Chapter 4 : Signal conditioning

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Chapter 4 Signal conditioning

• 4.1 Introduction to signal conditioning
• 4.2 Bridge circuits
• 4.3 Amplifiers
• 4.4 Protection
• 4.5 Filters

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Introduction
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ELECTRICAL MEASUREMENT SYSTEM
WHY?

• Easy to transmit signal from measurement site
• the data collection site
• Easy to amplify, filter and modify
• Easy to record the signal

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Signal conditioning

• Used in factory or machine automation to
  convert sensor or transducer measurement signal
  levels to industry standard control signals
• Provide computer and control system manufacturers
  a common communication method to effectively
  receive and transmit measurement and control data
• Examples of measurement data temperature or
  AC/DC voltage/current signals from various
  transducers
• Examples of control data on/off signals for a
  heating element or proportional signals for a
  valve actuator.

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Signal conditioning
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Bridge circuits
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Bridge circuits

• Used to convert impedance variations into voltage
  variations
• Can be design so the voltage produced varies
  around zero
• Amplification can be used to increase voltage
  level for increased sensitivity to variation of
  impedance

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Wheatstone bridge

• D voltage detector

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Exercise 1

• Determine
• R4 if a Wheatstone bridge nulls with
  R1 1000 O, R2 842 O, and R3 500 O.
• The voltage offset if the supply voltage is 10.0
  V. The resistors in a bridge are given by R1 R2
  R3 120 O and R4 121 O.

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Galvanometer detector
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Exercise 2

• A bridge circuit has a resistance of
  R1 R2 R3 2.00 kO and R4 2.05 kO
  and a 5.00 V supply. If a galvanometer with a
  50.0 O internal resistance is used for a
  detector, calculate the offset current.

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Bridge resolution

• Resolution function of detector to determine
  the bridge offset
• Resistance resolution resistance change in 1
  arm bridge that causes an offset voltage equal to
  detector resolution
• Detector can measure change of 100 µV

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Resolution

• The smallest discernible change in input the
  smallest change in input that manifests itself as
  perceptible change in output that can be measured
  (example 0.000 1 mm)
• Primary factor in deciding precision
• Good resolution does not imply in good precision

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Current balance bridge
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Current balance bridge

• Used current to null bridge

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Exercise 3

• A current balance bridge has a 10 V supply
  voltage and resistors R1 R2
  10 kO, R3 1 kO, R4 950 O, R5
  50 O and a high impedance null detector.
  Determine the current required to null the bridge
  if R3 increased by 1 O.

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Potential measurements using bridges
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Potential measurements using bridges
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Exercise 4

• A bridge for potential measurement nulls when
  R1 R2 1 kO, R3 605 O, and R4 500 O with a
  10.0 v supply. Determine the unknown potential.

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Exercise 5

• A current balance bridge is used for potential
  measurement. The fixed resistors are R1 R2 5
  kO, R3 1 kO, R4 990 O, and R5
  10 O with a 10 V supply. Calculate the current
  necessary to null the bridge if the potential is
  12 mV.

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Amplifiers
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Op amp characteristic
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Summing amplifier
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Noninverting amplifier
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Exercise 7

• Design a high impedance amplifier with a voltage
  gain of 42 if R1 1 kO is chosen.

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Differential amplifier

• The transfer function
• Common mode rejection

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Voltage-to-Current converter
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Current-to-Voltage converter
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Exercise 8

• For a voltage-to-current converter using an
  op-amp, show that the relationship between
  current and voltage is given by
  
• .

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Integrator
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Exercise 9

• Use an integrator to produce a linear ramp
  voltage rising at 10 V per ms. Determine the R
  and C.

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Differentiator
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Linearization
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Linearization
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Filters
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Filters

• Filter a circuit that is designed to pass
  signals with desired frequencies and reject or
  attenuate others
• 4 types of filters
• Low-pass filter passes low frequencies and stops
  high frequencies
• High-pass filter passes high frequencies and
  rejects low frequencies
• Band-pass filter passes frequencies within a
  frequency band and blocks or attenuates
  frequencies outside the band
• Band-reject filter passes frequencies outside a
  frequency band and blocks or attenuates
  frequencies within the band

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Low-pass RC filter
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Low-pass RC filter

• Critical frequency
• Output-to-input voltage ratio

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Exercise 10

• A measurement signal has a frequency less than 1
  kHz, but there is unwanted noise at about 1 MHz.
  Design a lowpass filter that attenuates the noise
  to 1 if a capacitor 0.01 µF has been used. What
  is the effect on the measurement signal at its
  maximum of 1 kHz?

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High-pass RC filter
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High-pass RC filter

• Critical frequency
• Output-to-input voltage ratio

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Exercise 11

• Pulses for a stepping motor are being
  transmitted at 2000 Hz. Design a highpass filter
  to reduce 60 Hz noise and reduce the pulses by no
  more than 3 dB.

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Design Methods

1. Determine critical frequency, fc
2. Select standard capacitor (µF pF)
3. Calculate required resistance (1 k? - 1 M?)
4. Use nearest resistance standard value to
   calculated value
5. Consider tolerance in resistors and capacitors

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Practical considerations

1. Very small resistance -gt lead to large currents
   and loading effects -gt avoid large capacitance
   (R kO -MO, C µF pF)
2. The exact fc is not important, choose R and C of
   approximately to the fc
3. Isolation filter input/output with voltage
   follower
4. Cascade RC filters to improved fc sharpness -gt
   consider loading

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Band-pass RC filter
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Band-pass RC filter

• Critical frequency
• Output-to-input voltage ratio

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Exercise 12

• A signal conditioning system uses a frequency
  variation from 6 kHz to 60 kHz to carry
  measurement information. There is considerable
  noise at 120 Hz and at 1 MHz. Design a bandpass
  filter to reduce the noise by 90. What is the
  effect on the desired passband frequencies if r
  0.01? Determine all the resistors and capacitors.

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Band-pass RC filter
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Band-reject RC filter
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Twin-T notch filter
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Twin-T notch filter

• Critical frequency
• Grounding resistor and capacitor

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Exercise 13

• A frequency of 400 Hz prevails aboard an
  aircraft. Design a twin-T notch filter to reduce
  the 400 Hz signal if 0.01 µF has been used and
  calculate the grounding resistor and capacitor.
  What effect would this have on voice signals at
  10 to 300 Hz? Determine the higher frequency when
  the output is down by 3 dB.

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Example
An amplifier produces an output of 4 V when the
input is 4 mV. What is the gain G and the
decibel gain GdB?
Solution
Output 5 Volts Vo Input 5 ?V 5?10-6 volts
Vi
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Example
A noninverting amplifier like shown in Figure is
to be constructed with a mA741C op-amp. It is to
have a gain of 100. Specify values for the two
resistors.
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Solution
Since R1 and R2 typically range from 1k? to 1M?,
we arbitrarily choose R299k? ? R1 1k?
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Example
A recording device has a frequency response which
shows that the output is down 2 dB at 200 Hz. If
the actual input is 5.6 V at 200 Hz, what will
be the expected error of the voltage reading (in
volts)
Solution
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Example
A simple Wheatstone bridge as shown in Fig is
used to determine accurately the value of an
unknown resistance R1 located in leg 1. If upon
initial null balance R3 is 127.5 W and if, when
R2 and R4 are interchanged, null balance is
achieved when R3 is 157.9 W, what is the value
of unknownn resistance R1
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Solution
Null balance first achieved R1.R4R2.R3 With
R3127.5 W Then R1.R4/R2127.5 Null balance
after R2 and R4 interchange R1.R2R3.R4 With
R3157.9 W Then R1.R2/R4157.9 R4R1.R2/157.9 The
n R1.R1.R2/R2.157.9127.5 R1141.8 W
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Another example
Solution