Transducers and signal conditioning
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
Transducers
• Convert physical quantity to
electrical signal, or vice
versa
• May need to amplify or
offset signal (signal
conditioning)
• Once it is in the range we
want (e.g., 0..5V), we can do
analog to digital conversion
Microcomputer Architecture and Interfacing
Potentiometer – angle measurement
Colorado School of Mines
Professor William Hoff
2
Transducer Examples
• Linear position sensor
• VS is the difference
between the top
secondary voltage and
the bottom secondary
voltage
• When the core is
displaced, the sign
(phase) of the output
voltage determines the
direction of the
displacement and the
amplitude indicates the
amount of displacement
• Advantages:
• Absolute position
• Non contact
• Linear output over
a wide range
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
3
Transducer Examples
• Strain gauge
– Resistance varies with strain
– Can measure force (e.g.,
bathroom scale)
• Wheatstone bridge
– Can be used with strain gauge
– Measures small changes in
resistance
– Doesn’t depend on constant
voltage source
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
4
Transducer Examples
• Thermistor
Thermistor
– Resistance varies with
temperature
– Slow response time
• Thermocouple
– Voltage is generated when
temperature different from a
reference
– Need to know reference
temperature
• Non-linear relationship to
temperature – need
calibration or look-up table
Thermocouple
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
5
Non-linear Output
• Calibrate sensor
and use a look-up
table
• Should interpolate
between entries
in the table
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
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Examples
• Phototransistor
– Light falling on base allows
conduction
– Acts like a variable current
source
• Photocell or photoresistor
– Cadmium sulfide (CdS)
– Light changes resistance
– Response can be slow
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
7
Signal Conditioning
• May need to offset and scale transducer output to range 0..5V, to
match full range of A/D converter
• Op-amp circuits can do this
• Example
VIN
+
OP AMP
R1
VOUT
R2
Figure 12.6 A voltage scaler
Voltage gain:
AV = VOUT  VIN = (R1 + R2)  R1
= 1 + R2/R1
Microcomputer Architecture and Interfacing
Colorado School of Mines
• Ideal op amp has infinite
input impedance
• So current flowing through
R2 is the same as the
current flowing through R1
• Also, voltage at the two
inputs is the same
VIN = VOUT R1/ (R1 + R2)
Professor William Hoff
8
Example - scaling
• Choose appropriate values of R1 and R2 to scale a voltage in the
range of 0..200mV to 0..5V
VIN
+
OP AMP
R1
VOUT
Voltage gain:
AV = VOUT  VIN = (R1 + R2)  R1
= 1 + R2/R1
R2
Figure 12.6 A voltage scaler
• We want a gain of (5V)/(200 mV) = 25
• So R2/R1 = 25-1 = 24
• We can use say, R1 = 10 KW and R2 = 240 KW
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
9
Other op-amp circuits
• Summing circuit
• Inverting voltage follower
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
10
Level shifting and
scaling
• Example: shift the input voltage from -1.2V..+3.0V to 0V .. 5V
• Solution:
0 = – 1.2 (Rf/R1) – (Rf/R2) V1
5 = 3.0 (Rf/R1) – (Rf/R2) V1
We can pick R0 = R1 = 10 KW, R2 = 50 KW, Rf = 12 KW, and V1 = -5V
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
11
Summary / Questions
• Sometimes we have to perform signal
conditioning to transform the output voltage of
a sensor to get it into the range 0..5V.
– Why do we want to do that?
• Signal conditioning includes amplifying,
offsetting, and filtering the signal.
• Op amps can be used to do this.
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
12