 1/?2 ( ?3
dB)
At cut-off frequency ZR Zc. Therefore
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High pass filter
Am
0.707Am

• A high pass filter is device that significantly
  attenuates the low-frequency content of a signal
  while passing the high-frequency content with
  minimal attenuation.
• A filter designed to pass all frequency above a
  given cut-off frequency
• High-pass filters invert the frequency
  characteristics of low-pass filters
• Approximate low frequency with ?? 0 and high
  frequency with ? ? ?
• at low frequency, gain 0
• at high frequency, gain 1

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High pass filter
Vo(t)
Vi(t)
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High pass filter

• Cut off frequency is where the gain 1/?2 ( ?3
  dB)

At cut-off frequency ZR Zc. Therefore
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Effect on filtering the high frequency signal
Noise is a high frequency signal
Only noise is allowed to pass through the high
pass signal
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Filters

HPF
LPF







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Band pass filter

• Designed to pass all frequency that fall between
  fc1 (HPF) and fc2 (LPF)
• High pass filter followed by Low pass filter
• Gain

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Band pass filter
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Band Pass Filters
Cut off frequency fc1 10 Hz fc2 100 Hz
100 Hz
10 Hz
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After passing the High Pass filter
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After passing the Low Pass filter
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THANK YOU