Download Basic Architecture of Electronics Instrumentation Measurement System

Additional information on Passive Probes (10x)
Take note that the scope’s input capacitance, Cin must falls within the
probe’s compensation range
For example, Agilent’s 8000 Series oscilloscope’s input characteristics
show that at 1 M input it exhibits 13 pF of typical input capacitance. And
the probe you’re going to use with this scope has 6-15 pF of
compensation range.
Active oscilloscope probes:
• For connecting fast rising and high frequency signal, active probe is
used - range of frequency up to 2 GHz
• As indicated by the name, this type of scope probe has active
components incorporated within the probe itself.
• This enables greater levels of functionality and higher levels of
performance to be attained.
• However they are much more expensive and normally reserved for
more exacting or specialist requirements.
Probe tip
Block Diagram of an Active Probe
FET acts as a source follower. The stage that follows the FET provides bipolar
transistors wired as emitter follower.
The FET provides a high-input resistance for low frequency signals and a low
capacitance for high frequency signals to the circuit being probed, and the bipolar
transistors produce a current gain.
Normally active probe is used for
i. high input signal impedance – normally around 1 M
ii. high frequencies signals
iii. low signal level
At low frequency
1 M
+
+
10 M
Vin
High
Impedance –
no current
flow
-
Vout
-
Circuit under
test
At high frequency
1 M
+
Vin
10 M
Low capacitance
(2 pF) - small
reactance
-
Circuit under
test
The common collector provides a current source with small
output impedance to reduce the loading effect
BASIC ARCHITECTURE OF
ELECTRONICS INSTRUMENTATION
MEASUREMENT SYSTEM
MEASURAND
TRANSDUCER
SIGNAL
CONDITIONING
DISPLAY
RECORD
SENSORS AND
TRANSDUCERS
TRANSDUCERS
• a device that converts a primary form of energy into a
corresponding signal
• take form of a sensor or an actuator
SENSORS
a device that detects/measures a signal
ACTUATOR
a device that generates a signal
Example, a Heater is an actuator while a Thermometer is the sensor
Electronic sensors
• Generally electronic sensor consists of a primary transducer:
changes “real world” parameter into electrical signal for
example, heat, sound, etc
• for example, a microphone (input device) converts sound
waves into electrical signals for the amplifier to amplify (a
process), and a loudspeaker (output device) converts these
electrical signals back into sound waves
Quantity being
Measured
Light Level
Temperature
Input Device
(Sensor)
Light Dependant Resistor (LDR)
Photodiode
Photo-transistor
Solar Cell
Thermocouple
Thermistor
Thermostat
Resistive Temperature Detectors
Force/Pressure
Strain Gauge
Pressure Switch
Load Cells
Position
Potentiometer
Encoders
Reflective/Slotted Opto-switch
LVDT
Speed
Tacho-generator
Reflective/Slotted Opto-coupler
Doppler Effect Sensors
Sound
Carbon Microphone
Piezo-electric Crystal
• Input type transducers or sensors, produce a voltage or signal
output response which is proportional to the change in the
quantity that they are measuring .
• The type or amount of the output signal depends upon the type
of sensor being used.
• The types of sensors can be classed as two kinds, either
Passive Sensors or Active Sensors.
Active Sensors
Generally, active sensors require an external power supply to
operate, called an excitation signal which is used by the sensor to
produce the output signal.
Active sensors are self-generating devices because their own
properties change in response to an external effect producing for
example, an output voltage of 1 to 10V DC or an output current
such as 4 to 20mA DC.
EXAMPLE 1 – STRAIN GAUGE
It does not generate an electrical signal
itself, but by passing a current through it
(excitation signal), its electrical resistance
can be measured by detecting variations in
the current and/or voltage across it.
Force
The Wheatstone bridge provides a way to convert these changes in
resistance to changes in voltage, which are easy to work with.
• R2 in the diagram is set at a value equal to the strain gauge resistance with no
force applied.
• R1 and R3 are set equal to each other.
• Thus, with no force applied to the strain gauge, the bridge will be symmetrically
balanced and the voltmeter will indicate zero volts, representing zero force on the
strain gauge.
where R4 is the resistance of the strain gauge
So let say R1 = R3 = R2 = R
Which means that before any force is applied R4 also equals to R
Hence, any changes in R4 can be denoted as (R + R)
No. of coins
Output Voltage
(mV)
0
0
1
0.769
2
1.389
3
2.108
4
2.76
5
3.38
6
3.98
7
4.64
8
5.32
9
6.35
10
7
REF: http://www.slideshare.net/umangIITD/transducer-andinstrumentation
Consider that the excitation voltage applied is 10 V and the value of R is 120 .
i.
ii.
What is the new resistance value of the strain gauge for Vo = 5.32 mV?
Calculate the percentage of increment of the resistance
Answers: 120.26 , 0.22 %
Relationship with Change of resistance, R = RoG
Where Ro = initial resistance when there is no applied stress, G = gauge
factor and  is the strain unit deformation and
 = / E
Where  = the mechanical stress (N/m2)
E = Young’s Modulus which is specific for each type of material
Continue from previous example:
Given G = 2.12, and the Young’s Modulus of aluminium is 72 GPa.
Calculate the mechanical stress.
Answers: 74.3 MPa
Derive the output voltage equation
Answers: Vo = (R / 2R) Vi
Passive Sensors
Unlike an active sensor, a passive sensor does not need any
additional energy source and directly generates an electric signal
in response to an external stimulus.
For example, a thermocouple or photo-diode. Passive sensors are
direct sensors which change their physical properties, such as
resistance, capacitance or inductance etc.
EXAMPLE 1 – THERMOCOUPLE
Operation:
• a conductor generates a voltage when subjected to a temperature gradient.
• a second conductor material will also generates a different voltage under
the same temperature gradient.
• The voltage difference generated by the two materials can then be
measured and related to the corresponding temperature gradient.
• thermocouples can only measure temperature differences and need a
known reference temperature to yield the absolute readings.
Where, SA and SB are referred to as Seebeck Coefficients (unit is V/K) and
with the assumption that the coefficient remains constant through out the
metal
Ttip
Tref
Material
Selenium
Tellurium
Silicon
Germanium
Antimony
Nichrome
Iron
Molybdenum
Cadmium, tungsten
Gold, silver, copper
Rhodium
Tantalum
Lead
Aluminium
Carbon
Mercury
Platinum
Sodium
Potassium
Nickel
Constantan
Bismuth
Seebeck coefficient
relative to platinum (μV/K)
900
500
440
330
47
25
19
10
7.5
6.5
6.0
4.5
4.0
3.5
3.0
0.6
0 (definition)
-2.0
-9.0
-15
-35
-72
The thermocouple below is using Metal A as copper and Metal B as
constantan. If the measured voltage in the following circuit is 3.53 mV, what is
the temperature of the hot junction (in °C), if the cold junction is at 0°C?
Ttip
Answer: 85°C
Tref