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Analog Sensing 101WITH P14452
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Agenda
Analog Signal Characteristics
Common Problems with A/D Conversion Clipping
Small Signals
Aliasing
Analog Signal Conditioning
“Digital” Sensors
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Analog Signal Characteristics
Two Key Parameters
Voltage Range
i.e. +/- 50V
Frequency Range
i.e. < 5 KHz
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Common Problem: Clipping
Cause:
A/D Converter is unable to record voltages outside of operating range (i.e. 0-5V)
Solution:
Attenuation
Reduce the voltage range of the signal, without corrupting the information
Scale (in software) the digital representation to reflectthe true value
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Common Problem: Small Signals
Cause:
Analog Signal has a voltage range much lower than that of the A/D converter
Solution:
Amplification
Increase the voltage range of the signal, without corrupting the information
Scale (in software) the digital representation to reflect the true value
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Common Problem: Aliasing
Cause:
Violation of the Nyquist limit by
Incorrect sampling frequency
High Frequency Signal Noise
Solution:
Low Pass Filter
Remove all frequencies above the frequency range of the sensor before capture of the signal
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Analog Signal Conditioning
Process:
Attenuate Large Signal
Amplify Small Signal
Convert Differential Signal to Single Ended Signal
Apply DC Bias
Remove High Frequencies
Convert to Digital Value
Scale Digital Value according to:
Attenuation
Amplification
DC Bias
Conversion Factor ( i.e. Celsius / mV )
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“Digital” Sensors What are they?
Analog Sensor with Signal Conditioning and A/D conversion in the same package
Advantages
Standard Communication Protocol
i.e. I2C, SPI, CAN
Preset to Optimally Condition and Sample
Minimized PCB Requirements
Inexpensive
Accurate
Disadvantages
Software Requirement
Typically Fixed Sampling Rate
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Case Study: Temperature Sensor
ADT7301 Temperature Range
-40 C to 150 C
Accuracy +/- 0.5 C
Precision 13-bit
0.03125 C / 1 bit increase
Max Sampling Rate 83.33 Hz
Interface SPI
2.7-5.25V Supply
Cost: ~$1.21
OMEGA TTIN-18 Type T Thermocouple Temperature Range
-185 C to 300 C
Additional Components Needed
Instrumentation Amplifier
INA128 - $11.30
Low Pass Filter
3 Op-AMPS
LM741 - $0.70
Analog to Digital Converter
Cost: $21 + $11.30 + $2.10 = $33.40
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Case Study: Pressure Sensor
Honeywell SSCDANN150PG2A3 Pressure Range
0 – 150 PSIG
Accuracy +/- 1% (2% band)
Precision 14-bit
0.0092 PSI / 1 bit increase
Max Sampling Rate ~2 kHz
Interface I2C
3.3V or 5V Supply
Cost: ~$36
Ashcroft G17M0242F2200# Pressure Range
0 – 200 PSI
Accuracy
+/- 0.5%
Voltage Supply: 9-24V
Additional Components Needed
Attenuator (2 Op-amps) ~$1.40
Instrumentation Amplifier ~$11.30
Low Pass Filter (3 Opamps) ~$2.10
Analog to Digital Converter
Cost: $175.50 + $20.80 = $196.30
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Case Study: Vibration Sensor
Freescale Xtrinsic MMA8451Q 3-axis Vibration Range
+/- 8g
Accuracy +/- 1% (2% band)
Precision 14-bit
0.000977g / bit
Max Sampling Rate =< 800Hz
Interface I2C
1.95V to 3.6V Supply
Cost: ~$1.67
Omega ACC310 1-axis Vibration Range
+/- 75g
Precision: 100mV/g
Output Range: +/- 7.5V
Voltage Source: 24-36V
Additional Components Needed
Attenuator (2 Op-amps) ~$1.40
Instrumentation Amplifier ~$11.30
Low Pass Filter (3 Opamps) ~$2.10
Analog to Digital Converter
Cost: $175 + $20.80 = $195.80