1. Data Acquisition Electronics Unit – Lecture 6
Sensors and Transducers
Signal conditioning
Data sampling and recording
Prepared by Jim Giammanco
LSU 09/23/2013
Electronics 6

2. Data Acquisition SENSOR TRANSDUCER SIGNAL CONDITIONING DATA COLLECTION
AND STORAGE
ANALOG-TO-DIGITAL
CONVERSION
READOUT
AND/OR DISPLAY
LSU 09/23/2013
Electronics 6

3. Sensors and Transducers
Sensor – responds to a physical variable
Transducer – converts a sensor response to an electrical signal
Often the sensor and transducer are integrated into a single unit – example, the thermistor converts temperature into resistance.
LSU 09/23/2013
Electronics 6

4. Some Physical Variables (possible sensor inputs)
Temperature Pressure Force
Humidity Light Intensity Position
Radioactivity Acceleration Attitude
Magnetic field strength
Electric field strength
Chemical composition
LSU 09/23/2013
Electronics 6

5. Some Electrical Signals (possible transducer outputs)
Resistance
Voltage
Current
Pulse frequency
Pulse width
LSU 09/23/2013
Electronics 6

6. Temperature Measurement
Convert temperature to resistance? Use…
Thermistor, RTD (resistance temperature device)
Thermistor has negative TC, RTD has positive TC
Convert temperature to voltage? Use…
Thermocouple (Seebeck effect)
Convert temperature to current? Use …
Semiconductor junction devices
TC is “temperature coefficient.” The resistance of an RTD increases with temperature, that of a thermistor decreases.
Thermocouple outputs are only a few millivolts.
Semiconductor temperature sensors usually output a few microamps per kelvin.
SkeeterSat uses a thermistor to control the pitch of the audio pulses.
LSU 09/23/2013
Electronics 6

7. Silicon Diode Temperature Sensor
Forward voltage drop of Si Diode is a function of temperature. dV/dT is approximately -2.1 mV/oK above about 20 oK.
TC is “temperature coefficient.” The resistance of an RTD increases with temperature, that of a thermistor decreases.
Thermocouple outputs are only a few millivolts.
Semiconductor temperature sensors usually output a few microamps per kelvin.
SkeeterSat uses a thermistor to control the pitch of the audio pulses.
Lakeshore Instruments
LSU 09/23/2013
Electronics 6

8. Silicon Diode Temperature Sensor
Constant current source (CCS) should provide small current to minimize self-heating of the diode.
1N457A Si Diode at 1mA for example.
Cheap CCS: Voltage source and Big Resistor. R = E/I
Better CCS: LM234
(see datasheet for details)
TC is “temperature coefficient.” The resistance of an RTD increases with temperature, that of a thermistor decreases.
Thermocouple outputs are only a few millivolts.
Semiconductor temperature sensors usually output a few microamps per kelvin.
SkeeterSat uses a thermistor to control the pitch of the audio pulses.
LSU 09/23/2013
Electronics 6

9. Pressure Measurements
Solid state pressure transducers
Can sense gauge or absolute pressure
Output is usually a voltage signal
Usually a hybrid electromechanical device
Temperature compensation is essential
LSU 09/23/2013
Electronics 6

10. ICS1230 Pressure Sensor 1.5 mA excitation
Pressure is proportional to difference of V2 and V1.
At 0 millibars V2-V1 = 0.
At 1000 millibars V2-V1 ~ mV.
See datasheet 1230-A015-3L
LSU 09/23/2013
Electronics 6

11. Signal Conditioning Filtering to reduce noise or interference
Amplification or Attenuation
Level shifting
Span and Base adjustment
Impedance transformation
LSU 09/23/2013
Electronics 6

12. Span and Base Example: Temperature sensor: 450 mV to 800 mV
BASE = 450 mV, SPAN = 350 mV
Signal Conditioning Output: 0V to 3V (to match ADC)
BASE = 0V, SPAN = 3V
11.6 mA is 7.6 mA above the base. (7.6 mA)/(16 mA) represents 47.5% of the output span.
47.5 % of the temperature span is 80.8 degrees, so the temperature is 80.8 degrees above the base temperature (-20), or 60.8
LSU 09/23/2013
Electronics 6

13. Span and Base Gain = 8.571 Offset = -3.857
Design and Build a circuit that performs this transfer function.
What do we need?
11.6 mA is 7.6 mA above the base. (7.6 mA)/(16 mA) represents 47.5% of the output span.
47.5 % of the temperature span is 80.8 degrees, so the temperature is 80.8 degrees above the base temperature (-20), or 60.8
LSU 09/23/2013
Electronics 6

14. Operational Amplifier - Opamp
Ideal Opamp Parameters:
▪Input Z = ∞ ▪Out Z = 0
▪Gain = ∞ ▪BW = ∞ ▪Vos = 0
LSU 09/23/2013
Electronics 7

15. Opamps - The Golden Rules
The output attempts to do whatever is necessary to make the voltage difference between inputs zero.
The inputs draw no current.
The output influences the input pins via the external feedback network. Use the Rules and Ohm’s Law to analyze or synthesize opamp circuits.
LSU 09/23/2013
Electronics 7

16. Voltage Follower - Unity Gain Buffer
High input impedance Low output impedance.
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17. Non-Inverting Amplifier
Feedback network is a voltage divider.
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18. Non-Inverting Amplifier
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19. Inverting Amplifier
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20. Inverting Amplifier
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21. Summing Amplifier
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22. Differential (difference) Amplifier
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23. Operational Amplifier - Opamp
Available in Single, Dual and Quad Packages
Bipolar power supply (typ. +/- 12V)
Single supply operation (typ. +12V,GND)
V- < Vout < V+
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24. BalloonSat as a Data Acquisition System
BASIC Stamp BS2P24
24LCxx EEPROM
Real Time Clock
4 channel A/D converter
Voltage reference for ADC
Temperature sensor
4 LED indicators
LSU 09/23/2013
Electronics 6

25. Activity EEPROM - Read, Write
Real-Time-Clock - Set Time/Date, Read Time/Date
ADC - Select Channel, Read ADC
Read ADC, store data and time stamps to EEPROM. “Post-flight” readback of data
Coming Soon….Build (and document!) a data acquisition system using your BalloonSat
LSU 10/10/2013
Electronics 7