SIGNAL CONDITIONING

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CHAPTER 5

• SIGNAL CONDITIONING

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Structure For Chapter 5

• INTRODUCTION TO SIGNAL CONDITIONING
• BRIDGE WHEATSTONE BRIDGE
• Balanced Condition
• Unbalanced Condition
• AMPLIFIER
• PROTECTION
• FILTERS Low Pass, High Pass Band Pass

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Introduction

• Signal conditioning refers to operations
  performed on signals to convert them to a form
  suitable for interfacing with other elements in
  the process-control loop.
• The signal may be

4
Bridge

• Definition Bridges are electrical circuits for
  performing null measurements on resistances in DC
  and general impedances in AC.
• Bridge circuit are used to convert impedance
  variations into voltage variations.

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Wheatstone Bridge

• DC Bridge (Wheatstone Bridge)

• Two conditions to be considered
• Balance Condition
• Ig 0
• Unbalance Condition
• Ig ? 0

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Balance condition, Ig 0

• The current through the galvanometer, Ig 0

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Derivation
Vcb Vdb ? I3R3 I4R4 Vca Vda ? I1R1 I2R2
Therefore,
I1 I3 I2 I4
Using voltage divider,
If R4 is unknown resistor
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Derivation
Apply KCL at node C I1-I3-Ig 0, since Ig0,
I1I3 Apply KCL at node D I2-I4Ig 0, since Ig
0, I2I4
Vcb Vdb ? I3R3 I4R4 Vca Vda ? I1R1 I2R2
(I4R4)/(I2R2) (I3R3)/I1R1 ? Solve for R4
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Unbalance condition, Ig ? 0

• The current flowing through the galvanometer, Ig.
• Ig is determined using a Thevenin eq. circuit.

c
d
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Derivation
Vth Vcd VR1 VR2 or
VR4 VR3
VR1, VR2, VR3, VR4 Voltage divider
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Derivation (contd)
Apply KVL in the RED loop Vth -VR2VR1VR1-VR2


-
-
Apply KVL in the BLUE loop Vth VR4-VR3


-
-
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Derivation (contd)
OR
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Derivation Sensitivity (contd)
Rg, the internal resistance of the galvanometer
is neglected
If a load is connected across the output terminal
Zth
Total deflection of galvanometer
D S Ig
S sensitivity of the galvanometer
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Derivation (contd)
Zth
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Example 1
If a Wheatstone bridge, as shown in Figure 2,
nulls with R11000O, R2842O, and R3500 O, find
the value of R4
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Example 2

• The resistors in a bridge are given by
  R1R2R3120O and R4121O. If the supply is 10.0
  V, find the voltage offset.

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Example 3
The Wheatstone bridge circuit as in Figure 3 is
unbalanced when R130kO, R210kO, R3 2kO, R4
5kO and E 5V. The internal resistance of
galvanometer, Rg 100O. (i) Calculate the
value of Ig that will flow through the
galvanometer. (ii) Determine the deflection
of galvanometer if its sensitivity is 1mm/µA.
(iii) If R4 is replaced with a new Rx, obtain
the value of Rx when the bridge is balance.
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Example 4
At 20oC, the Wheatstone bridge as shown in Figure
4 is in balance condition when R11000O, R2842O,
and R3500 O. Meanwhile, R4 is copper Resistance
Temperature Detector (RTD).The internal
resistance of galvanometer, Rg100O and the
temperature coefficient of the RTD, a0.0042O/oC.
If the RTD is dipped into boiling water (100oC),
determine the deflection of galvanometer if its
sensitivity is 1mm/µA