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EKT 314
ELECTRONIC INSTRUMENTATION
Elektronik Instrumentasi
Semester 2 2012/2013
Chapter 3
Analog Signal Conditioning
Session 1 & 2
Mr. Fazrul Faiz Zakaria
school of computer and communication engineering. universiti malaysia perlis
1
Wednesday, March 27, 13
Signal Conditioning
The connection bet ween a sensor and an interface circuit. In the image on the left, the sensor has voltage
output. In the image on the right, the sensor has current output.
Sensors produce different kind of signals
Voltage output or Current output
Can’t necessarily take sensor output and put right into Microprocessor
ADC or logic input
Signal may need:
High to low impedance buffer, current to voltage conversion, gain,
detection, & filtering.
Wednesday, March 27, 13
Wednesday, March 27, 13
The Objectives
Explain the purpose of analog signal conditioning.
Design a Wheatstone bridge circuit to convert resistance change
to voltage change.
Design RC low-pass and high pass filter circuits to eliminate
unwanted signals.
Understanding the schematics of four common op amp circuits
and provide the transfer functions.
Explain the operation of an instrumentation amplifier and draw
its schematic.
Design an analog signal-conditioning system to convert an input
range of voltages to some desired output range of voltage.
Design analog signal conditioning so that some range of
resistance variations is converted into a desired range of voltage
variation.
Wednesday, March 27, 13
The Principles
Convert signal to a form suitable for interface with other elements
Signal level and bias
changes
Linearization
Signal Conditioning
Conversion
Filtering and
impedance matching
Concept of loading
Wednesday, March 27, 13
Signal Level and bias changes
adjusting the level (magnitude) and bias (zero value)
of some voltage representing a process variable
amplification vs attenuation --> uses amplifier
Linearization
The purpose of linearization is to provide an output that varies
linearly with some variable even if the sensor output does not.
Wednesday, March 27, 13
Conversion
Convert one type of electrical variation into another.
Signal Transmission (Voltage to current, Current to
Voltage converter)
Digital Interface (ADC requires 0-5V input)
Filtering & impedance matching
Filtering-Eliminate unwanted signals in the
process-control loop
Impedance matching-transducer internal
impedance or line impedance can cause error
in measurement of a dynamic variable.
Wednesday, March 27, 13
Concept of loading
Concern -loading of one circuit by another.
Thévenin's theorem for linear electrical net works states that any combination of
voltage sources, current sources, and resistors with t wo terminals is electrically
equivalent to a single voltage source Vth and a single series resistor Rth.
this equivalent voltage Vth is voltage obtained at the terminals A-B of the net work
with terminal A-B open circuits
this equivalent resistance Rth is the resistance obtained at terminal A-B of the
net work with all its current source open circuited and all voltage sources short
circuited
Wednesday, March 27, 13
Example 1.1
An amplifier outputs a voltage that is 10 times the
voltage on its input terminal. It has an input resistance
of 10 kΩ. A sensor outputs a voltage proportional to
temperature with a transfer function of 20mV/ °C. The
sensor has an output resistance of 5.0 kΩ. If the
temperature is 50 °C, find the amplifier output.
Wednesday, March 27, 13
Solution for Signal Conditioning
Passive Circuits
Divider Circuits
Bridge Circuits
RC Filters
Operational amplifier (OP-AMP)
Wednesday, March 27, 13
Divider Circuits
The simple voltage divider can often
be used to convert resistance
variation into voltage variation.
R total = R 1 + R 2
V
V
s
s
I total =
=
R total R 1 + R 2
R
2
VD = I total R 2 = Vs
R1 + R2
Demo
either R1 or R2 can be your sensor
Wednesday, March 27, 13
Example 3.2
the sensor system shown above has R1=10Kohm and
Vs=5V. suppose that R2 is our sensor whose resistance
varies from 4Kohm to 12Kohm as some dynamic
variable varies over a range . Find (a) the minimum and
maximum of VD (b) the range of output impedance and
(c) the range of power dissipated by R2
Wednesday, March 27, 13
Bridge Circuits
Bridge circuits are used to
convert impedance variations
into voltage variations
in the instrumentation
situation this bridge is operate
in the unbalance operation
voltage across the a & b point
depend on the resistance
ratio, when the ratio are equal
where R1R4=R2R3, in that
situation voltage across a & b
will be zero
any change on the resistance
value which cause the bridge
unbalance, will produce a
voltage across the a & b point.
Wednesday, March 27, 13
Bridge Circuits
TV = V a - V b
Va = potential of point a with respect to c
Vb = potential of point b with respect to c
R4
R3
Va = R1 + R3 : Vs and Vb = R2 + R4 : Vs
R4
R3
TV = R 1 + R 3 : V s - R 2 + R 4 : V s
R3 R2 - R1 R4
TV = T Q R 1 + R 3 V : Q R 2 + R 4 V Y V s
Demo
Wednesday, March 27, 13
Bridge Circuits
When a galvanometer is used for a null detector, it is
convenient to use the Thévenin equivalent circuit of the
bridge.
R3 R2 - R1 R4
Y Vs
V th = T
QR 1 + R 3 V : QR 2 + R 4V
Demo
R2 R4
R1 R3
R th =
+
R1 + R3 R2 + R4
Demo
V th
IG =
R th + R G
Wednesday, March 27, 13
Example 3.3
A bridge circuit has resistance of
R1=R2=R3=2Kohm and R4=2.05 Kohm and a 5V
of supply. If a galvanometer with a 50 ohm
internal resistance is used for a detector, find
the offset current.
Wednesday, March 27, 13
Example 3.4
A bridge circuit has resistance of
R1=R2=R3=R4=2Kohm and a 10V of supply.
clearly, the bridge is nulled. suppose a 31/2 digit
DVM on a 200mV scale will be used for null
detector. find the resistance resolution for the
measurements of R4
Wednesday, March 27, 13
Bridge Circuits
LEAD COMPENSATION
When bridge circuit may be located
at considerable distance from the
sensor whose resistance changes
are to be measured.
Problem many effect that change
the resistance.
any changes in lead resistance are
introduced equally into both arms
of the bridge circuit, thus causing
no effective change in bridge
offset
For remote sensor applications, this
compensation system is used to
avoid errors from lead resistance
Wednesday, March 27, 13
Wire 3, is the power lead and has no
influence on the bridge balance
If wire 2 changes in resistance-->
change R4. Wire 1 changes by the
same amount (& exposed to the
same environment) --> change R3
Example 3.5
A bridge circuit is used with sensor located 100
m away. The bridge is not lead compensated and
the cable to the sensor has a resistance of 0.45
ohm/ft. The bridge nulls with R1=3400 ohm,
R2=3445 ohm and R3=1560 ohm.What is the
sensor resistance ?
Wednesday, March 27, 13
Bridge Circuits
Current Balance Bridge
This method uses a current to
null the bridge
R 4 >> R 5
(R
2
+ R 4 ) >> R 5
Now ;
Vb =
V (R 4 + R 5 )
R2 +R4 +R5
+ IR 5
so ;
# R
&
R
+
R
4
5
3
(( − IR 5
ΔV = V %%
−
$ R1 + R 3 R 2 + R 4 + R 5 '
Wednesday, March 27, 13
Example 3.6
A current balance bridge circuit has resistors
R1=R2=10 Kohm, R3=1 Kohm R4=950 ohm and
R5=50 ohm and high-impedance null
detector.find the current required to null the
bridge if R3 change by 1 ohm. the supply
voltage is 10 V
Wednesday, March 27, 13
Bridge Circuits
Primary application of
bridge circuits
Bridge off-null voltage is clearly nonlinear for
large-scale changes in resistance.
To Convert variations of
resistance into
Variations of voltage
If the range of resistance
variation is Small and
centered about the null
value Then then
nonlinearity of voltage
Resistance is small.
However, for small ranges of resistance
change, the off-null voltage is nearly linear.
Wednesday, March 27, 13
RC Filters
To eliminate unwanted noise signals from
measurements, it is often necessary to use
circuits that block certain frequencies or
bands of frequencies.
Wednesday, March 27, 13
RC Filters
Low Pass RC Filter
fc = 1
It blocks high
frequencies and
passes low
frequencies.
V out
=
V in
Wednesday, March 27, 13
2 πRC
1
!
$
f
1+ #
&
" fc %
2
RC Filters
Design Guideline
find the critical frequency that will satisfy the design
criteria
Wednesday, March 27, 13
Example 3.7
A measurement signal has a frequency < 1 kHz,
but there is unwanted noise at about 1MHz.
Design a low pass filter that attenuates the
noise to 1%. what is the effect on the
mesurement signal at its maximum of 1 kHz
Wednesday, March 27, 13
RC Filters
High-Pass Filter
-Passes High
frequencies
-Blocks low
frequencies
Wednesday, March 27, 13
fc = 1
V out
V in
2 πRC
!
$
f
# f &
"
c %
=
2
!
$
1+ # f
&
f
"
c %
Example 3.8
Pulse for stepping motor are being transmitted
at 2000 Hz. Design a filter to reduce 60 Hz
noise but reduce the pulses by no more than 3dB
Wednesday, March 27, 13
Wednesday, March 27, 13